The Japan Meteorological Agency (JMA) completed its second global atmospheric reanalysis, the Japanese 55-year Reanalysis (JRA-55). However, the horizontal spatial resolution of JRA-55, TL319 (about 55 km), is insufficient for representing the hilly topography of the Japanese islands. Therefore, to reproduce extreme events caused by the hilly topography and their long-term climatological change in Japan, JMA has conducted a dynamical regional downscaling, called DSJRA-55, based on JMA's operational mesoscale model, which has a horizontal resolution of 5 km. DSJRA-55 receives its initial field and boundary conditions from the JRA-55 reanalysis. DSJRA-55 is historically the first products in the world that covers very long term for 55 years with very high resolution in 5 km. Furthermore, DSJRA-55 does not perform data assimilation; instead, initial field and boundary conditions are given at frequent intervals to the downscaled model and short-range forecasts are performed. Then, successive forecasts are connected continuously to create the DSJRA-55 product. In early evaluation results, DSJRA-55 was able to reproduce observed temperature and precipitation during 1958-2012. Although it showed a systematic temperature bias in some regions and seasons and it underestimated the frequencies of heavy-rain days and heavy-rain hours, DSJRA-55 reproduced the overall distribution of orographic precipitation well. DSJRA-55 is therefore expected to be useful for analyzing past extreme events and for statistical studies of long-term climate.
This study analyzed the oceanic convective systems that induced heavy rainfall over the western coast of Sumatra on 28 October 2007. The convective systems that satisfied the definition of a mesoscale convective complex (MCC), as identified by infrared satellite imagery, developed repeatedly for 16 hours over the Indian Ocean near Sumatra. The MCC developed from midnight on 27 October until the early morning of 28 October, and it was intensified by the land breeze from Sumatra. New convective systems around the decaying MCC were generated during the daytime of 28 October, and they propagated to the western coast of Sumatra in the evening because of a divergent outflow from a cold pool. The combination of the land breeze from Sumatra and cold pool outflows from the decaying MCC was a significant factor in the formation of the convective system that induced strong rainfall up to 46 mm h−1 over the western coast of Sumatra.
Multipoint videosonde launchings into the same stratiform cloud system were carried out in Okinawa during the Baiu rainy season for the first time. Six minutes after the first videosonde launching from Cape Zanpa (26.44°N, 127.71°E), the second videosonde was launched from Onna (26.50°N, 127.84°E), which is located 15km northeast of Cape Zanpa. Microphysical features observed by simultaneous videosonde soundings revealed the well-known horizontal homogeneity of the stratiform cloud, but local differences in cloud microphysics were found in the cloud, which are expected to be related to the developing/dissipating processes of the stratiform cloud. This new observation technique using the videosonde will be useful in investigating three-dimensional microphysical structures that change over a short period of time, such as short periods of intensive heavy rain.
We investigated features of the atmosphere and ocean to seek a possible candidate that suppressed the growth of the El Niño event in 2014. In the boreal summer-fall season, equatorially antisymmetric sea surface temperature (SST) anomalies with a positive (negative) sign to the north (south) of the equator prevailed in the central and eastern tropical Pacific. In association with the SST anomalies, cumulus convective activity was enhanced in the region of the climatological Intertropical Convergence Zone (ITCZ). Anomalous southerly surface winds flowing across the equator toward the ITCZ induced upward latent heat flux anomalies and lowered SST in the near-equatorial region. These coherent spatial patterns between SST, wind, and latent heat flux anomalies suggested that the wind-evaporation-SST (WES) feedback sustained the suppression of the El Niño growth. A linear baroclinic model experiment indicated that the enhanced convective heating in the ITCZ also contributed to sustain the anomalous surface southerlies across the equator by the intense meridional atmospheric circulation over the equator. These results indicate that the anomalous southerlies across the equator sustained by the WES feedback and intense convective heating in the ITCZ contributed to the suppression of the El Niño growth.
Dry ice (DI, solid carbon dioxide) and liquid carbon dioxide (LC) are both used in airborne glaciogenic cloud seeding, but no studies have objectively compared the efficacies of those two seeding agents. This paper describes a numerical model that involves the microphysical processes associated with airborne cloud seeding, and describes numerical simulations done to compare the efficacies of airborne DI and LC seeding in enhancing precipitation. Simulation results suggest that seeding effects appear faster for DI, and that the total amount of surface precipitation is almost the same for DI and LC seeding. Our results are counter to previously published assertions that LC is generally superior to DI in enhancing precipitation through airborne seeding in any type of clouds.
In this study, we conducted an energetics analysis of the atmospheric general circulation for up-to-date reanalyses, JRA-55 and ERA-Interim, and compared the result with that of the old reanalysis, JRA-25. Since three-dimensional normal mode functions are used, we can separate the energy spectrum into Rossby and gravity components. According to the comparison between JRA-55 and ERA-Interim, it is found that the characteristics of the energy spectrum and the energy interaction are quite similar in zonal wavenumber and vertical mode domains. However, kinetic energy of Rossby modes for JRA-55 is larger than that for ERA-Interim in all wavenumbers. On the other hand, kinetic energy of gravity modes for JRA-55 is smaller than that for ERA-Interim in small wavenumbers. Therefore, one of the features of JRA-55 is that the wind and geopotential fields are close to the geostrophic balance. According to the comparison among JRA-55, ERA-Interim and JRA-25, the imbalance for JRA-55 at zonal wavenumbers 10 to 25 is similar to ERA-Interim and is smaller than that for JRA-25. It is also found that the interactions of available potential energy in the zonal wavenumber domain for JRA-55 is comparable to that for ERA-Interim and is larger than that for JRA-25.
The quantitative understanding of aerosol-cloud-precipitation interactions is still insufficient despite substantial amounts of previous efforts to solve this problem since it has inherent complexity. Probably we might need overwhelming aerosol forcings well beyond cloud and precipitation variabilities in order to identify and attribute its discernible effect on clouds and precipitation. Korea recently suffered from severe haze episodes that appear to be largely long-range transported from China, which could be made the best use of to evaluate the hypothesis of enhanced aerosol impacts on clouds and precipitation. A couple of severe hazes in January 2013 originated from eastern China were also observed in the mid-Korean peninsula. The cloud systems overlapped with aerosol plumes seemed to be modified such that drizzle-type light precipitation lasted longer within half a day than the operational weather forecast because precipitation might be extended at a less rate due to increases in number concentration of smaller cloud droplets as shown by a sensitivity test using the WRF model. This study shows a possible evidence of inadvertent weather modifications by enhanced aerosols. It also implies that Korea would be a better testbed to investigate aerosol impacts on weather.
El Niño Southern Oscillation (ENSO) influences on rainfall extremes around Sulawesi and the Maluku Islands in the eastern Indonesian Maritime Continent were investigated focusing on spatial and seasonal aspects using daily rainfall data at 23 stations during 1972-2012. The results show that interannual variations in rainfall extremes were strongly correlated with the ENSO phases. Wetter (drier) conditions were associated with La Niña (El Niño) events, in terms of total precipitation, rainy days, and consecutive dry days at more than 90% of the stations. Dry days tend to increase more than 2 months in the El Niño than La Niña years causing severe droughts in the region. Frequency and number of stations of heavy rainfalls increased (decreased) during La Niña (El Niño) events, whereas ENSO influences were weak (strong) on severest (moderately intense) rainfall events. ENSO influences on rainfall amount and number of rainy days vary spatially and seasonally. They were predominant during July–November but less during December–February. Heavy rainfall frequency was significantly higher during La Niña than El Niño years in transitional seasons.
Westerly (WWEs) and easterly (EWEs) wind events, short-lived anomalous westerly and easterly surface winds, are known to occur with unusual large magnitude over the equatorial Pacific. As their relative frequency of occurrence and dependence on background conditions are yet to be fully clarified, we analyzed daily surface winds for 1982-2013 from which WWEs and EWEs are detected. Both types of events appear over the Pacific warm pool, where sea surface temperature (SST) is sufficiently high for active deep convection, and favorably occur with increasing Niño4 SST. However, the frequency of occurrence of EWEs is less than that of WWEs, resulting in asymmetry in wind amplitude. Local and remote anomalous convections are equally important in exciting these events, but different local development processes cause the asymmetry in the frequency of occurrence. These results can also be seen in wind stress anomalies, albeit obscured due to nonlinearity therein.
The seasonal and interannual variations of the gravity waves (GWs) in the Antarctic mesosphere were examined using horizontal wind data at altitudes of 50-100 km above the medium frequency (MF) radar at Syowa Station (69.0°S, 39.6°E). The climatology of the GW variance reached a maximum in winter over a wide altitude range of 64-90 km, and the interannual variability was large below 80 km in spring and autumn. These features are consistent with observations from previous studies. In addition, we detected a weak but significant maximum in the GW variance in summer from 70-78 km as well as large interannual variability. Moreover, we examined three possible mechanisms underlying the GW interannual variability observed in summer. The first mechanism was modulation by stratospheric sudden warmings in the Arctic through inter-hemispheric coupling, and it was not clearly observed at Syowa Station. The second mechanism was modulation of the vertical filtering of GWs in association with the breakdown of the polar vortex in the Southern Hemisphere, and it was identified as a potential mechanism. The third mechanism was tropical convection and propagation to the Antarctic region, and it was identified as another likely mechanism of interannual variability in the GWs. This result was supported by the consistency between years with strong tropical precipitation and years with large GW variances at Syowa Station.
Detailed observations of temperature evolution are required to clarify the mechanism of the nocturnal evolution of urban heat island. We conducted spatial and temporal high-density observation of temperature and longwave radiation in Kyoto City, Japan. The results suggest that the time evolution associated with temperature phase shift is one type of urban heat island evolution. Moreover, the phase shift appears to be closely related to a change in wind direction. Two phases, before and after the phase shift, are suggested to be controlled by two different mechanisms: radiative cooling and advection.
Global wind profile observation is important to improve initial conditions for numerical weather prediction (NWP), general circulation model, and various other meteorological studies. A space-borne Doppler wind lidar (DWL) is one of promising remote sensing techniques for global wind measurement. We describe a study based on simulated satellite measurements for assessing the measurement performances of a Japanese coherent DWL. Global simulations are performed using pseudo-truth atmospheric model of an observing system simulation experiment (OSSE) conducted using the global NWP system of the Japan Meteorological Agency. Wind profile retrieval simulations have been performed for 1 month (August, 2010) and the results show that the percentage of good quality estimates is 40% below 8 km, and it decreases to 10% at 8-20 km in the southern hemisphere and is 20-50% in the northern hemisphere. Expected line-of-sight wind speed errors for good quality estimates are 0.5 m s−1 below 8 km and 1.1 m s−1 at 8-20 km. In the future, the simulated observations will be used in the OSSE to quantitatively infer the potential impacts on NWP accuracy. To illustrate such analysis, results are shown from an initial validation test using a simple wind measurement model instead of the realistic DWL simulations.
The eastward propagation processes of the Madden-Julian oscillation (MJO) was examined from a case study of the first MJO generated in late October during CINDY2011. The eastward propagation of the MJO was found to be synchronized with an extratropical cyclone in the Southern Hemisphere. The synchronized propagation of the MJO and the extratrpical cyclone was associated with the ridge and trough pair meridionally extending between 30°S∼15°N. The convection center of the MJO shifted eastward as a result of the westerly winds in the tropics, expanding eastward by the zonal pressure gradient force between the ridge and trough.
The transport of chemical compounds from surface emissions into the stratosphere is very important for stratospheric, and even global, climate change. However, the lack of observational data makes it difficult to trace these emissions back to specific regions. This study uses numerical simulations to investigate the transport of surface emissions from high population density regions into the stratosphere. In March, April and May, Southeast Asia and Australia tracers contribute ∼1/3 and ∼1/4 of total tracers entering the stratosphere, respectively. In June, July and August, Southwest Asia contributes ∼1/2 of the total, which is far more than the contribution of all other source regions. In September, October and November, South America and Southeast Asia each accounts for ∼1/4 of the total tracer budget. In December, January and February, Australia and Southeast Asia each accounts for ∼1/4 of all tracers entering the stratosphere. A further quantitative estimation illustrates that the average proportion of a tracer entering the stratosphere compared with its total release is 2.6% for Southeast Asia, followed by 1.7% for Australia, 1.4% for Southwest Asia, 1.0% for Africa, 1.0% for South America, 0.9% for East Asia, 0.7% for North America, and 0.3% for Europe.
Climatology of precipitation and associated circulations over southern Zhejiang, northern Fujian and eastern Jiangxi (ZFJ) of China is examined based on the analysis of observational precipitation data and NCEP Reanalysis data from 1971 to 2013. The maximum rainfall over ZFJ occurs in the second ten days of June, which is associated with southwesterly winds in the lower troposphere. The analysis of rainfall amount averaged over ZFJ in the second ten days of June shows the seven strong rainfall years, in which rainfall amount anomalies are larger than one standard deviation. The rainfall in the second ten days over ZFJ for the strong rainfall years is associated with enhanced southwesterly winds in the lower troposphere and strengthened westerly winds in the upper troposphere. Torrential rainfall events occur more frequently during 1991-2013 than during 1971-1990.
The impact of additional radiosonde observations during the Vietnam-Philippines Rainfall Experiment 2010 (VPREX2010) was investigated by performing observing system experiments using the local ensemble transform Kalman filter (LETKF) and the atmospheric general circulation model for the Earth Simulator (AFES). During the experimental period from 15 September to 15 October, 2010, a westward-propagating disturbance was developed in the South China Sea and caused heavy rainfall on the east coast of Vietnam and Hainan Island. By assimilating the additional radiosondes, significant increases in wind speed, temperature and specific humidity were detected in the lower troposphere around the disturbance. In addition, the analysis ensemble spread for meridional wind decreased by 5-25% across the Indochina Peninsula, Philippines Sea and western Pacific to the south of Japan. Moreover, winds became stronger around the disturbance due to the additional observations, and the ensemble spread for wind speed became larger. The results show that the disturbance in an early stage of development was not well detected in the South China Sea without the use of additional radiosonde observations. Therefore, it is suggested that continuous and intensive radiosonde observations in Vietnam and the Philippines are essential for the improvement of the objective analysis of such disturbances.
In data assimilation for weather forecast, ensemble Kalman filter assumes linearity of the observation operator and Gaussianity of the probability distribution function (PDF) to explicitly solve the analysis. As a method avoiding errors based on these assumptions, we describe a four-dimensional ensemble-based variational method (4D-EnVAR) with observation localization. This formulation differs from that of the four-dimensional local ensemble transform Kalman filter (4D-LETKF) only in two points: (1) not assuming linearity of the observation operator and (2) calculating it globally. Using single-observation assimilation experiments and the observation system simulation experiments with a low-resolution atmospheric general circulation model, we demonstrate that 4D-EnVAR with observation localization has an advantage over 4D-LETKF because the observation operator is globally calculated in EnVAR.
Himawari-8 is a Japanese geostationary weather satellite that was launched in October 2014 and has been in operation since July 2015. Himawari-8 is equipped with an outstanding high-performance imager that has 16 spectral channels (3 for visible, 3 for near-infrared and 10 for infrared wavelengths) with a 10-minute observation interval. We retrieved aerosol optical thickness (AOT) from visible and near-infrared multispectral observations of Himawari-8 and assimilated the AOT data into a global aerosol forecast model with an ensemble Kalman filter system. The data assimilation result was validated by comparison with conventional products derived from polar-orbiting satellite aerosol observations (i.e., Moderate Resolution Imaging Spectroradiometer (MODIS) AOT) of an Asian dust storm in June 2015. The Himawari-8 AOT data assimilation successfully produced an analysis and forecast of the Asian dust that was comparable or superior to those of the MODIS AOT data assimilation. The Himawari-8 aerosol product has a much higher temporal coverage than that of polar-orbiting satellites, which is promising for aerosol data assimilation. This study is a first step in the application of geostationary satellites for aerosol research.
We revealed the difference in the ice cloud microphysical properties of high clouds between the western Pacific (WP) and eastern Pacific (EP) regions, based on satellite retrievals. The effective particle radius (re) was analyzed by using active sensors on board the CloudSat and CALIPSO satellites. We focused on ice clouds, defined as clouds with cloud top temperatures of less than 0°C. These ice clouds are classified into five types defined by the cloud optical thickness (COT). Mean cloud top heights of high cloud in WP were higher than those in EP by about 2km. The re of optically thin clouds (0 < COT < 0.3) showed weak temperature dependency over both regions. For optically thick clouds (3 < COT), re increases with temperature (T). In the WP, re at lower temperatures (T < −40°C) is larger than that in the EP, whereas in the EP, re at higher temperatures (T > −40°C) is larger than that in the WP. The difference in re may be caused by differences in moisture convergence and upward motion.
This study calculated the Walker circulation index and then discovered strengthening of the circulation in recent years. Thus, this study analyzed a difference of averages between 1999-2013 and 1984-1998 periods to determine the cause of the strengthening of the Walker circulation in recent years. With regard to 850 hPa stream flows, analysis on difference between two periods showed that anomalous easterlies (anomalous trade wind) were dominant due to strengthening of anomalous anticyclonic circulations at the subtropical Pacific of both hemispheres. In order to determine whether upward flows are strengthened at the tropical western Pacific and downward flows are strengthened at the tropical central and eastern Pacific in recent years, a difference in zonal atmospheric circulation averaged over 5°S-5°N between two periods was analyzed. The Walker circulation, in which air ascended at the equatorial western Pacific was descended at the equatorial central and eastern Pacific, was strengthened more in recent years.
Typhoon Songda (2004), while undergoing an extratropical transition and reintensification over the Sea of Japan, spawned serious damages over forest areas in the northern part of Japan. To clarify influences of anticipated global warming on the typhoon hazard in high-latitude regions, we carried out numerical experiments on Typhoon Songda in a current climate and a pseudo global warming climate and examined the impacts of climate change on the typhoon intensify and windstorms for the region. The typhoon intensity at its maturity becomes stronger in a warming climate, and the typhoon while passing over the Sea of Japan weakens more rapidly in the warming climate. Thus the strong wind over the northern part of Japan in the warming climate also becomes weaker. We examined atmospheric environmental factors in the higher-latitude region: sea surface temperature, vertical shear, and meridional gradient of temperature. It was found that the environmental meridional gradient of temperature at the 500 hPa level is smaller in the warming climate than in the current climate, which indicates that the baroclinicity becomes weak under global warming. The weak baroclinicity makes the typhoon environment unfavorable for the extratropical transition, and therefore, possibly reduces the severity of typhoon wind in northern Japan.
The annual variation of extreme precipitation in Japan was analyzed using hourly data at 981 stations for 36 years (1979-2014). Except over the Nansei Islands, the intensity of hourly extreme precipitation (Pext) defined by upper 0.1% cases is found to show a unimodal annual variation with a peak in midsummer, unlike total precipitation amount which has a bimodal annual variation with two peaks of Baiu and Shurin. The dependence of Pext on mean temperature is close to or slightly lower than the Clausius-Clapeyron (CC) rate of 6-7%/K, indicating that the annual variation of extreme precipitation is strongly linked to that of saturation vapor amount. On the other hand, Pext is different according to regions by several tens of percent for a fixed temperature. This fact suggests the influence of geographical factors on its spatial distribution. As for 24-hour precipitation, Pext shows larger deviation from the CC scaling and larger regional differences than that of hourly precipitation, implying stronger control of synoptic effects in generating long-lasting heavy rainfall.
A role of environmental shear on the organization mode of warm-season quasi-stationary convective clusters (QSCCs) was statistically investigated by using operational weather radar and radiosonde data from May to October during 2005-2012. With the use of an ellipse-fitting method, the total number of QSCCs whose shape was determined was 2549. It was found that 87% of the QSCCs have an aspect ratio of larger than 1.4, suggesting that the elongated mode is dominant during the warm season in Japan. The elongated QSCCs were mostly oriented southwest–northeast. The analyses of the environmental shear direction with respect to the orientation of the elongated QSCCs showed that the wind shear direction at the lower troposphere is mainly parallel to the orientation of the elongated QSCCs. A comparison between the elongated and the circular QSCCs with the environmental parameters showed that the lower convective instability and stronger intensity of the low-level shear clearly characterize the elongated QSCC environments. A parameter combining convective instability and shear, bulk Richardson number, characterizes the environmental conditions for determining the organization mode of the QSCCs, suggesting that a back-building mechanism should play a role in generating the elongated QSCCs.
Using wind data over three years from July 2012-June 2015 from the PANSY radar, an MST radar, newly installed at Syowa Station (39.59°E, 69.0°S), statistical characteristics of vertical winds and vertical momentum fluxes in the Antarctic lower troposphere are examined. Frequency spectra covering a wide frequency range from (30 d)−1 to (8 min)−1 are divided into three frequency regions obeying power laws with different scaling exponents. The transition frequencies are different between horizontal and vertical wind spectra. Vertical fluxes of horizontal momentum were estimated for two wave period ranges of 8 min–2 h and 2 h–1 d which have almost equal logarithmic scales. The momentum fluxes are larger for longer period components. There are evidences showing that the vertical wind disturbances in the lower troposphere are due to gravity waves forced by topography aligned in the north-south direction. First, the strong disturbances are observed when horizontal winds are strong near the surface. Second, zonal winds tend to almost zero around the top of the disturbances. Third, frequency spectra are large at a wide range of frequency below a critical level, as is consistent with the phase modulation of mountain waves by unsteady mean flow.
Persistent and/or recurring large-/synoptic-scale atmospheric flow patterns can cause severe weather events in surrounding areas. This study first classify the large-/synoptic-scale patterns in the ERA-Interim 500 hPa geopotential height over East Asia in extended winters of 1979/80-2013/14 and then assess their predictability on medium-range timescales for the extended winters of 2006/07-2013/14 and 1985/86-2013/14, using operational ensemble forecasts and ensemble reforecasts, respectively. The winter monsoon, western Pacific (WP), high and low pressure, and southerly flow (SF) patterns are detected as dominant patterns. Some transitions among these patterns occur more frequently, leading to preferred winter circuits of patterns. The occurrence of El Niño/La Niña can also significantly increase or decrease the frequency of patterns. Models have broadly similar biases in the transitions and frequencies of patterns, but some models show different biases with lead time. Verification of probabilistic flow-pattern forecasts reveals that the forecasts made by state-of-the-art models are useful up to a lead time of 14 days on average, and that forecasts related to WP (SF) tend to show higher (lower) skills than the other forecasts. This suggests that models find it difficult to predict the movement of low pressure systems south of Japan.
The radiative and chemical interactions of stratospheric water vapour (SWV) mean that SWV has a significant influence on the climate. Based on the Stratospheric Water and OzOne Satellite Homogenized (SWOOSH) SWV data (100-1 hPa, 60°S to 60°N), variations in the middle and upper SWV are mainly investigated over the past two decades. Water vapour variations below 10 hPa are found to result from upward transport of the lower SWV. Using empirical orthogonal function analysis and regression analysis of the anomalous upper SWV variability, it is found that upper SWV variations have an 11-year period controlled by the solar cycle as well as a 2-year cycle regulated by the quasi-biennial oscillation (QBO). Our results also show that state-of-the-art chemistry–climate models perform poorly in simulating the upper SWV variations. It is possibly because the effect of lower–middle SWV changes on the upper SWV variations is too strong in the simulations.
This study exemplifies a capability of upper tropospheric Atmospheric Motion Vectors (AMVs) derived from successive imagery of MTSAT-2 images to detect the intensification of Typhoon Danas (1324). The evolution of the wind field around the cloud top captured by the AMVs revealed two remarkable features: increases of radial outflow, representing secondary circulation in the form of the two convective bursts (CBs), and increases of tangential winds after the first CBs and during the second CBs. It is suggested that the updrafts and latent heat release associated with the first CBs induced increases of lower tropospheric convergence and absolute angular momentum. This preconditioned the axisymmetrization and increase in tangential velocity and inertial stability during the second CBs that led to tropical cyclone (TC) intensification. These results show that upper tropospheric AMVs can be used to detect TC intensification and thereby improve TC forecasts.
Supercooled liquid droplets (SLDs) not only play important roles in precipitation formation processes but they also affect the radiation budget. Therefore, it is important to clarify the distribution and quantity of SLDs. Hydrometeor videosonde observations were performed in February 2011 at Rikubetsu in inland Hokkaido, Japan. Five hydrometeor videosondes were released in ice precipitations in stratiform clouds and SLDs were detected in three cases. The clouds in these three cases had SLDs at the cloud tops. The microphysical quantities of the SLD layers were within the ranges of those observed in Arctic mixed-phase clouds. The cloud-top SLDs had potential to cause radiative cooling, which contributed to the formation of upward motion generating ice precipitation. Small water vapor amounts above the cloud tops cannot contribute to moisture supply. Vertical profiles of temperature and moisture showed that the SLD layers were decoupled from surface moisture sources. The absence of additional moisture supply was consistent with short lifetimes, compared with persistent Arctic mixed-phase clouds.
By the GEOS-Chem simulation with fixed anthropogenic and biomass burning emissions, this study exhibits the differences in interannual variations (IAVs) of surface-layer PM2.5 concentrations among three populated regions, the eastern United States (US), eastern China, and Europe driven by variations in meteorological parameters. In the eastern US, PM2.5 concentrations have relatively small IAVs with no explicit seasonality, with the absolute percent departure from the mean (APDM) values of 4-16% in four seasons. The IAVs of PM2.5 are found to be large in North China and the northwestern Europe during winter and spring. The APDM values are 24-28% in winter and 32-36% in spring in eastern China, and 32-36% in winter and 20-24% in spring in Europe. Additionally, we obtain the key meteorological parameters that drive the IAVs of PM2.5 by the stepwise multiline regression model (SLR) containing 8 meteorological variables. The most important meteorological variables over the eastern US, eastern China, and Europe are, respectively, the westerly at 850 hPa, surface wind speed, and the planetary boundary layer height in winter, and precipitation, relative humidity, and surface temperature in summer.
A future change in the soil-frost depth in eastern Hokkaido was assessed with multiple local climate scenarios. The bias-corrected downscaled data with an aid of a simple snow-depth model were applied to the empirical estimation from cumulative freezing degree-days (CFD) for days when the snow depth is less than 20 cm. Consequently, as the response to global warming climate that was defined as +2-K world in this paper, the maximum soil-frost depth significantly decreased in the coastal regions along the Pacific and in central Tokachi subprefecture with a slight variation among climate scenarios. Parameter-sweep experiments for two observed cases clarified that the temperature increase would bring the soil warming in the environment where the insulation effect of snow cover does not work at present, while it would bring the soil frosting in the environment where the insulation effect works.
We investigated the relationship between the atmospheric variability in El Niño conditions and Arctic Oscillation (AO) during the period from late autumn to early spring, focusing on the vertical linkage between the troposphere and the stratosphere, based on a composite analysis. The results of the composite analysis indicate that the vertical linkage is the clearest in late winter to early spring, particularly in the three-month of February – April (FMA). In FMA, the upper tropospheric patterns and upward propagation of planetary waves with zonal wavenumber 1 are enhanced and contribute to a negative phase of the stratospheric Northern Annular Mode (NAM) in El Niño conditions. The results also indicate that the stratospheric negative potential vorticity (PV) anomalies associated with the negative phase of NAM induce a lowered tropopause, vertical compression of the tropospheric column, positive surface pressure anomalies in the polar region and hence the negative phase of the AO. This vertical linkage and the impact of sea level pressure near the pole are consistent with a quantitative estimation based on geostrophic and hydrostatic adjustment associated with the stratospheric PV anomalies.
The 5-km-mesh, Non-Hydrostatic Regional Climate Model was used to simulate snow depths in Japan and to project their changes in the future. The simulated snow depths had large biases, and bias corrections were required to project future snow depths accurately. We developed a new method of bias correction that is accurate and easily implemented for automatic use on a computer. Three classification methods of regional frequency analysis were tested in nine regions of Japan. The classification method based on the second order of L-moments (L-cv) was the best bias correction method among those tested. We checked that this bias correction was useful method for future climate projections by using the test sample estimate. Snow depth in the future climate was projected to decrease by about 50 cm, such that the average snow depth over Japan was about 30 cm in the future climate. The projected decrease in the maximum snow depth was large in the Nagano and Gifu regions and small in the Hokkaido region compared with other regions.
Long-term (2005-2015) tropospheric nitrogen dioxide (NO2) column data recorded by the satellite-borne Ozone Monitoring Instrument (OMI) in East Asia were analyzed to investigate annual trends on an national basis and their potential causes. We found an evident decrease of 6% year−1 in the NO2 level over China after 2011. The grid-basis trend analysis implies that the rapid decrease occurred on a provincial or larger spatial scale and was likely due to a nationwide action such as the widespread use of denitrification units. In Japan and South Korea, a turnaround indicating an increase was observed after 2013 and 2012, respectively. As a consequence, the tropospheric NO2 pollution level in East Asia was found to be recovered to the 5-year-ago level in 2015.
41% of Earth's land area is drylands, including the arid region of China and Mongolia. Evaporation per unit area of arid region is relatively low, but the surface area of the dryland region gives it a regional significance in the context of climate variability. We used the eddy covariance method to estimate heat fluxes in the Gobi Desert steppe of Mongolia from May to October 2014. Observation period averaged net radiation, sensible heat flux, and latent heat flux were 85 W m−2, 58 W m−2, and 11 W m−2, respectively. The sensible heat flux was larger than the latent heat flux throughout most of the study period, but the latent heat flux exceeded the sensible heat flux for a couple of days after rainfall events. Total evaporation estimated by the eddy covariance method almost equaled total rainfall and decreased the soil water content. Discussion is extended to the imbalance problem, especially the latentheat heat flux compared with precipitation and the soil moisture change.
Greenhouse gases Observing SATellite (GOSAT) is the operational satellite dedicated to atmospheric CO2 observations. Assimilation of data provided by GOSAT is expected to yield reliable CO2 fluxes in semi-arid regions because of frequent observations owing to clear skies. Here we estimated net CO2 flux over semi-arid regions of the Southern Hemisphere using the GOSAT column averaged CO2 (XCO2) and surface CO2 measurements. Assimilation of GOSAT XCO2 indicated that semi-arid regions are integral components of recent terrestrial CO2 uptake, accounting for 44% globally. Compared with estimates assimilated from surface measurements, estimates by GOSAT XCO2 suggest a 50% reduction in the semi-arid CO2 uptake, amounting to 1.1 Pg C yr−1. Significant estimation differences occurred for South America and South Africa, where the GOSAT makes frequent measurements but where surface CO2 measurements are limited. In comparison, the two estimates varied less in Australia, where more surface measurements are available. These results suggest that GOSAT XCO2 is effective at regulating excess estimates of semi-arid CO2 uptake in regions that are less constrained by surface CO2 measurements. To promote understanding of climate change effects in semi-arid regions, it is important to continue monitoring trends in CO2 uptake with GOSAT.
Aeolian dust, resulting from wind erosion, is controlled by two major factors: aeolian erosivity (i.e., wind speed) and aeolian erodibility (i.e., land surface conditions). Erodibility factors include a number of land surface parameters that interact in complicated manners. Thus far, the extent to which each of the factors contributes to dust outbreaks, which vary regionally and seasonally, remains unclear. As such, we present a novel map of the controlling factors for dust outbreaks in dryland of East Asia by quantifying the relative contributions of the erosivity and various erodibility parameters to inter-annual dust variations on a station basis during the period of 1999 to 2013. Erosivity controls dust outbreaks in the Taklimakan Desert, west of the Hexi Corridor, and on the north side of the Altai Mountain. On the other hand, dust outbreaks are dependent on erodibility in steppe regions: lower precipitation or less abundant vegetation during the previous summer was found to be related to dusty springs in the Mongolian steppe, whereas the less abundant spring vegetation and reduced snow cover enhanced dust outbreaks in the Inner Mongolian steppe. Anthropogenically restored vegetation in desertified areas was found to be likely to suppress dust outbreaks in the Loess Plateau.
Intraseasonal variability of precipitation over the Indonesian Maritime Continent (IMC) is related mainly to the Madden–Julian oscillation (MJO). The MJO enhances and suppresses precipitation around the IMC according to its phase. This study examined the relationship between the MJO and the stable isotope of the precipitation over the IMC. Observations and simulation data from an isotope circulation model and colored moisture analysis (ICM/CMA) were used. Temporal analyses of the observed and simulated data showed that an active MJO stage, associated with a positive precipitation anomaly over the IMC, was correlated with the depletion of δ18Op at all stations. Spatial analysis of the total column precipitable water from the ICM/CMA revealed that during the MJO phase 3-5, water of Indian Ocean origin reached as far as western parts of Borneo and Bali. For other part of the IMC in these phases, vapor was refreshed from the South Maritime Continent Ocean and North Maritime Continent Ocean. Water that originated from the Pacific Ocean traveled westward during MJO phases 3-5, and it retreated eastward during phases 6-8.
The Transformed Eulerian-Mean (TEM) equations derived by D. G. Andrews and M. E. McIntyre are powerful tools for diagnosing the meridional circulation and wave-mean interaction in the troposphere and/or middle atmosphere. However, the TEM equations cannot properly treat the lower boundary and unstable waves. The Mass-weighted Isentropic zonal Mean (MIM) equations derived by T. Iwasaki are the equations that overcome those problems and are recently used for analyzing polar cold air outbreak. On the other hand, the MIM equations have not been extended to three dimensions (3D), especially for wave activity flux, although the TEM equations have been extended by several studies. The present study formulates the 3D wave activity flux describing wave propagation in the mass-weighted isentropic time mean equations. A dispersion relation for inertia-gravity waves and Rossby waves on those equations is also derived and used to relate the 3D wave activity flux to the group velocity. Finally, we use the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-analysis (ERA-Interim) data and report the comparison result of 3D wave activity fluxes between TEM and mass-weighted isentropic time mean equations.
Typhoon Fengshen (2008) was marked by a persistent track toward the northwest, which was poorly predicted by an operational hydrostatic model, which indicated a significant northward bias. Using a global nonhydrostatic model with finer grid spacing, we have simulated a reliable track of this typhoon. The purpose of this study is to clarify the causes of the northward bias by comparing the output of the two models. This typhoon was marked by the asymmetry of rainfall concentrating in the downshear side. While both models could reproduce the asymmetric structure, a significant difference between them was found in the vertical structure. In the hydrostatic model, the vortex tilted to the downshear side with a displacement from lower to upper levels exceeding 100 km. This tilt was related to weak updrafts of, at most, 0.5 m s−1. Diagnosis using vorticity budget demonstrated that the tilt of the vortex resulted from a lack of vertical coupling that was too weak to withstand differential advection between the lower and upper levels. These results suggest the importance of reproducing inner-core updrafts for better track prediction of a typhoon in an environment with strong vertical shear.
Himawari-8, a next-generation geostationary meteorological satellite that has been in operation since July 2015, incorporates significant improvements in resolution, scan frequency, and number of bands, bringing new capabilities to weather forecasting. By taking advantage of the availability of high-frequency data with high spatial resolution, an ensemble Kalman filter implemented with a mesoscale regional model assimilated rapid-scan atmospheric motion vectors (RS-AMVs) from Himawari-8. Data assimilation and ensemble forecast experiments were conducted for a heavy rainfall event that occurred in September 2015 in the Kanto and Tohoku regions of Japan. The results showed that the inclusion of RS-AMVs improved precipitation scores, especially for weak and moderate rainfall. In addition, the subsequent model forecast simulated successfully the band of heavy rainfall. Ensemble-based probabilistic forecasts showed that when RS-AMVs were assimilated, the results captured the occurrence of torrential rainfall with a relatively high probability. The ensemble-based correlation analysis indicated that the strong rainfall was related to advection of moisture at low to mid levels and moisture flux convergence at lower levels. Simulations with a higher resolution model initialized by nested data assimilation showed that the assimilation of frequent RS-AMVs improved the forecast results.
This study investigated the impact of observation operators on low-level wind speed analysis. An evaluation of wind speeds retrieved by variational multiple-Doppler analyses using radial velocities (Vr) based on the formats of both a Plan Position Indicator (PPI) (hereafter, PPI-VAR) and a Constant Altitude Plan Position Indicator (CAPPI) (hereafter, CAPPI-VAR) was performed for comparison with wind speeds observed by a wind profiler during the warm season of three consecutive years. The statistical analysis showed that PPI-VAR was more accurate than CAPPI-VAR at 500 m above ground level (AGL). The error of CAPPI-VAR at 500 m AGL was caused by a representative error of CAPPI-formatted Vr derived from a certain radar whose beam height was far from the analysis level, and this error became more obvious the greater the vertical difference in wind speed across the analysis level. CAPPI-VAR uses CAPPI-formatted Vr from each radar equally; thus, the representative error might cause performance degradation of CAPPI-VAR at 500 m AGL. Conversely, PPI-VAR uses PPI-formatted Vr from each radar with appropriate weighting based on the beam height distance from the analysis level. PPI-VAR showed better results at 500 m AGL because the observation grid points were dense around 500 m AGL.
A previous study shows that tropical convective systems share a similar structure regardless of horizontal scale: lower-level horizontal convergence precedes maximum precipitation and this convergence rises and tilts over time. We conduct a series of aqua-planet experiments (APE) using the Non-hydrostatic Icosahedral Atmospheric Model (NICAM) to investigate whether this structure is maintained under different conditions with an Earth-like APE (CTL-exp) and a radiative convective equilibrium (RCE-exp) where sea surface temperature is uniform and no planetary rotation is applied. The experiments are conducted with the 56 km mesh size with explicit convective calculation without a cumulus parameterization scheme. CTL-exp shows a well-known multi-scale convective structure where a smaller convective system propagates westward along the equator whereas a larger convective system propagates eastward. In RCE-exp, the simulation also has a multi-scale structure but the larger-scale convective system no longer propagates in a preferred direction. The convective systems in CTL-exp have a similar tilting structure to tropical convective systems regardless of horizontal scale, while the larger scale convective system in RCE-exp show a smaller tilting structure. We speculate that the difference in CTL-exp and RCE-exp structures comes from the propagation speed of the convective systems.
We describe two unprecedented large (100-member), long-term (61-year) ensembles based on MRI-AGCM3.2, which were driven by historical and non-warming climate forcing. These ensembles comprise the “Database for Policy Decision making for Future climate change (d4PDF)”. We compare these ensembles to large ensembles based on another climate model, as well as to observed data, to investigate the influence of anthropogenic activities on historical changes in the numbers of record-breaking events, including: the annual coldest daily minimum temperature (TNn), the annual warmest daily maximum temperature (TXx) and the annual most intense daily precipitation event (Rx1day). These two climate model ensembles indicate that human activity has already had statistically significant impacts on the number of record-breaking extreme events worldwide mainly in the Northern Hemisphere land. Specifically, human activities have altered the likelihood that a wider area globally would suffer record-breaking TNn, TXx and Rx1day events than that observed over the 2001-2010 period by a factor of at least 0.6, 5.4 and 1.3, respectively. However, we also find that the estimated spatial patterns and amplitudes of anthropogenic impacts on the probabilities of record-breaking events are sensitive to the climate model and/or natural-world boundary conditions used in the attribution studies.
As nonhydrostatic models have higher resolution, a topographical representation scheme is desirable as an alternative to the terrain-following approach, which is unstable for steep topography. We developed a conserved topographical representation scheme using a thin-wall approximation in z-coordinates (the CT scheme). This scheme is formulated by the flux-form finite-volume method with a flux limiter, so that the total integrals over the entire domain of prognostic variables are conserved: this is advantageous compared to the conventional thin-wall approximation method. The CT scheme is easily implemented for existing models that use the finite-volume method. We constructed the scheme to satisfy conservation of mass, horizontal momentum, and total energy. We compared the results of the CT scheme for an isolated mountain case with those of a step-mountain (SM) method. The CT scheme represents the propagation of gravity waves more accurately than the SM method. The upward flux of horizontal momentum becomes more vertically uniform for the CT scheme than for the SM method over time. In addition, the horizontal momentum budget shows that the total momentum is reduced by reaction force at the lower boundary with changes due to numerical damping in the upper layers and numerical filters in the free layers.
The climatological features of the pole-ward, equator-ward edges, and the width of the tropical Hadley Cell (HC) during the annual cycle in each hemisphere are studied based on six reanalyses datasets (i.e., NCEP/NCAR, NCEP-DOE, ERA40, JRA25, ERA-Interim, and CFSR). The result indicates that the amplitude of HC in the Northern Hemisphere (NH) is much intense than that in the Southern Hemisphere (SH). And the amplitude of equator-ward edge is much larger than that of the pole-ward edges in both hemispheres. In addition, it is found that the ERA40, CFSR (for the pole-ward edge of HC in the SH) and JRA25 (for the pole-ward edge of the HC in the NH) show big inconsistency comparing with the other datasets in depicting the locations of pole-ward edges of HC.
In this study, we investigate the impact of interannual variability of boreal summertime (June-September) north Indian Ocean (NIO) sea surface temperature (SST) on the distribution of tropical cyclone (TC) genesis over the western North Pacific (WNP) using observational datasets. In the boreal summers with warm (cold) SST in the NIO, fewer (more) than normal TCs form over the entire WNP, with fewer (more) TCs forming north of 10°N and more (fewer) TCs forming south of 10°N. The warm (cold) SST in the NIO induces anomalous anticyclonic (cyclonic) vorticity north of 10°N and cyclonic (anticyclonic) vorticity south of 10°N, which contributes to the meridional seesaw-like distribution of WNP TC genesis. This study provides a new perspective to understand TC activities over the WNP and may help seasonal TC prediction.
Tropical cyclone (TC) intensity forecasts issued by the Regional Specialized Meteorological Centre (RSMC) Tokyo - Typhoon Center are systematically compiled to analyze the long-term behavior of errors and to explore the potential for improvement in the forecast accuracy using a statistical correction approach. In this study, a dataset is constructed from annual statistics and every single forecast listed on annual reports on the activities of the RSMC Tokyo. This study found that (1) the accuracy of annual mean forecast has not improved over 26 years and that (2) forecast errors tend to be larger in the rapidly developing TCs. Further analysis reveals that recent forecast output (2008-2014) contains biases associated with the magnitude of the vertical shear of horizontal wind, convective available potential energy, upper ocean temperature, maximum potential intensity (MPI) and ocean coupling potential intensity (OC_PI). To evaluate the adverse effect of such biases in the current forecast system, a simple statistical correction is applied. It improved TC intensity forecast by 7.8-16.9% when an OC_PI is employed.
The rare snowfall event in 2012 over North China due to the development of an unusual cyclone was analyzed. The results show that the cyclonic vorticity occurs first at the middle troposphere, and then extends to both the upper and lower troposphere. The trigger mechanism for the cyclone genesis is the baroclinic forcing with the upper unusual vorticity advection promoting its rapid development. The efficient deployment of the high- and low-level jets forms a favorable environment for its development and also transports a great number of vapor and the instability energy into the area of blizzard, resulting in the occurrence of the rare event.
Synoptic conditions causing an extreme snowfall event in the Kanto-Koshin district occurred on 14-15 February 2014 are investigated through a reanalysis data set. Associated with a developing cyclone passing the south coast of Japan, persistent snowfall exceeding more than 24 hours over the Kofu-Basin resulted in 112 cm snowfall at Kofu. Slow progress of the south-coast cyclone also contributed to the long snowfall duration. An anticyclone developed over the northern Japan (∼1032 hPa) also contributed to this extreme snowfall. This anticyclone brought warm and moist air inflow by southeasterlies forming moisture flux convergence over the Kanto-Koshin district on the morning of 14th when snowfall started in the Koshin district in spite that the south-coast cyclone was located to the south of Kyushu. Further, ageostrophic cold northerlies with high pressure extension from the anticyclone by “cold-air damming (CAD)” would suppress warming with the approaching south-coast cyclone and keep snowfall until the morning of 15th. In other four heavy snowfall events at Kofu, snowfall durations were almost 12 hours. Although anticyclone to the north and CAD were identified in each case, the moisture transport from the southeast was not evident and moisture flux convergence was not formed earlier.
We conducted observations using four shipborne global navigation satellite system (GNSS) receivers on three research vessels and one passenger ferry to assess the real-time practicality of measuring GNSS-derived precipitable water vapor (PWV) over the ocean. A kinematic precise point positioning strategy was used for the GNSS analysis with a real-time GNSS satellite ephemerides (orbit and clock information). The analyzed time series of PWV was contaminated with unrealistic sharp variations that occasionally occurred. Periodic occurrence of a spiky variation with a cycle of one sidereal day, along with post-fit phase residuals averaged at each elevation and azimuth, indicated that one of the causes of the unrealistically large time variation was interference of reflected signals (multi-path). A simple quality control (QC) procedure based on the amount of PWV time variation was proposed. After the QC was applied, the retrieved PWVs had 3.4-5.4mm root mean square (RMS) differences against radiosonde observations, and 2.3-3.7mm RMS against those retrieved at nearby ground GNSS stations. The proposed QC procedure rejected more than 60 percent of retrieved PWV on research vessels and 6-11 percent on a passenger ferry. The results demonstrate the great potential of the real-time ephemerides and the necessity for careful consideration of the observation environment.
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