The possible impact of the winter monsoon intensity and sea surface temperature (SST) gradient on the activity of explosively developing extratropical cyclones around Japan is investigated using the Weather Research Forecasting model. Two independent long-term integrations over 18 winters from 1993/94 to 2010/2011 are conducted using prescribed observed SST data (OS run) and spatially smoothed data (SS run). The OS run is successful in reproducing the spatial distributions of the explosive cyclone activity in the vicinity of Japan under both strong and weak winter monsoon conditions. Under strong monsoon conditions, the Kuroshio, the Kuroshio Extension, and the Japan Sea subpolar fronts give rise to enhanced near-surface baroclinicity through the increase in heat and moisture fluxes from the ocean surface, resulting in frequent occurrence of the explosive cyclone activity along those fronts.
To accurately simulate the present and project the late 21st century climates of Japan, we carried out 20-year integrations with a 5-km-mesh, non-hydrostatic regional climate model (NHRCM) nested within a 20-km-mesh Atmospheric Global Circulation Model (AGCM20). The NHRCM simulated monthly mean snow depths more accurately than AGCM20 which used to drive NHRCM. However, NHRCM underestimated snow depths on the Japan Sea side of northern Japan. Because a sample size was insufficient for correcting bias with a cumulative distribution function mapping (CDFM) method at one site, we applied a regional frequency analysis method to group AMeDAS sites in Hokkaido. We tested two kinds of bias corrections: CDFM and a bias correction (designated PBC) based on equating the histograms of model results and observational data. Both methods greatly reduced the biases of simulated snow depths, but PBC produced a greater reduction in the root mean square error in almost every region. The results of PBC indicated that snow depth will decrease more than 80 cm in western parts of Hokkaido in the future.
It is shown that there are two types of wave solutions trapped at the boundaries which owe to the Coriolis force proportional to the meridional component of the earth's rotation vector (hereafter referred to as the fH force) under the nontraditional approximation (non-TA). One is a type of Kelvin waves (non-TA Kelvin waves) trapped on the eastern and western boundaries. Unlike traditional Kelvin waves (TA Kelvin waves), non-TA Kelvin waves trapped on the western (eastern) boundary can have northward (southward) phase and group velocities in the Northern Hemisphere (NH). The other is a type of Rossby waves trapped on the ground. The external Rossby waves can have wave structure in the vertical and amplitudes decaying with height. Moreover, the fH force modifies even the characteristics of TA Kelvin waves trapped on the southern and northern boundaries: In the NH, the Kelvin waves trapped on the southern boundary have an upper limit (kc) to the zonal wavenumber (k), and those with large k (> kc) trapped on the northern boundary have eastward phase velocity in the NH. The latter is regarded as the third type of edge waves unique to non-TA.
The relationship between the twilight sky and atmospheric state has been studied for the past 50 years, providing a good understanding of the general picture of the radiative transfer process under twilight conditions. However, tropospheric aerosols may strongly affect the spectral characteristics of the twilight glow near the horizon under clear-sky conditions, and the mechanism of this phenomenon is not well understood because of its high complexity. In this study, we observed the twilight sky using a digital camera and calculated the spectral radiance of the twilight sky using a radiative transfer model (RTM). Photographic observations aided by several ground-based radiometric measurements revealed that tropospheric aerosols significantly influenced the twilight sky color. In particular, the sky near the horizon became darker and bluer with increasing aerosol optical depth (AOD), which was consistent with the RTM simulations. The RTM simulations also showed for the first time that bluish multiply scattered light dominated the twilight sky near the horizon and reddish singly scattered light decreased with increasing tropospheric AOD. These simulation results suggest that photographic observations have the potential to characterize tropospheric aerosols under twilight conditions.
We propose a new scenario for the seasonal migration of the Baiu frontal zone over the East China Sea in which this migration is affected by variations in the sea surface temperature (SST). Using atmospheric and oceanic objective analysis datasets, a relationship was determined between the seasonal migration of the Baiu frontal zone and the decaying process of the cold high over the East China Sea. Before the middle of June, the cold high, cooled by the low SST, is present over the continental shelf, and the position of the Baiu frontal zone corresponds to that of the Kuroshio Front. After the middle of June, the cold high decays and is shifted northward in association with the warming SST over the shelf. As a result, the Baiu frontal zone migrates northward and ends in the middle of July due to the dissipation of the cold high.
Changes of a surface wind under the future climate condition in and around Japan were investigated by using a 5-km-mesh non-hydrostatic regional climate model (NHRCM05). In the present climate, monthly mean surface wind speeds are well reproduced by NHRCM05. Additionally, the NHRCM05 reproduces the seasonal change of mean surface wind speeds fairly well: the wind during the cold season is calculated to be stronger than that in the warm season. Under the future climate, monthly mean surface wind speeds show little change. In the warm season, according to a 20-km-mesh atmospheric general circulation model (AGCM20), the Pacific anticyclone is projected to weaken near Japan, and the sea level pressure over the seas around the Philippine Islands is projected to rise, so the frequency of the westerly wind is projected to increase across Japan by NHRCM05. During the cold season, according to NHRCM05, the wind speeds around Hokkaido are projected to increase a little value due to the decrease of the sea ice in the Sea of Okhotsk. Further, the frequency of southeasterly wind is projected to increase significantly. In the future, the Monsoon Index is projected to be lower than in the present climate by AGCM20, resulting in southeasterly wind inflow to Japan.
The two-way relationship between tropical cyclones over the Bay of Bengal in late fall and stationary Rossby wave propagation along the Asian jet is examined using the Japanese long-term Re-Analysis project (JRA-25) and the Japan Meteorological Agency Climate Data Assimilation System (JCDAS) data. As a precursory signal of the maximum development of the tropical cyclones migrating northward over the Bay of Bengal, a stationary wavetrain pattern prevails from the Mediterranean to the western Indian subcontinent. A cyclonic circulation anomaly to the northwest of the Bay of Bengal, which is part of the wavetrain pattern, facilitates the northward migration and enhancement of the tropical cyclones over the Bay of Bengal through the change in the mid-tropospheric mean flow and the equatorward advection of higher potential vorticity, whereas those cyclones can in turn give rise to the downstream development of the stationary Rossby wave packets along the Asian jet. Such a teleconnection further triggers surface cyclogenesis in the vicinity of Japan by leading to the intrusion of higher potential vorticity in the upper troposphere toward the Japan Sea.
Decadal variability of Antarctic sea ice and a role of ocean dynamics are examined with sea ice concentration (SIC) and sea surface temperature (SST) derived from satellite microwave observation and obtained by a high resolution coupled ocean-atmosphere-ice general circulation model (CGCM). Sea ice observations revealed a circumpolar variability of sea ice edge (SIE) on quasi-decadal time scale. SST also showed variation on similar time scale with warm (cool) anomaly roughly corresponding to retreat (extension) of SIE at negative (positive) southern annular mode (SAM). CGCM run without anthropogenic forcing and volcanic eruptions revealed that the leading mode of SIC is quasi-circumpolar pattern with a dominant time scale of 12-17 years and the leading mode of SST also has a similar pattern with the SIC showing a high degree of inverse correlation. The modeled SAM significantly correlates with these leading modes of both SIC and SST, representing the same structure with the observations. This indicates that the oceanic natural variability is the key to understand the quasi-decadal variability in sea ice.
This study focuses on improving quantitative precipitation forecast (QPF) related to a tropical cyclone by optimal estimation of two parameters of the Kain-Fritsch convective parameterization scheme in a high-resolution regional model - the Weather Research and Forecasting (WRF). The micro-genetic algorithm (GA) is employed for optimization, and a QPF skill score is used as a fitness function. The target parameters include the autoconversion rate (c) and the convective time scale (Tc). An interface between the micro-GA and WRF is developed and applied to an extreme heavy rainfall case in Korea, related to Typhoon Rusa (2002), at a grid spacing of 10 km. To produce the best QPF skill for this tropical cyclone case, the default parameter values are adjusted by significant amount. Our results indicate that the micro-GA is effective to retrieve the optimal parameter values, which are especially important in improving forecast skill of heavy rainfall events.
Understanding of the uplifting of Asian pollution plumes into the free atmosphere is key to evaluating the impact of trans-Pacific transport on hemisphere-scale chemical compositions. In this study, a regional chemical transport model combined with lidar and surface observations off the northwestern Pacific Rim was used to investigate the uplifting mechanisms of Asian pollutants in the spring of 2011. The potential source regions of anthropogenic fine particulates in the boundary layer and free atmosphere in western Japan were also indentified. The model reproduced accurately the observed surface anthropogenic PM2.5 with correlation coefficient ranging from 0.5-0.65 and its vertical profiles in East Asia. Long-range transport from the Asian continent was responsible for the high anthropogenic PM2.5 concentrations in the free atmosphere over northwestern Pacific Rim in spring, with a contribution of 55-70% in selected five cases. The rarely-reported local weak trough and the saddle field over northeastern Asia were found to be important uplifting mechanisms from the boundary layer to the free atmosphere in addition to the well-known warm conveyor belt mechanism (WCB). It is suggested that more studies on these two mechanisms are needed.
We present surface CO2 flux estimates obtained by an inverse modeling analysis from column-averaged dry air mole fractions of CO2 (XCO2) observed by the Greenhouse gases Observing SATellite (GOSAT) and ground-based data. Two inversion cases were examined: 1) a decadal inversion using ground-based CO2 observations by NOAA from 1999 to 2010 to derive CO2 flux interannual variability, and 2) an inversion using NOAA plus NIES GOSAT XCO2 data from June 2009 to October 2010. We used single-shot GOSAT data and individual NOAA flask data for the inversions. Our results show differences in estimated fluxes between the NOAA data inversion and the NOAA plus GOSAT data inversion, especially in Northern Eurasia and in Equatorial Africa and America where the ground-based observational sites were sparse. Uncertainty reduction rates of 40%-70% were achieved by inclusion of GOSAT data, compared to the case using just the NOAA data. The inclusion of GOSAT data in the inversion resulted in larger summer sinks in northwest Boreal Eurasia and a smaller summer sink in southeast Boreal Eurasia, with a clear uncertainty reduction in both regions. Adding GOSAT data also led to increase in Tropical African fluxes in boreal winter beyond interannual variability from NOAA data inversions.
Infrared observations from the Multi-functional Transport Satellite (MTSAT)-1R and latent heating profiles from the Tropical Rainfall Measuring Mission (TRMM) satellite are statistically analyzed to delineate the temporal variation of heating profiles associated with isolated cold cloud systems in the life cycle stages from the intermediate between initiation and mature to dissipating over tropical oceans. Clear temporal variations are confirmed for convective heating profiles, while those of the stratiform heating profiles are negligible. The resulting total profiles show that heating occurs at all levels throughout the lifetime of cold cloud systems, and the peak shifts from 5 to 8 km with elapsed time. Because that the area of stratiform rainfall in a system and its temporal variation are large, the total heating contribution released by a cold cloud system to the surrounding atmosphere during these life cycle stages is dominated by changes in the rainfall area and the ratio of the stratiform rainfall, rather than by changes in the shape of convective heating profiles.
A field experiment to clarify the characteristics of temperature distribution near an asphalt car road was carried out at the Meteorological Instruments Center in Tsukuba, Japan. Fifteen thermometers equipped with artificially ventilated radiation shields were installed on a wide grass field within a distance of 10 m from edges of the road. At a height of 0.5 m above the ground, the temperature on the leeward side of the road was found to show substantial bias from that on the windward side of the road. The biases were positive values of 0.2-0.4°C on the average and larger when the thermometers was nearer to the road or in cases of lower wind speed. The temporal variation of the biases showed a diurnal change and had a maximum peak in the evening and negative values during some hours of the day. Smaller positive biases around 0.1°C were also found at a height of 1.5 m during some time of the day whereas small negative biases were seen at a height of 2.5 m in summer. These results indicate complicated distribution of roadside temperature, although they can partly be interpreted by advection of air heated over the road.
In this study, the Nested Air Quality Prediction Model System (NAQPMS) with an online air pollutant tagged module was employed to investigate the contribution of regional transport from the North China Plain (NCP) to Northeast (NE) China during a severe haze episode in the winter of 2010. For simulating particulate matter with a diameter of less than 10 μm (PM10), the NAQPMS model was in good overall agreement with observations in NCP and NE China. Using the tagged module, contributions from regional transport and local sources to the ground PM10 concentration at four representative sites, namely Yingkou, Liaoyang, Tieling, and Jilin, along the haze transport pathway in NE were estimated. The results showed that the regional contribution from the Beijing-Tianjin-Hebei area and Shandong Province accounted for 33-87% of the PM10 pollution on average at the above four sites, with the contribution decreasing along the pathway from NCP to NE. The impact of regional transport on the peak PM10 concentrations at the four sites was also investigated. The contribution of the Beijing-Tianjin-Hebei area to PM10 pollution was 73% at the nearest site Yingkou and 30% at the farthest site Jilin, which is approximately 1500 km from Beijing. Our results demonstrate that Beijing-Tianjin-Hebei may significantly contribute to haze pollution in NE, particularly under favorable meteorological conditions. Thus, efforts to improve the air quality in NE China need to focus on controlling both local and regional emissions, particularly from the downwind NCP.
An extreme heat wave hit western Russia in the summer of 2010. To investigate the contribution of anthropogenic climate change to this event, 100-member ensembles of atmospheric general circulation model (AGCM) experiments, with and without possible human-induced changes in sea surface temperature (SST) and sea ice, were generated. The AGCM can reproduce monthly surface air temperature (SAT) anomalies for the past 30 years over the continental area, indicating a significant influence of the anomalous boundary conditions on the surface climate variability. While the ensemble average does not capture the extremely high SAT anomaly over western Russia observed in August 2010, the ensemble covers the anomaly with the probability of occurrence at 3.3%. Without the anthropogenic change in SST and sea ice, the ensemble fails to capture the observed SAT anomaly, reducing the probability of occurrence to 0.6%. The atmospheric response to the tropical precipitation change associated with anthropogenic SST increase leads to warming over Eurasia through northward temperature advection, consistent with the observed upward SAT trend. Drying of the land surface in spring may also have favored the summer warming over western Russia.
Near real-time week-long forecasts of Madden-Julian Oscillation (MJO) events using a regionally stretched global nonhydrostatic model were executed for the intensive observation period of the Cooperative Indian Ocean experiment on intraseasonal variability in the year 2011 (CINDY2011)/Dynamics of the Madden-Julian Oscillation (DYNAMO) field program. Forecast skill was validated using a real-time multivariate MJO (RMM) index and the forecast skill scores. The scores indicated a dependency on the initial MJO amplitude and phase. The errors were smaller in forecasts initialized in the active period of the MJO. A tendency for over-amplification (under-amplification) in forecasts initialized at phases 2-6 (7-8, 1) and earlier (delayed) phase propagation in the forecasts initialized at phases 1-3 (5-6) was identified. This was related to systematic positive biases in RMM1 and RMM2. The RMM1 error dominated in wind fields without significant contributions from convection. The RMM2 error grew in both convection and wind anomalies after the 5-day lead time, causing reduction in the MJO skill score. These results suggest that improvement in the basic model fields, i.e., large-scale circulations and a link between convection and dynamics, should extend the predictability of MJO in this framework.
The seasonal water cycle features over the maritime continent were determined using water sources from seven regions produced by global Rayleigh-type circulation model. The model output was validated statistically to reproduce stable isotopes by the observed δ18O and δD content of precipitation at eight stations. The model explains the Asian-Australian monsoon circulation well and demonstrates the seasonal changes of the water origin on the basis of three climatic patterns as a signature of rainy and dry season: (1) the semi-annual pattern, seasonal changes are indicated by the raise of water vapor from Indian Ocean for rainy season and southern maritime continent for dry season (2) the anti-monsoonal pattern, represented by the alternating raise and retreat of water vapor from the southwest Pacific Ocean, southern and tropical maritime continent seas, and (3) the monsoonal pattern, characterized by the raise water vapor from the Indian Ocean and northern maritime continent sea for rainy season and southern maritime continent seas for dry season.
An extreme precipitation/flood event that occurred in the Indonesian capital of Jakarta on Java Island in the middle of January 2013 coincided with an active phase of the Madden-Julian Oscillation (MJO) with the enhanced convective phase centered on the western Pacific. Analysis of upper-air sounding data showed that strong to moderate upper westerly to northwesterly winds persisted over the island prior to and during the heavy rain event, which were caused by the active phase of the MJO, while northwesterly winds occurred near the surface. Meteorological radar observations indicated regular genesis of convection at night over the sea to the northwest of the island, and southeastward propagation over the island from the nighttime to early morning. The movement of the precipitation systems was dominated by the upper northwesterly winds. The results suggest that the eastward propagation of an active phase of the MJO exerted a strong influence on the formation of extreme heavy rain over western Java Island.
This paper investigates the effect of the Australian-maritime continents on the Indian Ocean Dipole (IOD) mode by using a coupled general circulation model (CGCM) under an idealized land—sea configuration. In such idealized CGCM experiments, the presence of the Australian-maritime continents increases the amplitude of the IOD between July and September. To determine the possible mechanisms associated with IOD enhancement by the Australian-maritime continents, the mean state during the boreal summer (austral winter) was compared from model runs both with, and without, the Australian-maritime continents. The presence of the Australian-maritime continents reduces rainfall during the austral winter, and generates an easterly (westerly) wind anomaly over the equatorial Indian Ocean (Pacific). These conditions are favorable for the development of a cooler SST and shallower thermocline, and hence enhance IOD amplitude through the thermocline feedback. In addition, a linear baroclinic model (LBM) experiment showed that the negative heating caused by the reduced rainfall over the Australian-maritime continents generates a significant easterly (westerly) wind anomaly over the equatorial Indian Ocean (Pacific) through the Matsuno—Gill response.
The impact of a tropical cyclone on the northward migration of the Baiu frontal zone (BFZ) is investigated in the case of the tropical cyclone MAWAR (2012) using a global cloud-system resolving model, called NICAM. From 4 to 6 June in 2012, the BFZ rapidly shifts northward with MAWAR. A simulation with the initial data of 29 May reproduces the northward migration of the BFZ and the tropical cyclone. Strong southerlies on the eastern side of the tropical cyclone transport moist and high-temperature air into the BFZ. This horizontal advection affects the northward migration of the BFZ. In contrast, the BFZ stagnates to the south of Japan in another simulation with the initial data of 30 May because the tropical cyclone track is diverted eastward. Thus, realistic reproducibility of a tropical cyclone is needed for better simulations and prediction of the BFZ migration.
The winter response of the Asian jet stream to global warming is investigated using the Coupled Model Intercomparison Project Phase 5 (CMIP5) multi-model dataset under the RCP4.5 scenario. We first evaluate model performances in reproducing the current climatology in the upper troposphere and select the best 27 models. A multi-model ensemble projection by the selected models indicates that the jet stream over the Indochina peninsula and the South China Sea is intensified on its equatorial side in the late 21st century, while the jet stream shifts poleward over the Eurasian continent and the North Pacific. The strengthening of the jet stream on the south side is associated with cyclonic (anticyclonic) circulation anomalies in the upper (lower) troposphere over the southern part of China and decreased upper tropospheric divergence over the Maritime Continent. The strength of the upper tropospheric divergence and the amplitude of the anticylonic eddy streamfunction are strongly correlated. These findings suggest that future changes in the jet stream are related to the weakening of a Matsuno-Gill response to tropical heating.
The Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) standard product version 7 (V7) is evaluated, particularly in terms of extreme event estimation, using rain gauge data of high-temporal-resolution over southern Japan. A 1-minute rain gauge dataset within PR's field-of-view (FOV) is desirable for the evaluation. However, such dataset included only one data match with near-surface rain rate estimates (NSR) higher than 50 mm h-1 (extreme NSR). By relaxing the spatial and temporal matchup conditions, several tens of matches with extreme NSR are obtained, and the bias ratio of extreme NSR to corresponding gauge value is calculated to range from +41% to +94%. Considering that the relaxed matchup conditions may exaggerate the positive bias, the conclusion can be made that extreme NSR has a positive bias of less than +50% in V7 and that estimates are largely reliable. A similar evaluation is performed for extreme NSR in the TRMM PR standard product version 6 (V6), which showed a bias ratio of greater than +100%. Many false extreme NSR are estimated in V6, but a large part of them are diminished in V7.
Modulation of the Pacific tropical instability waves (TIWs) is investigated from 1950 to 2007 using high-resolution ocean data assimilation. So far, detecting their multi-decadal variations is difficult due to the lack of a fine-scale observation network. The results of our high-resolution general circulation model indicate that the Pacific TIW activity increased rapidly after the early 1990s, because a precedent intensified subsurface temperature front north of the cold tongue associated with the mid-1970s' climate shift positively influenced the baroclinic source. This intensified subsurface meridional temperature gradient related to a warm phase of the tropical Pacific decadal variability associated with cold anomalies along the subsurface thermocline which contribute to decay of the surface warm phase, characterized by wide-spread meridional warm anomalies and upwelled cold anomalies along the equator.
Clarification of the year-to-year variations and long-term trends of the timings of the start of leaf-expansion (SLE) and end of leaf-fall (ELF) is an important and challenging task because these timings affect spatial and temporal variations in water, heat and carbon cycles. Here, (1) we examined the relationships between daily mean air temperatures and the timings of SLE and ELF by using digital camera images in a cool-temperate deciduous broad-leaved forest in Japan from 2004 to 2011, and constructed a simple statistical phenology model based on their relationships, and (2) we then evaluated year-to-year variations and long-term trends in the timings of SLE and ELF over the past 51 years (1961-2011) by using that phenology model. We found that (1) the year-to-year variations in daily mean air temperatures over 2°C and below 18°C well affected those in the timings of SLE and ELF, respectively, and (2) a significant long-term linear trend in the timing of ELF (it moved later; 2.7 days decade-1) and the length of the leafy period were observed (it prolonged; 3.4 days decade-1), but no long-term trend was evident in the timing of SLE.
A new atmospheric humidity variable called the modified relative humidity (MRH) is proposed based on properties of the Johnson's SB distribution function. The frequency distribution of MRH can be roughly approximated by the normal distribution, while other variables such as relative humidity and the water vapor mixing ratio cannot. This characteristic suggests that MRH is convenient for statistical variable controls such as data assimilation and climatological grid data controls. Super-saturation and negative water vapor states induced by positive and negative humidity increments are avoided by using MRH. Three types of MRH, Types-I, -II, and -III, were examined. Type-III, with three fixed parameters, was the best function for approximating to the normal distribution. However, Types-I and -II, each with two fixed parameters, were beneficial for stable statistical humidity variable estimations.
Teleconnection patterns in boreal winter were sought by using the streamfunction based on one-point covariance analysis. As a result, five patterns with two new patterns, i.e., the North and South Pacific Oscillation pattern (PO) and the Middle Atlantic pattern (MA), were obtained. The PO is an interhemispheric pattern with major centers in the Pacific, related to the El Niño/La Niña. The MA has a similar structure with the East Atlantic pattern (EA), though the orientation of the pattern and the difference in the sustaining mechanism, as well as a relatively low correlation between the two indices, distinguish the MA from the EA, with the primary importance of the MA from several pieces of evidence. The PO and MA have significant relations to the surface air temperature mostly through the temperature advection. By comparing the result of the combined EOF for the zonal and meridional winds, the appropriateness of the use of the streamfunction in association with wind variations was also confirmed.
The factors of tropical cyclone (TC) genesis that are associated with African easterly waves (AEWs) were analyzed. We detected AEWs that passed over the West African coast (WAC) using ERA-interim data from July to September 2000-2010 and examine differences between the characteristics of AEWs that either develop or do not develop TCs. We first examined the environmental conditions of the AEWs that develop TCs and their dependencies on genesis location. We found that the mid-level relative humidity near the WAC is most strongly related to the genesis location among the factors contributing to TC genesis. Composite maps of the AEWs show that wave trains at 600 hPa are west-north-westward for AEWs that develop TCs and westward for non-developing AEWs. The location of maximum of water vapor at 400 hPa coincides with the stream function center for cases of TC development, while it is shifted to the southeast for cases of non-development. We focused on a case of a non-developing AEW with high relative humidity near the WAC, and found that, among other possible suppression mechanisms, a dry shallow vortex originating from the Sahara Desert had an additional effect of suppressing TC genesis.
A non-negative filter is developed to avoid negative values that may be produced in the interpolation of a non-conservative semi-Lagrangian advection scheme of tracers. Non-negativity is realized by a variable transformation with a function using arcsine. One-dimensional advection tests show that the proposed filter eliminates negative values of the mixing ratio and reduces error, whereas other non-negative filters (e.g., the global hole filling method) increase error. The variable transformation with the arcsine function efficaciously reduces the error in the region where the mixing ratio is zero, with a small counter effect to non-zero values. The proposed filter is very simple, so that it can be used in multi-dimensional schemes without modification and can be combined with other filters, such as a mass fixer and a monotonic filter.
Diurnal variation of precipitation is frequently observed over southern China in summer. We investigate it using the regional version of the nonhydrostatic icosahedral atmospheric model (NICAM), with particular focus on the interactions among land, cloud, and radiation processes. The NICAM almost exactly reproduces the diurnal variation of precipitation but slightly overestimates its amplitude. The simulated spatial distribution of precipitation is very similar to that observed by the Tropical Rainfall Measuring Mission 3B42 product. Precipitating region propagates inland from the coast during 14-20 LT. We perform sensitivity experiments to study the effects of surface conditions, cloud, and radiation interactions, and the results are as follows. 1) When volumetric soil moisture is reduced, surface temperature increases and surface evaporation decreases. Water vapor inflow increases owing to enhanced land-sea contrast; however, the decrease in surface evaporation counteracts this increase, thus decreasing precipitation. 2) When cloud amount is reduced, net downward radiation and surface temperature increase. Precipitation does not appear to vary with cloud conditions. The zone of the horizontal convergence of water vapor flux propagates inland in the afternoon, although local evaporation has a greater impact than water vapor inflow on the diurnal variation of precipitation.
In this study, the usefulness of the Quasi Static Variational Assimilation (QSVA) method in assimilating radar data for heavy rainfall cases, which are related to a high degree of nonlinearity, is investigated by comparing it with the single or multiple outer-loop Four Dimensional Variational (4D-Var) method. In the QSVA method, the length of the assimilation window is gradually increased, and the starting point of the current minimization task comes from the minimizer of the previous one. In some cases, the rainfall and wind forecasts are improved to the greatest extent through the use of the multiple outer loops. On the contrary, in the other cases, the use of the QSVA method improves the rainfall and wind forecasts the most significantly. According to the analyses of the nonlinearity of the minimization problem, the cases, in which the QSVA method is the most effective in assimilating radar data, are related to a relatively high degree of nonlinearity. Owing to the quasi-static adjustment of the QSVA method, the possibility of getting trapped near the local minimum is reduced, and the QSVA method results in better analyses and forecasts.
Fog frequency at Kushiro, Hokkaido Island, Japan, during midsummer depends strongly on the advection of sea fog from the ocean onto the land, which itself depends on the behavior of the North Pacific High (NPH). This study investigates the relationship between unusually high fog frequency at Kushiro during early autumn in 2012 and the NPH extension. The total fog frequency during mid-August through mid-September in 2012 was the highest over the past 32 years. A northwestward expansion of the NPH caused anomalous southerly winds, that transported warm moist air onto the southeast coast of Hokkaido Island. This warm moist southerly flow favors the onshore advection of sea fog, as it does during midsummer. During early autumn in 2012, the warm advection also results in a large temperature difference between atmosphere and sea over the cold Oyashio current, which suggests that an increase in sea fog generation is another important factor for the high fog frequency at Kushiro.
Atmospheric temperature and water vapor profiles are retrieved from thermal infrared spectral radiances measured with the Thermal And Near infrared Sensor for carbon Observation-Fourier Transform Spectrometer (TANSO-FTS). An initial validation of these retrievals was carried out with dedicated radiosonde measurements over the land and the ocean. Comparisons are made between measurements that are matched closely in both time (one hour) and space (<110 km). The differences in individual matchups for water vapor and temperature are within 50% and 3 K, respectively, in most regions of the vertical profile. For the water vapor retrieval, the mean difference from radiosonde data is less than 20%, and the standard deviation of the difference is smaller than 25% for both land and ocean scenes. For the temperature retrieval, the comparison with the radiosonde profiles indicates that the mean difference is better than 1.5 K with <1.2 K standard deviation. We find that uncertainty in surface emissivity could make water vapor and temperature retrievals in the lower troposphere difficult. We also demonstrate a negative correlation between the water vapor in the atmospheric column and the precision of the retrievals.
We estimated the snow water equivalent (SWE) of snowpack in central Japan from September 2006 to August 2008 by using a 3.3 km-mesh regional climate model with two land-surface models: Noah land-surface model (Noah LSM), and Noah land-surface model with multiparameterization options (Noah MP). The model validation for temporal variations of SWE at the Tohkamachi station and the comparison of modeled maximum SWE with estimated that from observed maximum snow depth at ten sites showed that Noah MP could simulate spatiotemporal variations of SWE better than Noah LSM which underestimated SWE. Simulated SWE in central Japan peaked in March, but the difference of SWE between the two land-surface models was greatest in April. SWE determined using Noah LSM (Noah MP) in analysis domain reached 18.1% (28.5%) of the total storage capacity of high dams in Japan in March 2007, whereas it reached 32.4% (44.1%) in March 2008. The difference of SWE between the two land-surface models was particularly high under warm conditions, that is, during the snowmelt season, and during a warmer than normal winter. Our results indicate that the choice of land-surface model for estimates of SWE is important under warm climatic conditions.
On 26 August 2011, the merger of two misocyclones was accompanied by surface wind gusts in western Tokyo, within the Tokyo Metropolitan Area Convective Study region. In this study, using data from a high-density ground weather observation network stations and the Doppler Radar for Airport Weather system at Haneda Airport, we clarified the merging process of the two misocyclones and the structure of the resulting vortex.
The Pseudo-Global-Warming Downscaling (PGWDS) method is a simple way to downscale future climate change using the reanalysis data added by the long-term mean difference between present and future climate data projected by a general circulation model (GCM). The PGWDS method has three substantial advantages as compared to a conventional dynamical downscaling method, (1) removing GCM biases, (2) reducing the amount of required GCM output, and (3) shortening the simulation duration. We have focused on the third advantage and applied the PGWDS method to a local area in Japan in January. The future changes in monthly mean precipitation, snowfall, and surface air temperature estimated by the 10-year averages can be regarded as the future climatic changes estimated by the 30-year averages because most of the 10-year averages are within the standard error of the 30-year averages. Meanwhile, the future frequencies in the 10 years are often larger than the error range of those in the 30 years in the extreme events. Short sampling duration seems to be a primary cause. It is necessary to check the possibility of shortening the simulation duration because the estimation of future change is not appropriate in some cases.
Decadal-scale modulation (DSM) appears in the atmospheric circulation change around the onset (July 20) of the western North Pacific summer monsoon (WNPSM). This work examined the DSM associated with the onset precipitation increase (OPI), using global reanalysis datasets from 1979 to 2010 (32 years). The years 1985-1993 had positive decadal anomalies, and the onset precipitation increased with the development of specific meridional circulation similar to the Pacific-Japan pattern. In the subsequent negative term of the years 1994-2000, the atmospheric circulation change was distinguished by a smaller OPI, southwestward extension of the WNPSM, and an anomalous wave train emanating northeastward from the Philippines. The patterns in the following positive decadal term of the years 2001-2007 were similar to those in the preceding negative term but with opposite signs. These changes in the DSM were coherent with decadal sea surface temperature (SST) anomalies before the onset around the region of 28-29°C in SST in the western North Pacific due to vertical instability under the trade wind inversion.
A classification system for rain clouds was developed using ground-based radar reflectivity and infrared brightness temperature (TBB) data from multifunctional transport satellites (MTSAT) and applied to the Phimai radar station, Thailand. The proposed method can classify cloud types into convective rain, stratiform rain and non-rain for areas covered with cumulus and/or cirrus clouds by applying a statistical integration analysis of rain gauges, ground-based radar, and MTSAT data. The classified precipitation areas were used to estimate quantitative precipitation amounts over Phimai. To merge different rainfall data sets derived from these three sources, the bias among the data must be removed. A combined correction method was developed to estimate the spatially varying multiplicative biases in hourly rainfall obtained from the radar and MTSAT using the rain gauges. This consecutive analysis was applied to the rainy season (July to September) in 2009 to obtain the multiplicative bias correction and to combine the data sets. The correlation coefficient, root mean square error, and mean bias were used as indicators to evaluate the performance of our bias-correction method. The combined method is simple and useful. The combined rainfall data were more useful than the data of TRMM 3B42 V7 and ground-based radar estimates.
Ensemble data assimilation methods have been improved consistently and have become a viable choice in operational numerical weather prediction. A number of issues for further improvements have been explored, including flow-adaptive covariance localization and advanced covariance inflation methods. Dealing with multi-scale error covariance is among the unresolved issues that would play essential roles in analysis performance. With higher resolution models, generally narrower localization is required to reduce sampling errors in ensemble-based covariance between distant locations. However, such narrow localization limits the use of observations that would have larger-scale information. Previous attempts include successive covariance localization by F. Zhang et al. who proposed applying different localization scales to different subsets of observations. The method aims to use sparse radiosonde observations at a larger scale, while using dense Doppler radar observations at a small scale simultaneously. This study aims to separate scales of the analysis increments, independently of observing systems. Inspired by M. Buehner, we applied two different localization scales to find analysis increments at the two separate scales, and obtained improvements in simulation experiments using an intermediate AGCM known as the SPEEDY model.
In the ensemble Kalman filter, covariance localization plays an essential role in treating sampling errors in the ensemble-based error covariance between distant locations. We may limit the influence of observations excessively, particularly when the model resolution is very high, since larger-scale structures than the localization scale are removed due to tight localization for the high-resolution model. To retain the larger-scale structures with a limited ensemble size, the dual-localization approach, which considers two separate localization scales simultaneously, has been proposed. The dual-localization method analyzes small-scale and large-scale analysis increments separately using spatial smoothing and two localization scales. These are the control parameters of the dual-localization method, and this study aims to investigate the parameter sensitivities by performing a number of observing system simulation experiments using an intermediate AGCM known as the SPEEDY model. Two smoothing functions, the spherical harmonics spectral truncation and the Lanczos filter, are tested, and the results indicate no significant difference. Also, sensitivity to the two localization parameters is investigated, and the results show that the dual-localization approach outperforms traditional single localization with relatively wide choices of the two localization scales by about 400-km ranges. This suggests that we could avoid fine tuning of the two localization parameters.
A new temperature-dependence correction (T-D correction) for Meisei RS2-91, RS-01G, and RS-06G radiosonde relative humidity (RH) measurements has been developed recently to remove the artificial stepwise change of ∼3% RH at 0°C associated with the present (original) correction. These radiosondes have been used at most of the Japanese upper-air stations since the 1990s. The historical radiosonde humidity records at Sapporo and Tateno stations on the 925, 700, and 500 hPa pressure levels show apparent large downward trends between 1999 and 2009. This is because the original T-D correction has only been applied since February 2003 after a moist bias was discovered. The new T-D correction is found to result in a much smaller downward RH trend at Sapporo and almost no trend at Tateno.
We investigated ocean mixed layer temperature (MLT) balance using moored buoy data together with satellite and atmospheric reanalysis datasets in the eastern Indian Ocean for 2002-2012, a period during which six positive and two negative Indian Ocean Dipole (IOD) events were observed. We focused on the location 5°S, 95°E where in situ measurements of more than 10 years by a TRITON buoy were available. Heat balance analysis demonstrated that horizontal heat advection mainly produced MLT anomalies and that air-sea heat fluxes had a damping effect during the development phase of IOD events. In contrast, during the decay phase, air-sea heat fluxes had a primary role in suppressing MLT anomalies, causing decay of the anomalous IOD conditions. The contributions of horizontal heat advections differed in events, which may be attributed to diversity of IOD evolutions. These results suggest that IOD involves variety of feedback processes during the development and decay phases.
This study explores how north-south displacements of the Baiu front influence isotopic variation in Baiu precipitation. Deuterated water vapor (HDO), which was measured continuously in central Japan during the rainy season in 2010, showed clear intraseasonal variations associated with north-south migration of the Baiu front. Because water vapor transport by lower tropospheric southwesterlies from the subtropical high is characterized by relatively high HDO/H2O isotopic ratios, the highest HDO/H2O isotopic peaks were observed when the Baiu front migrated to the north of the observation site. Although these data were collected over only a one-year period, the findings can be used to explain interannual isotopic variations in Baiu precipitation during the period 1961-1978 at the Tokyo station. The isotopic content of June precipitation showed a positive correlation with the ratio of warm rainfall formed by subtropical marine air to total rainfall in June. This suggests that the isotopic content of Baiu precipitation is higher than that of a normal year, for years when the Baiu front shows prolonged stationary patterns in regions towards the Sea of Japan. Thus, we conclude that water isotopic ratios are a powerful tool for reconstructing past shifts in the position of the Baiu front.
The title in the citation information was wrong.
Wrong: Statistical analysis of heating profile changes associated with developing process of isolated tropical cold cloud systems by using satellite observations.
Right: Statistical analysis of temporal variation of heating profiles associated with isolated tropical cold cloud systems by using satellite observations.
The corrected PDF file can be downloaded from the following link button.
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