Both the observed background circulation and the northwest Atlantic sea surface temperature anomalies (SSTA) associated with the circulation anomaly over the Ural Mountains during early winter (October-December) are investigated, and it is shown that a positive height anomaly over the Urals is remotely linked to a positive SSTA by an upper wave-train-like anomaly chain across the North Atlantic and coastal Europe. To investigate whether and how the SSTA affects the circulation over the Urals, large-ensemble atmospheric general circulation model (GCM) experiments are conducted, and the results show that the SSTA forces a similar wave-train-like anomaly chain, resulting in a positive geopotential height anomaly over the Urals. The mechanism that maintains the response is diagnosed by investigating the roles of anomalous diabatic heating, and transient vorticity forcing, via a linear baroclinic model (LBM). The results suggest that the two upstream anomalies in the chain are largely maintained by anomalous transient vorticity forcing, although it is modulated by anomalous diabatic heating. In contrast, the Ural response is largely maintained by anomalous diabatic heating. To mimic the initial mechanism of the response, an idealized heating representing the initial SSTA-induced heating is prescribed. The LBM response to the idealized heating is obtained, and then transient feedback to the heating-induced anomalous flow is simulated, via a linear storm track model (STM). The LBM responses to the anomalous transient vorticity forcing resulting from the idealized heating resembles the GCM simulation upstream, but is not significant over the Urals. This suggests further that the Ural response is triggered, and maintained, by anomalous diabatic heating.
Realistic vertical heating and drying profiles in a cumulus scheme is important for obtaining accurate weather forecasts. A new empirical cumulus parameterization scheme, based on a procedure to improve the vertical distribution of heating and moistening over the tropics is developed. The empirical cumulus parameterization scheme (ECPS) utilizes observed profiles of Tropical Rainfall Measuring Mission (TRMM) based apparent heat source (Q1) and the European Centre for Medium-Range Weather Forecasts (ECMWF) analysis based apparent moisture sink (Q2). A dimension reduction technique through rotated principal component analysis (RPCA) is performed on the vertical profiles of heating and drying over the convective regions of the tropics, to obtain the dominant modes of variability. Analysis suggests that most of the variance associated with the observed profiles can be explained by retaining the first three modes. The ECPS then applies a statistical approach, in which Q1 and Q2 are expressed as a linear combination of the first three dominant principal components which distinctly explain variance in the troposphere as a function of the prevalent large-scale dynamics. The principal component (PC) score, which quantifies the contribution of each PC to the corresponding loading profile, is estimated through a multiple screening regression method which yields the PC score as a function of the large-scale variables. The profiles of Q1 and Q2 thus obtained are found to match well with the observed profiles. The impact of the ECPS is investigated in a series of short range (1-3 day) prediction experiments, using the Florida State University global spectral model (FSUGSM, T126L14). Comparisons between short range ECPS forecasts and those with the modified Kuo scheme, show a very marked improvement in the skill in ECPS forecasts. This improvement in the forecast skill with ECPS emphasizes the importance of in- corporating realistic vertical distributions of heating and drying in the model cumulus scheme. This also suggests that in the absence of explicit models for convection, the proposed statistical scheme improves the modeling of the vertical distribution of heating and moistening in areas of deep convection.
We investigated seasonal transition of dominant modes of sea surface temperature anomalies (SSTAs) in the tropical Indian Ocean, analyzing the National Centers for Environmental Prediction/ National Center for Atmospheric Research reanalysis products (NCEP/NCAR reanalyses), the Global sea-Ice and SST dataset (GISST2.3b), and the Simple Ocean Data Assimilation (SODA). During the coincidence years when the Indian Ocean Dipole (IOD) is followed by the major El Niño during boreal autumn-winter season, surface dipole structure in the tropical Indian Ocean tends to turn into the basinwide warm pattern in the November-December period. In contrast, the subsurface dipole keeps its structure from boreal autumn to winter. Such a surface-confined transition of SSTA is induced by latent heat flux anomalies in the eastern Indian Ocean. These latent heat flux anomalies are associated with changes in scalar wind speed anomalies. The zonal direction of climatological surface winds changes from easterly into westerly over the eastern Indian Ocean in November-December, while the anomalous Walker circulation during the El Niño induces easterly surface wind anomalies to persist there. As a result, deceleration of scalar wind speed takes place during boreal winter, and leads to warming of SST through suppressed evaporation. In addition to these latent heat flux anomalies, incoming solar radiation anomalies contribute to the net surface warming during this period. Furthermore, we discuss the role of the ocean dynamics for keeping the warm SSTAs in the western Indian Ocean.
Interannual variability of the Ogasawara (Bonin) high in August is examined in relation to propagation of stationary Rossby waves along the Asian jet using monthly averages from the NCEP/NCAR reanalysis dataset for 52 years. The perturbation kinetic energy at 200 hPa is used as a measure of the activity of stationary Rossby waves along the Asian jet. Composite maps of five relatively wavy-jet years with close phases show an enhanced anticyclone over Japan. This anomalous ridge has a maximum amplitude at 250 hPa and extends throughout the troposphere with little zonal and slight northward tilts. Wave-activity and isentropic potential vorticity analyses clearly show that the ridge is created by the propagation of stationary Rossby waves to Japan. The anomalous ridge accompanies a positive temperature anomaly over Japan in the entire troposphere. A negative temperature anomaly to the east of Japan is also created in the lower troposphere by the northerly flow between the anomalous ridge and trough. By contrast, the equivalent-barotropic ridge over Japan is very weak in the zonal-jet years. Although Rossby waves are as strong as those in the wavy-jet years near the source, they are found to converge to the southeast of its source with little further downstream propagation. This contrast in the behaviour of Rossby waves is consistent with the intensity of the Asian jet to the east of 90°E. The composite analysis suggests that the enhancement of a deep ridge near Japan is regulated by the intensity of the Asian jet. The composite analysis study conducted here emphasizes the importance of the propagation of stationary Rossby waves along the Asian jet for the late summer climate in northeastern Asia.
The case study in this paper describes the characteristics of the observed internal structure of the ITCZ precipitating system and related environmental factors in the western Pacific. We deployed the R/V Mirai for a 10-day period in June 2000 at (7 N, 140 E) to determine the detailed structure of the western Pacific ITCZ by obtaining various data including C-band Doppler radar data and 3-hourly radiosonde data. In the present case, four mesoscale convective systems (MCSs) passed over the vessel continuously. The leading edge of the MCSs elongated meridionally, which was orthogonal to the low-level wind shear. The internal structure was similar to the typical two-dimensional structure, including a leading convective edge and a following stratiform precipitating area with a melting convergence. However, the detailed structure was rather three-dimensional. The subsystem in the northern end of the MCSs included an earlier-dissipating weak stratiform precipitating area. This could have occurred as a result of the introduction of the subtropical dry air to the northern end of the MCSs, by the combined effect of the melting convergence and large-scale wind profile. The initialization processes of the MCSs are also discussed. These results suggest that, to form and maintain the western Pacific ITCZ MCSs, the combination of the environmental meridional moisture contrast and the dynamics in the stratiform region of the MCSs is also important as well as the large-scale disturbances.
During the field experiment of winter mesoscale convective systems over the Sea of Japan in 2001 (WMO-01), several snowbands associated with a cold front developed remarkably at 10∼30 km off the coast of Hokuriku district on 28 and 29 January 2001. Dual-Doppler radar analysis was made to investigate the mechanisms responsible for the successive development of the snowbands. Derived wind fields revealed that a mesoscale convergence line was formed between the prevailing westerly and shallow (∼1 km depth), cold southwesterly winds that blew from inland toward Toyama bay. As the downshear-tilting convective cells in the snowbands approached the convergence line, they became almost upright and their height increased from 4 km to 6∼7 km. The edge of the southwesterly cold air temporarily became indistinct because of the entrainment of the air into the convective cells that developed aloft. However, the subsequent advection of the cold southwesterly reestablished the convergence line, and enabled the successive development of following snowbands.
In this paper a method is proposed to compress the multi-channel sounder data for global greenhouse gases monitoring into a small number of hypothetical channels loosing a negligible information content that the original channel has. Recent Earth observation satellites provide radiance data of thousands of channels at one observation point, piling up gigabyte of data per day. Meanwhile it has recently been recognized that the atmospheric field analysis for the numerical weather prediction produces a better result, when it uses the radiance data of satellite sounder rather than such the retrieved physical parameters, as the temperature and humidity. The difficulty of using radiance data of large number channels is that it uses too much resources in the assimilation loop of three, or four-dimensional variational formulation. In the present work we propose a method to compress a large number of original channels to a small number of hypothetical channels, whose weighting functions are the eigenvectors made from the matrix of original weighting functions for various atmospheric and observational conditions. The original weighting functions of an arbitrary spectrum concerned are, in turn, expanded with the eigenvectors obtained as above. Using this relation, the radiances of hypothetical channels are obtained by a linear combination of originally observed radiances. The error covariance matrix of the hypothetical channels is also obtained from measurement error of original channels. Since, in the present method of hypothetical radiance, much more channels of original spectrum can be incorporated in the analysis, almost all the information that the original spectrum has is maintained. An example of the application is shown for the simulated radiance data of a remote sounder of the atmospheric greenhouse gases, which observes the solar radiation reflected in the Sun glint region of the water surface, with a high spectral resolution. The result showed that the 240 channels of original radiance data can be condensed to 5 channels or less of hypothetical radiances, with loosing negligible information that the original data has.
The mesoscale structure and evolution of a rainband that occurred on 24 June 2001 in the downstream region of the Yangtze River and produced heavy rainfall as much as 110 mm within only two hours in a limited area were studied through detailed analyses of upper-air, surface, and triple-Doppler radar data. The rainband evolved on the north side of a surface Meiyu front. It was oriented in a northeastsouthwest direction nearly parallel to the mean storm motion and the low-level vertical shear. The rainband lasted more than 5 hours and propagated slowly southeastward. The environmental conditions ahead of the rainband were characterized by extraordinary stability below and significant conditional instability in a deep layer above the frontal surface that was approximately 0.5 km above the ground. It was found that significant amount of convective available potential energy (1179-370 J kg-1) could be realized by lifting air parcels in a deep layer (0.7-0.5 km) above the frontal surface. The rainband formed as a core of the storm-relative northeasterly inflow associated with the largescale environment appeared on the backside around an altitude of 4 km. The rapid development of the rainband was associated with the surge of another storm-relative northeasterly inflow from the rear around an altitude of 2 km. The latter inflow appeared to be related to the development of a mesoscale vortex after the formation of the rainband. The air that fed the updrafts of the rainband came from an elevated layer above the frontal surface, rather than from the boundary layer. On the other hand, downdrafts at low-levels were shallow and weak during the formation and development of the rainband and strong outflows under the rainband were absent. It is likely that the horizontal convergence enhanced by the northeasterly inflow around an altitude of 4 km was important in the formation of the rainband by lifting the elevated air that possessed conditional instability. Meanwhile, the lifting of the most unstable air in the vertical column of the atmosphere by the northeasterly inflow around an altitude of 2 km would have aided the rapid development of the rainband and the heavy rainfall.
The diurnal variation of snow precipitation in the west coastal area near Wakasa Bay has been investigated for two winter seasons of 2001 and 2003 using surface precipitation radar, rain gauge, and satellite infrared data. Radar reflectivity-derived precipitation intensity shows a clear diurnal cycle within Wakasa Bay for both years, although the cycle is clearer in 2001 than in 2003. The precipitation maximum occurs in the early morning, and the minimum occurs in the evening. Using radar data collected during January 2003, the precipitation diurnal cycle within the Bay is compared with three nearby regions: offshore—open water region to the north, inland—land region to the northeast, and inshore—coastal region to the northeast. It appears that all the other three regions have the precipitation maximum during the day and the minimum during night, although the diurnal variation inland (over land) is very small. Additionally, in the offshore region, there exist two precipitation maxima and minima during a 24-hour day. Analysis of precipitation data from AMeDAS (Automatic Meteorological Data Acquisition System) rain gauge stations basically agrees with the radar observations. It shows that a similar diurnal cycle, found in the radar data within Wakasa Bay, can also be found for coastal stations, although the different patterns are shown depending on the location and time period in 2003. When averaging data from both January and February during 2001 and 2003, the diurnal cycle tends to be smoothed out. Cloud top temperature and cloud fraction derived from satellite infrared data do not show a clear diurnal cycle within Wakasa Bay, nearby inshore and inland regions, although the decreases of brightness temperatures are seen around noon in the offshore region for both cases of 2001 and 2003. Analysis using collocated satellite and radar data indicates that cloud top temperature has little skill in reflecting surface precipitation for the winter convective clouds associated with cold air outbreaks. Finally, possible causes of the diurnal variation are discussed, including local land-sea breeze and mountain wind effects, as well as the radiative cooling effect.
Operational radiosonde data at San Cristo´bal (0.90°S, 89.62°W; eastern Pacific) are compared with those at Singapore (1.37°N, 103.98°E; western Pacific) intending to explore the differences in the dynamical properties of the tropopause region and in the activities of atmospheric waves between the eastern and the western tropical Pacific. The interannual variations in the meteorological parameters of the tropopause region are found to be almost synchronized between the two stations and do not show direct correspondence with the time evolution of El Niño. These evidences, as opposed to the general expectation of the strong influence of sea surface temperature variations on the tropopause properties, suggest that the tropospheric dynamical forcing is not a prevailing factor that drives the interannual variation of the tropical tropopause properties for these stations. The dynamical parameters of the tropopause region are affected by passages of vertically propagating atmospheric waves that characterize the time evolution of daily sounding data. The variations with the time scales of 15 to 20 days are identified as equatorial Kelvin waves of zonal wave number 1, which is consistent with the observed out-ofphase relationship between the two stations. The perturbations in the tropospheric temperature and wind are also brought about by those waves with the typical time scales of several days. These waves are more pronounced over San Cristo´bal than in Singapore. As the convections are more active and reach higher altitude in the western than in the eastern Pacific, this evidence implies that the altitude rather than the strength of convection is more important in characterizing the daily fluctuations of the tropical troposphere. The differences between the two stations in the time evolution and magnitude of the quasibiennial oscillation in the equatorial stratosphere are found to be marginal, although occasional differences in the onset time and the magnitude of zonal wind acceleration could be pointed out.
Number-size distribution of aerosols and its spatial variation, were observed in the free troposphere up to 11 km altitude over the northwestern Pacific Ocean, in the Pacific Atmospheric Chemistry Experiment (PACE)-7 campaign in February 2000. Characteristics of the size distributions were observed in the relation to the different air mass. The spatial difference in the features of size distributions was clearly divided by the location of subtropical front. Over the extratropical region, the mode radii were large (0.03-0.06 mm), and the concentrations of aerosols in the accumulation mode (0.15 < r < 0.5 μm) were high, suggesting a strong influence from anthropogenic particles from the Asian continent. On the other hand, the size distributions in the subtropical air mass had the mode radii of 0.01-0.03 mm, and showed low concentrations of accumulation mode particle. Moreover, high number concentrations of very fine particles (0.004-0.01 mm radius), were distributed at upper altitudes (> 8 km) over 15-31°N latitude. These aerosols were assumed to be mainly sulfuric acid particles transported from the tropics. Size distribution at the center of subtropical jet streams was slightly aged, suggesting influence from the continent. However, it is basically resemble to the size distributions measured in the upper troposphere over the subtropical region. On the basis of the trajectory analysis, these aerosols would have been transported from the tropical upper troposphere, to the middle latitudes by Hadley circulation, and then mixed into the subtropical jet streams.
Aerosol-induced atmospheric optical phenomena were analyzed at four sun photometer sites in Japan, viz., Ryori (39°02´N, 141°50´E), Tsukuba (36°03´N, 140°08´E), Yonagunijima (24°28´N, 123deg;01´E), and Minamitorishima (24°18´N, 153°58´E), over the period MarchHApril 2002 to elucidate the causes of such phenomena, and explain regional differences. Using observed optical properties, the phenomena were classified into three kinds of aerosol events: Type A, Type B, and Type C, which, respectively, correspond to relatively clear conditions, typical Kosa conditions, and haze conditions. The frequency of occurrence of these aerosol events was found to be significantly different at the four observation sites in Japan during the observation period. Type B events were found to occur rather frequently in Ryori in northern Japan. The severe Kosa events observed at Ryori on April 10, 2002, which constitute a case study, passed over Northeast China where dust storms have rarely been reported before 2000. The smallest mean aerosol optical depth at 500 nm (τ500) and Ångström exponent (α) were observed in Minamitorishima. However, according to other studies, the mean τ500 in Minamitorishima was larger than those in the eastern Pacific, which showed that the influence of the Asian outflow on atmospheric optical properties was still significant even 3,000 km away from the Asian continent. The mean τ500 and α, as well as the frequency of occurrence of Type C events, were the largest at our observation site in Yonagunijima. Based on the trajectory analysis obtained during the observation period, the high frequency of Type C events at Yonagunijima was ascribed to the haze from biomass burning in Southeast Asia and/or urban pollution in coastal areas in Southeast China. The frequency of Type C events in Ryori and Tsukuba indicated that haze is not an uncommon occurrence, not only in southern Japan, but also in central and northern Japan, and that, along with Kosa events, they may affect atmospheric optical properties over Japan in the spring.
Based on the reanalysis data from NCEP/NCAR and other observational data, interannual variability of the Mascarene high (MH) and Australian high (AH) during boreal summer from 1970 to 1999 is examined. It is shown that interannual variability of MH is dominated by the Antarctic oscillation (AAO), and MH tends to be intensified with the development of the circumpolar lows in high southern latitudes. On the other hand, AH is correlated with AAO as well as El Niño and Southern Oscillation (ENSO), and tends to be intensified when El Niño occurs. Since AH, especially MH, is positively correlated with AAO, composite analysis on the difference between the positive and negative MH is performed to reveal the physical mechanism responsible for the East Asian summer monsoon anomalies associated with AAO. The result shows that, with the intensifi- cation of MH, the Somali jet, and Indian monsoon westerlies, tend to be strengthened. Accordingly, AH and the associated cross-equatorial flow, become stronger whereas the trade wind over the tropical western and middle Pacific become weaker. In association with the above changes, convective activities near the Philippine Sea are largely suppressed, as a consequence, exciting a negative convection anomaly, and a Rossby wave train from East Asia via the North Pacific to the western coast of North America (a negative Pacific-Japan pattern). Corresponding to the negative Pacific-Japan pattern, an anomalous rainfall pattern appears in East Asia. Correlation analysis between AAO and sea level pressure, 500 hPa geopotential height further indicates that AAO is a strong signal influencing the climate anomaly in both hemispheres, including East Asia. Due to the seasonal persistence, AAO and the related MH and AH in boreal spring, may provide some useful information for the East Asian summer monsoon prediction. With the intensification of MH during boreal spring through summer, the Meiyu/Baiu rainfall from the Yangtze River valley to the Japan Islands tends to increase, while less rainfall is found outside of this region. In contrast with MH, the effect of AH on summer rainfall is confined to southern China.
In this paper, future climate changes over East Asia (20-50°N, 100-145°E) are projected from multimodel ensembles (MMEs) of selected coupled atmosphere-ocean general circulation model (AOGCM) simulations based on Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A2 and B2 scenarios. Analyzed variables include annual and seasonal near surface temperature and precipitation over East Asia. Before projecting future climate, model performances are evaluated on the simulation of the present-day climate (1961-1990) with bias, root-mean squared error (RMSE), and the Taylor diagram analysis. The result of model evaluation shows that CSIRO Mk2, ECHAM4/OPYC3, GFDL_R30_c, and HadCM3 exhibit higher performance. In order to test the sensitivity of projection results, four MMEs are defined: simple arithmetic averages of all seven AOGCM simulations (MME7) and four skillful AOGCM simulations (MME4), and skill-weighted averages of seven AOGCM simulations based on the skill scores of the Taylor diagram (MME_S1 and MME_S2). While the weighted MMEs (MME_S1 and MME_S2) have similar performances in the present-day climate simulation to that of MME7, MME4 constructed by four skillful models reveals higher performance than the other MMEs. The overall projection results from four MMEs show that East Asia will experience warmer and wetter climate in the 21st century. The projection results are not sensitive to the MME method. Areaaveraged temperature changes for three 30-year periods of 2020s, 2050s, and 2080s simulated by MME7 A2 [B2] scenario ensembles are 1.2 [1.4], 2.5 [2.4], and 4.1°C [3.2°C] increase, and precipitation changes are 0.4 [1.4], 2.2 [2.6], and 5.0% [4.0%] increase, respectively. Spatial patterns indicate that both temperature and precipitation increases are larger over the continental area than the oceanic area, and that the areas of larger inter-model variability are in accord with those of stronger climate change. The intermodel variability (noise) in precipitation changes is as large as that of ensemble mean (signal), whereas noise is much smaller than signal in the projection of temperature changes. It is demonstrated that MME4 reduces the inter-model uncertainty about a half of MME7 in the temperature projection of the late 21st century, but not in the precipitation projection. A significant difference in projected patterns between A2 and B2 scenario ensembles (defined as a potential impact of greenhouse-gas mitigation) appears in the 2080s temperature field over the southwestern part of East Asia. However, no significant differences can be found between precipitation patterns of A2 and B2 scenario ensembles because of the dominant inter-model variability. It is also shown that the climate change over East Asia has a characteristic seasonal dependence, that is, larger increases of wintertime temperature and summertime precipitation, which implicates the possible change of the East Asian monsoon system by global warming.
After the climate shift of 1976/1977, associated with the interdecadal variability in the tropical Pacific Ocean, persistence barriers of El Niño/Southern Oscillation (ENSO) and tropical mean tropospheric temperature (TMTT) variations are detected in the boreal spring and autumn, respectively. Prior to the climate shift, however, the TMTT persistence barrier is almost non-existent, despite the prominence of the ENSO persistence barrier. Thus, the phase lag between ENSO and the TMTT variations is not fixed prior to the climate shift, while there is a fixed lag after the climate shift. This interdecadal variability is most remarkable for the TMTT anomalies in December. After the climate shift, the TMTT anomalies in December tend to persist 4 months later than prior to the climate shift. This is further examined in the comparison to the SST averaged over the strongly precipitating regions only, that is, the rainy-region SST. The SST variations in the rainy-region well correspond to those over the remote ocean basins, such as the Indian Ocean and the Western Pacific, that show a lagged response to the equatorial eastern Pacific SST anomalies. The TMTT anomalies are positively correlated with the rainy-region SST anomalies in both periods, prior to and after the climate shift. The interdecadal variability of the rainy-region SST persistence is similar to that of the TMTT persistence, although the variability is not as distinct as that of the TMTT.
Using satellite-derived daily sea ice motion, this paper presents maps showing the relationship between the ice motion and surface wind, and mean ocean current in the Southern Ocean. The daily ice motions were derived from Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I) images. The mean ice motion field is characterized by eastward circumpolar drift; northward drifts in the Weddell Sea, Ross Sea, and areas around 80°E; and narrow westward coastal currents. A linear formula is used to examine the relationship between the ice motion and wind. It was found that daily variation in the ice motion is closely correlated with wind speed fluctuation, and that features of mean ice motion are mainly attributable to atmospheric forcing. The speed reduction factor (ratio of ice speed to wind speed) is relatively small near the coast, and increases with distance from the coast. Surface ocean currents beneath the ice cover were derived by subtracting the wind effect from the ice motion. It is revealed that the ocean current field is associated with the bottom topography.
Temporal variation in vegetation over a large area induces temporal variation in the landatmosphere water budget through evapotranspiration activity. To examine this relationship interannually, normalized difference vegetation index (NDVI) and evapotranspiration (ET) values collected from 1982 to 2000 over northern Asia were investigated. Monthly global NDVI values were acquired from Pathfinder AVHRR land data. Monthly ET was estimated from NCEP/NCAR assimilated atmospheric data and CMAP global precipitation data. All calculations were made for 2:5 × 2:5-degree grid boxes in the study region on a monthly basis. The correlation coefficient of the 19-year time series of ET and NDVI anomalies showed the annual maximum correlation in June, and high correlation generally distributed over the region except for southern arid areas and the most northern areas. Although interannual variations in temperature and precipitation anomalies also showed some relation to ET in the warm season, they did not exhibit high correlation in a specific month. Since the vegetation is probably most active in June, it is considered that vegetation variability contributed significantly to ET through its transpiration activity.