In order to estimate MODIS-equivalent aerosol optical thicknesses (AOTs) for dust particles during the nighttime over East Asia, an Artificial Neural Network (ANN) model approach was used to combine MODIS-measured infrared (IR) brightness temperatures (BTs) and visible (VIS) AOTs. For training the ANN model, IR BTs were used together with surface type and geometrical information as inputs to predict MODIS-derived AOTs as target data during the daytime when VIS-based AOTs are available. The training was done exclusively over dust-laid pixels during the spring (March–May) of 2006 over the East Asian domain (20°N–55°N, 90°E–145°E). It should be noted that the obtained daytime AOTs from the ANN model are in good agreement with MODIS-derived AOTs, with a correlation coefficient of 0.77 over the analysis domain. Although a weaker correlation is found during the nighttime when derived AOTs are compared against AOTs from CALIPSO, the case study indicates that the developed ANN method appears to effectively depict both the evolutionary features and intensity of the Asian dust plume during the nighttime. Results further indicate that nighttime VIS-like AOTs can be readily used for monitoring dust movement and its intensity during the nighttime, filling the gap between consecutive daytime AOT distributions.
A spatial-temporal projection method (STPM) is developed to predict the spring (March–May, MAM) rainfall in northern Taiwan. Seven large-scale atmospheric and oceanic fields (925-hPa zonal wind, meridional wind, and moisture, 850-hPa, 500-hPa, and 200-hPa geopotential height, and sea surface temperature) with their temporal evolutions during the preceding 11 months are used as predictors. An optimal ensemble (OE) strategy is developed based on the best cross-validation results from each predictor over the training period. Some predictors adopted in the OE show the longest lead time of 10-month. The deterministic forecast result based on the OE approach indicates that the STPM predictions are skillful with an averaged temporal correlation coefficient of 0.6. However, the amplitude of the forecasted rainfall is underestimated, which is treated by introducing an amplifier coefficient. The STPM is also skillful for the probabilistic prediction of spring rainfall in northern Taiwan. The averaged Brier skill score reaches 0.37 for the below-normal categorical case.
Focusing on the use of desert targets for solar channel calibration of geostationary satellites located in the west Pacific region, a desert target in Simpson Desert, Australia, was selected and used for the development of a calibration algorithm based upon radiative transfer modeling. The surface spectral reflectance required for radiative transfer calculation was obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS)-bidirectional reflectance distribution functions after tuning against the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) spectral data. Top of the atmosphere radiance simulations were conducted over the desert target, and comparisons with MODIS- and Sea-viewing Wide Field-of-view Sensor (SeaWiFS)-measured visible channel radiances revealed that the developed calibration method using the target over the Simpson Desert can be used to calibrate the geostationary visible channel within a 5% level of uncertainty. Application of the developed algorithm to Multi-Functional Transport Satellite (MTSAT-1R) visible channel measurements suggested that the MTSAT-1R operational calibration for the visible sensor appeared to be within a 5% error, but over the lower count value range.
We analyze tropospheric column ozone (TCO) data observed by satellite instruments over East Asia for 15 years (from 1995 to 2009), and investigate the relationship between enhanced TCO (E-TCO) and ozone intrusion from the stratosphere near the subtropical jet (STJ). A belt of E-TCO is observed at mid-latitude over East Asia throughout the year; the belt is located at latitudes approximately equal to that of the STJ on seasonal, monthly, and daily timescales. The observed results are compared with a tagged tracer simulation by using a global chemical transport model. The simulation for East Asia indicates that the contribution from tropospheric origin to the enhancement of TCO is comparable to that from stratospheric origin at latitudes close to the STJ, resulting in the high correlation of the E-TCO belt and the STJ. The two origins of ozone cannot be differentiated in the tropospheric column ozone observed by a satellite, especially over East Asia where the anthropogenic source regions of ozone precursors are situated close to the latitudes of the STJ. Some occasional data, however, indicate split origins on a daily timescale, suggesting that the two origins really contribute to the enhancement of TCO. Our results strongly suggests an urgent need to develop a new satellite sensor and/or a new algorithm to distinguish boundary layer ozone from free tropospheric ozone in order to promote our understanding of atmospheric pollution over East Asia.
The influence of global warming on tropical cyclone (TC) intensity has been discussed extensively over the past decade and downscaling studies have been conducted by embedding high-resolution regional models in the large-scale environment derived from global warming experiments in previous studies. With theoretical models for TC movement and intensity, this downscaling study examines the impact of global warming on TC intensity in the western North Pacific basin with attention to the selection of climate models for the Intergovernmental Panel on Climate Change (IPCC) fourth assessment report. Individual contributions of SST, vertical wind shear and TC track change to TC intensity change are also discussed. The synthetic TC activity in the study is simulated with motion and intensity models respectively, while little change is assumed in TC formation due to uncertain predictions for regional changes in future TC formation. The downscaling models are first evaluated against the observations during 1965–1999, and then ten IPCC models are selected based on TC intensity downscaled with large-scale environments from their control runs. With an increase of 1.6–3.5 K in SST over the period 2065–2099, the TC peak intensity and number of intense TCs are projected to increase by 14% and 66% respectively, in agreement with previous studies. That is to say, it is likely that intense TC numbers in this region will increase in a warmer world unless the future TC frequency decreases considerably in the western North Pacific basin. Although SST warming plays a dominant role in affecting TC intensity in most of the climate models, this study suggests that changes in vertical wind shear and prevailing tracks can have significant influence on TC intensity change, sometimes comparable to or even larger than the SST change. However, changes in TC track and vertical wind shear do not have the same systematic effect on TC intensity as SST does, increasing the projection uncertainty.
This paper analyzed the downscaling products for Japanese climate using five regional climate models (RCMs) with horizontal meshsize of 20 km with boundary conditions given from JRA-25 reanalysis. The RCMs successfully reproduced the temporal variations and geographic distribution of temperature and precipitation. Skill scores for the surface temperature were improved by the downscaling. The JRA-25 underestimated the precipitation amount for summer and winter seasons, and the RCMs reduced the error, especially in winter. The RCMs showed common features, such as a warm bias in the areas with a monthly-mean temperature lower than the freezing point, an overestimation of weak rainy days, and an underestimation of heavy rainy days. The comparison among five RCMs suggests that the warm bias is due to the lack of model resolution and the precipitation bias is related to the convective parameterization. The multi-RCM ensemble mean has considerable advantages over the individual RCM in regards to the bias and skill scores of surface temperature and precipitation, although it still showed the warm bias in snow areas in winter. The data set of the multi-model dynamical downscaling is expected to contribute to impact studies on the forthcoming climate change in Japan.
The purpose of this study is to improve the initial field accuracy for the simulation of tropical cyclones (TCs) in their initial and developing stages through mesoscale data assimilation with Global Positioning System (GPS) radio occultation (RO) soundings. The mesoscale 4-dimension variational data assimilation (Meso 4D-Var) system of the Japan Meteorological Agency (JMA), modified for low latitudes, is used to investigate Typhoon Usagi in July 2007. The performance of the Meso 4D-Var system and the impact of the GPS RO refractivity are compared against the JMA’s global analysis by conducting forecasts using the JMA non-hydrostatic model. When the JMA global analysis is used for the initial field, no typhoon is formed in the forecast model. In contrast, when the global analysis is replaced by the Meso 4D-Var analysis, the generation of the TC is successfully simulated. With GPS refractivity assimilated, the intensity forecast is significantly improved. The results indicate that the mesoscale data assimilation system performs well in low latitudes and that GPS RO data, which have high accuracy and considerably robust characteristics, are beneficial for the TC forecast. It is expected that the operational use of the assimilation system with GPS RO will contribute to the mitigation of meteorological disasters in low-latitude regions.
The activity of explosively developing extratropical cyclones in the vicinity of Japan in intimate association with the occurrence of heavy snowfall on the Japan Sea side of central Japan is examined using Japanese long-term reanalysis projected data (JRA-25), with additional data from the Japan Meteorological Agency climate data assimilation system (JCDAS). On a monthly basis, the explosive extratropical cyclone tracks tend to concentrate around the Kuroshio Current off the Pacific coast of Japan and the Japan Sea at the heavy snowfall phase, whereas the tracks disperse over the broader areas in the light snowfall phase. The heavy and light snowfall phases correspond well to the strong and weak phases of the East Asian winter monsoon circulation respectively, and the monsoon variability influences the local monthly snowfall on the Japan Sea side of central Japan through change in the explosive cyclone activity. On a daily basis, stationary Rossby wave packets propagating eastward along the northern Eurasian and South Asian waveguides, i.e., subpolar and subtropical teleconnection types, contribute to the occurrence of extraordinarily heavy snowfall events through the development of explosive cyclones. The subpolar teleconnection type facilitates not only the intensification and southward migration of a cold continental high in East Asia, but also the rapid growth of an explosive cyclone around Japan by inducing the equatorward advection of higher potential vorticity from high latitudes. Both developments lead to the reinforcement of an east-west gradient in sea level pressure (SLP) across Japan, thereby providing a favorable condition for the heavy snowfall events. For the subtropical teleconnection type, the explosive cyclone system is primarily responsible for the occurrence of extremely heavy snowfall events through enhanced zonal SLP gradient. It is also found that the explosive cyclone activity differs in terms of geographical location and track between the subpolar and subtropical teleconnection types.
In this study, we revealed and discussed the role of mesoscale convective systems (MCSs) in the eastward expansion of an upper tropospheric high (UH, that is South Asian High or Tibetan High) with a timescale of a few days over the Asian monsoon region in the summers of 1999–2008. The MCSs was extracted using hourly satellite images. The distribution of mature MCSs was inhomogeneous in the Asian summer monsoon region. At three specific regions, i.e., around the eastern part of Tibetan Plateau (TP), over the Bay of Bengal, and in northern Vietnam, the percentage of MCSs larger than 1.2× 105 km2 to all mature MCSs counted in each area was higher and exceeded 25%. Focusing on these regions, the UH extension was examined around the day of large MCSs (LMCSs) formation using objective analysis data sets. Only the LMCSs of eastern TP areas contributed to a significant increase of the geopotential height at 200 hPa with a temperature increase in the mid-upper troposphere. At the same time, the zone of active cloud convection (ZACC) was formed along 30–35°N over central and east China. Cyclonic circulation was strengthened in the eastern part of TP by the LMCS formation and transported moist air north and northeastward toward the inland region, such as the northeastern part of the Sichuan Basin, in the lower troposphere. This moisture advection corresponded to the meandering of the ZACC. The ZACC was maintained until a few days after the LMCS formation, and was also a contributor of greater extension of UH to the east and eastward propagation of the positive anomaly area of soil moisture.
The effect of a capping inversion that lies above a neutrally stratified Ekman boundary layer (EBL) on the stability of the EBL is investigated by solving eigenvalue problems and initial value problems. If the inversion layer is sufficiently high, a growing normal mode exists that corresponds to the inflection point instability. This normal mode is similar to that in the case without an inversion layer. However, the origin of the growing normal mode changes into the Holmboe instability as the height where the inversion layer is imposed is decreased. The growing normal mode finally disappears if the inversion layer is below a certain level. Even if there is no growing normal mode, there may exist non-modal perturbations that can grow transiently. We calculate the optimal perturbation that grows the fastest over a short time period. Unlike the normal mode, the properties of this short-term optimal perturbation are not affected qualitatively by the presence of the inversion layer. These results are consistent with the fact that dominant structures do not emerge, but evanescent smaller-scale streaks do in large eddy simulations performed by other researchers for a neutrally stratified EBL with an inversion layer.