The development of surface pressure anomalies observed in January over the Arctic is investigated based on composite analysis of observed data. The monthly anomalies in January with a high degree of zonal symmetry are found to extend from the surface into the lower stratosphere, in a manner similar to the Arctic Oscillation (AO). The formation of the AO-like signature in January tends to follow the strengthening/weakening of upward propagation of planetary waves in November over the western portion of Eurasia, where temperature anomalies associated with a train of external Rossby waves modify the thermal structure of the climatological planetary waves. In the lower stratosphere the external Rossby wavepacket also modulates meridional winds associated with the climatological planetary waves, augmenting upward propagations of the planetary waves. Thus, modulated upward propagation of the planetary waves subsequently changes the intensity of the stratospheric polar vortex, leading to the formation of the stratospheric AO-like signals and their downward extension. This study presents an example in which regional modifications in the structures of the upward-propagating planetary waves due to a zonally-confined external Rossby wave train can change the intensity of the stratospheric polar vortex, which may possibly prolong the predictability of seasonal climate anomalies in the Arctic and surrounding regions.
We examined the decreasing trend in rainfall during the late summer monsoon season (September) in Thailand from 1951 to 2000 and associated changes in tropical cyclone (TC) activity. Thailand receives significant rainfall from May to October and experiences two rainy peaks in late May to early June and in September. A previous study reported a decreasing trend in September rainfall in Thailand and, based on a regional climate model, suggested that the trend was associated with local deforestation. However, the long-term trend may also be affected by changes in large-scale circulation. Thus, the purpose of this study was to investigate changes in large-scale circulation associated with the decreasing rainfall trend. Westward-propagating TCs from the South China Sea and the western North Pacific brought most of the rainfall over Thailand in September. TCs include tropical depressions, tropical storms, severe tropical storms, typhoons, and residual lows. 70% of the rainfall amount in September was estimated to be associated with TCs. The 50-year time-series of September rainfall over Thailand showed a significant decreasing trend. TC activity defined by 700-hPa relative vorticity, showed a weakening trend over the Indochina Peninsula. TC tracks also suggested the weakening of TC activity over this area. The long-term trend in rainfall during the late summer monsoon season was closely associated with changes in TC activity over the Indochina Peninsula; these changes were likely caused by changes in the major course of TCs. Concurrent with the changes in TC tracks, there was a change in the TC steering current around the Philippines archipelago and Taiwan. This led to the TC activity over the Indochina Peninsula being inactive, probably resulting in the long-term decrease in rainfall over Thailand.
Surface heat fluxes around 2°N, 138°E in the tropical western Pacific have been acquired by Research Vessel (R/V) Mirai for a total of 74 days during four observational periods. Significant fractions of the sensible and latent heat fluxes are found to occur during occasional passages of precipitating cloud systems which cause large surface fluxes. Based on the radar reflectivity image and in situ precipitation observation at the R/V Mirai, the contributions of the surface heat flux enhancements due to precipitating clouds to the total accumulated surface heat fluxes throughout the observational periods are estimated to be 41 and 10 percent for the sensible and latent heat fluxes, respectively (12 percent for their sum). Furthermore, each 20 percent increase in the radar echo area ratio leads to increases in sensible and latent heat fluxes by about 11 and 30 W m−2, respectively. The significant enhancements of the sensible heat flux are due to the combined effects of increased wind speeds and large air-sea temperature differences resulting from distinct drops in air temperature at the surface. On the other hand, the enhancements of the latent heat flux are primarily due to the increased wind speeds. This difference in the way the sensible and latent heat fluxes are enhanced by precipitating clouds leads to the differences in the contributions of these enhanced fluxes to the total accumulated fluxes as well as their frequency distributions. These results suggest that precipitating convective systems play an important role in determining the surface heat fluxes over the tropical western Pacific on temporal scales that range from the life cycle of an isolated cumulus to Madden-Julian oscillation.
We propose a model for large snowflakes based on the fractal nature of their particle shapes. Monte Carlo simulations were conducted to make particles with a fractal dimension of 1.8 to 2.4. The roundness parameter for the projected images of the modeled particles was derived, and an average roundness of approximately 0.4 for particles with fractal dimension 2.1 was indicated; this average roundness matched reported values measured for large ice aggregates. The finite difference time domain method was used to calculate the backscattering cross-sections of ice particles with a fractal dimension of 2.1 and a particle diameter of up to ∼20 mm at microwave frequencies of 95 GHz, 35 GHz, and 9.8 GHz. The results were compared with those of equivalent-volume spheres and randomly oriented equivalent-volume hexagonal columns. Our snowflake model had smaller values of radar backscattering cross-sections than did the equivalent-volume spheres in the size range out of the Rayleigh regime. Furthermore, large differences in backscattering cross-sections between the snowflake model and the equivalent-volume hexagonal column were confirmed, in particular at a frequency of 35GHz.