The cloud variations under subtropical high（STH） conditions during summers over a ten-year period are studied using combined data from the International Satellite Cloud Climatology Project and the National Centers for Environmental Prediction.The results reveal that clouds mainly experience an isolated evolution in the STHs,which is designated in this study by the 1540 gpm geopotential lines at 850 hPa.In the STH domain throughout the Northern Hemisphere,the average amount of total clouds exceeds 30%.Low clouds dominate in the STH domain,contributing over 60%of total cloud amount within the Pacific subtropical high and over 40%within the Atlantic subtropical high.The prevalence of low clouds in above regions is determined by the circulation pattern around 150°-180°E and 850 hPa,which suppresses both the upward development of the cloud tops and the water vapor divergences near the surface.Furthermore,clouds present great geographical incoherence within the STH domain.In the eastern STHs,the amount of middle and low clouds increases to peak in the early morning and decreases to a trough in the afternoon,while the amount of high clouds remains stable throughout the day.Conversely,in the western STHs,the diurnal amplitude of low and middle clouds is less than three,while high clouds dramatically reach the maximum in the afternoon and drop to the minimum in the evening.Among the nine cloud categories,stratocumulus clouds with greater optical thickness account for the most under STH conditions,no matter their occurrence or amount,causing more shortwave cloud radiative forcing to cool the local atmosphere and surface as a consequence.
Two monthly datasets of sea surface temperature (SST),TMI SST retrieved from satellite observations by Remote Sensing System and HadISST1 (Hadley Centre Sea-ice and Sea-surface Temperature Data Set Version 1) derived from in situ measurements by Hadley Centre,were compared on climatologic multiple time scales over tropical and subtropical areas from 1998 to 2006.Results indicate that there is a good consistency in the horizontal global distribution,with 1.0° resolution on multi-year and multi-season mean scales between the two datasets,and also in the time series of global mean SST anomalies.However,there are still some significant differences between the datasets.Generally,TMI SST is relatively higher than HadISST1.In addition,the differences between the two datasets show not only remarkable regionality,but also distinct seasonal variations.Moreover,the maximum departure occurs in summer,while theminimum takes place in autumn.For all seasons,over 30% of the regions in the Tropical and Subtropical areas have a difference of more than 0.3°C.EOF analysis of the SST anomaly field also shows that there are differences between the two datasets,where HadISST1 has more significant statistical characteristics than TMI SST.On the other hand,results show that the difference between the two datasets is related to the vertical structure of ocean temperatures,as well as other simultaneously retrieved parameters in TMI products,such as wind speed,water vapor,liquid cloud water and rain rates.In addition,large biases between HadISST1 and TMI SST are found in coastal regions,where TMI SST cannot be accurately retrieved because of polluted microwave signals.
The impact of the subtropical high (STH) on precipitation was investigated on a daily timescale using matched NCEP and the Global Precipitation Climatology Project (GPCP) datasets.Comparison of the conditional probability (intensity) of precipitation under STH condi-tions with that under non-STH conditions suggests that the presence of the STH conditions has a limited impact on local precipitation.In the West Pacific Subtropical High (WPSH) and the North Atlantic Subtropical High (NASH),precipitation was only 30% lower under STH conditions than under non-STH conditions.The STH conditions had somewhat more impact on precipitation intensity,but it was still 50% less than the intensity under non-STH conditions (mean of roughly 5 mm d 1).Pre-cipitation under STH conditions was found to be highly correlated with vertical motion.Active updrafts occurring even under STH conditions are essential for frequent oc-currences and moderate intensities of precipitation.
Using a microwave radiative transfer(MWRT) model with microwave brightness temperatures(TBs) observed from the Tropical Rainfall Measuring Mission(TRMM) Microwave Imager(TMI),an indirect approach evaluated hydrometeors generated from the Weather Research and Forecasting(WRF) model in the process of CHABA typhoon in August 2004.This study compares the simulated TBs generated from the microwave radioactive transfer model connected to the WRF model with the observed TBs derived from TMI and analyzes the differences between these TBs.The results indicate that the WRF model underestimates the amount and area of liquid and ice hydrometeors inside the typhoon center.The results also indicate relatively better agreement between the simulated and the observed TBs in the vertical polarization than in the horizontal polarization.
FU Yun-FeiHENG Zhi-WeiLI Tian-YiSHEN Zhong-PingWANG Yu
