熱帶海洋的降雨特徵

Abstract

本研究分析熱帶海洋的降雨特徵，從海表面溫度（Sea Surface Temperature, SST）及大氣熱力環境場改變兩部分著手，討論環境改變時降雨的變化。 在SST與降雨的關係上，過去研究中指出降雨與SST有正比與反比關係。當SST大於26℃，小於30℃，SST與降雨成正比關係，意味著SST在海氣交互作用中扮演主動的角色。而當SST超過30℃，SST與降雨成反比關係，SST則扮演被動的角色。然而過去研究降雨與SST關係多以月平均以及低解析度資料進行分析，並非接近實際情況的降雨與SST。在此，我們分析高解析的日平均資料顯示降雨與SST為正比關係，而降雨對SST的影響則有延遲的效應，因此，以月平均資料的分析中出現SST與降雨的反比關係。 大氣環境中，降雨與水氣的關係相當密切，而大氣中水氣的變化又與溫度有關。我們選取夏季與冬季兩個水氣條件有差異的區域：季風區，討論在溫度改變時環境對於降雨的影響。當溫度增加，水氣增加，接近Clausius-Clapeyron方程得出的7%，然而強降雨的增加率遠高於水氣的變化，主要與垂直速度的增加有關。而大氣溫度在垂直結構上的改變，使得大氣穩定度產生變化，可能改變垂直速度增加率。而模式的模擬中，不同模式模擬的垂直速度差異很大，可能是模式間模擬的降雨增加率有差異的原因。

The characteristics of precipitation in tropical oceans were examined in this study. We focused on the influence of the atmospheric thermal condition and the boundary condition, such as sea surface temperature (SST). Previous studies show that the relationship between precipitation and SST could be either positive or negative. When SST is between 26 ℃ and 30℃, the correlation is positive, which SST plays an active role in the air-sea interaction. On the other hand, a negative correlation appears with SST greater than 30℃, so SST plays a passive role. The relationship between precipitation and SST discussed above is usually obtained from monthly data with a relatively coarse resolution, so it might not be able to represent a realistic relationship between these two variables. Here we examine the relationship between precipitation and SST by using daily satellite data with a high spatial resolution. Only a positive correlation is found between SST and precipitation. Precipitation shows a delayed impact on SST, i.e. a cooling 2-3 days after the peak of precipitation. This delayed impact might be the reason causing the negative correlation between SST and precipitation in monthly data with a relatively coarse resolution. The strength of precipitation is related to water vapor, which is strongly linked to temperature. We examined the differences of precipitation between summer and winter. We focus on monsoon regions, where moisture presumably varies a lot between these two seasons. When temperature increases, water vapor in the atmosphere increases by about 7%/K, which is close to the Clausius-Clapeyron thermal expansion under the condition that the relative humidity is constant. The increased rate of extreme precipitation is much greater than the increase in water vapor, which implies that vertical velocity is more important. The vertical structure of temperature differences between summer and winter is consistent with the atmospheric stability, which can affect the strength of upward motion. In model simulations, changes in extreme precipitation vary a lot among models, which might be due to differences in vertical velocity.