[A-AS08] 湿潤大気の多様な現象

[AAS08-P06] インド洋ー太平洋暖水域における、対流ダウンドラフトによる大気境界層冷却量とその総観スケールの変動

*横井 覚¹ (1.海洋研究開発機構)

キーワード：対流ダウンドラフト、船舶観測、大気境界層準平衡過程

Tropical deep cumulus convective systems interact with underlying atmospheric boundary layer (BL). While temperature and moisture conditions in the BL are one of the essential parameters that determine intensity and characteristics of the convective systems, the convective systems cool and dry the BL, namely, decrease its moist static energy (MSE), mostly due to precipitation-driven convective downdraft. There have been many case studies and statistical studies of events of the convective systems, it is still unclear to what extent the convective systems collectively decrease the BL MSE in synoptic and longer timescales. We address this question through analysis of shipborne observation data over the Indo-Pacific warm pool, where vigorous deep cumulus convective systems often occur, obtained by Japanese Research Vessel (R/V) Mirai (192 days in total) and US R/V Roger Revelle (43 days). We take 9- to 12-day averages to examine synoptic-scale feature. Based on BL dry static energy and moisture budget analyses, we find that synoptic-scale BL MSE decrease rate due to the convective downdrafts has positive and significant correlation with intensity of deep convection accompanying precipitation around the vessel. The ratio of the BL MSE decrease rate to precipitation amount multiplied by specific heat of vaporization, which is considered to be equivalent to net heating of the atmosphere by condensation, is on average ~10 %, and this ratio tends to be low when lower free troposphere is wet, and convective-to-stratiform rain ratio is low. Furthermore, we compare the BL MSE decrease rate by convective downdraft with that by entrainment through the BL top, which is another major process for the BL MSE decrease, and find that the contribution of the convective downdraft is generally smaller than that of the entrainment.