Representative Height of the Low-Level Water Vapor Field for Examining the Initiation of Moist Convection Leading to Heavy Rainfall in East Asia

Abstract

 This study investigated the representative height of low-level water vapor field that can be used to examine the occurrence possibility of heavy rainfall in East Asia. First, cloud base heights (CBHs) of moist convection were statistically examined by performing simulations with a 1-km-resolution numerical model during April–August 2008, with a focus on Kyushu and Shikoku Islands, western Japan. CBHs of moist convection with strong updrafts were simulated mainly around 500 and 300 m heights above sea level over land and over the ocean, respectively. This result indicates that low-level humid air below a height of 500 m is very important for the initiation of strong moist convection. Moreover, the equivalent potential temperature θ_e at the CBHs was examined to clarify θ_e values of lifted air parcels initiating cumulonimbus development. This result showed that, below the CBHs, θ_e was usually around 355 K.

 Given these results for the CBHs, θ_e at 500 m height from 10-km-resolution objective analysis data was statistically compared with θ_e at various heights and pressure levels over the ocean south of 35°N in East Asia during June–September 2008. These comparisons showed that analyses at the 850-hPa level could not represent the low-level water vapor field, while the θ_e field at 850 hPa in the Baiu season was strongly influenced by convective activity over the Baiu frontal zone. The θ_e field at 925 hPa also could not adequately represent the low-level water vapor field, but the difference in θ_e between heights of 250 and 500 m was very small. Because high θ_e layers must have some thickness, data at 500 m height can be considered representative of the low-level water vapor field in analyses examining the initiation of moist convection leading to heavy rainfall.