北大西洋风暴轴高低空分布及其能量诊断

Patterns of North Atlantic Strom Track in the Upper and Lower Troposphere and Their Eddy Kinetic Energy Analysis

利用1979~2013年NCEP再分析数据,通过经验正交分解对比了前冬时期北大西洋风暴轴的高低空分布,并用涡动动能(Eddy Kinetic Energy,EKE)方程对风暴轴高低空分布型差异进行了诊断。研究结果表明:上层和下层第一空间分布型差异巨大,对流层下层风暴轴中心偏北,靠近极地,而上层风暴轴中心偏西南,靠近北美沿岸。EKE方程诊断结果表明:正压转换项在高低空符号相反,导致了EKE在上、下层分布出现显著差异,即上层正压转换项为负,在扰动发展中起能量耗散作用,而下层正压转换项为正,且极大值区域对应下层EKE极大值区域,为风暴轴下层向极区域增强的主要原因。而斜压转换和非地转位势通量散度在上层均为正,且远大于下层,为风暴轴上层涡动能量维持的原因,也从涡动能量收支上解释了风暴轴的主体出现在上层。

This study investigates the distribution of the North Atlantic Storm Track （NAST） in the upper and lower troposphere, and associated eddy kinetic energy （EKE） distribution in each layer. EKE is then isolated in each layer. The results shows that there is a great difference in NAST and EKE between the upper and lower troposphere. In the upper troposphere, the NAST stays a bit further southwest; in the lower troposphere, the NAST shifts poleward. By examining each term in the EKE equation, it is found that the barotropic conversion is opposite in the two layers, which could be responsible for the great difference in their storm track distribution patterns. That is to say, the barotropic conversion mainly damps EKE in the upper layer whereas it strengthens EKE in the lower layer because the barotropic conversion is negative in the upper layer but positive and shows the maximum value in the center of NAST in the lower layer. Furthermore, the baroclinic conversion and the ageostrophic geopotential flux divergence both contribute to the maintenance of EKE in the upper layer.