[AAS05-P02] An Initiation Process of TD-type Disturbances under an Influence of Upper-Level Troughs
Keywords:TD-type disturbance, Upper-tropospheric trough , Extratropical forcing
At the same time, though TDDs are thought to be originated from lower-tropospheric eddies in association with convection in principle, several studies have indicated that some of TDDs are initiated by upper-tropospheric forcing (e.g. Tam and Li, 2006; Heta, 1991). Despite the plausibility of the upper-level contribution on the initiation of TDDs, little research has been conducted on the statistical properties of TDDs induced by upper-level activities.
From these perspectives, using CLAUS brightness temperature and JRA-55 products, we statistically analyzed the relationship between TDDs’ occurrences and the upper-level perturbations to understand the mechanism of the initiation process of TDDs.
As a result, it is revealed that about 30% of TDDs are generated under an influence of upper-level troughs (so called Mid-Pacific Trough), especially in the off-equatorial region near the date line. The initiation of such trough-related TDDs is associated with a preceding dynamical updraft at the back of the upper-level troughs. It enables convections to become deeper, which results in the development of low-level disturbances via the moisture-convection feedback. Furthermore, according to the composite figures on the trough-related TDDs, it is shown that their environments are much drier than those of other TDDs, which indicates that the upper-level troughs play important roles at the initial stage of their occurrence, especially in relatively drier conditions.