Winter Westerly Disturbance Activity in High Mountain Asia: A Wave Tracking Approach

Abstract:

Extra-tropical cyclones, including Winter Westerly Disturbances (WWD) over central Asia, are fundamental features of the atmosphere that maintain energy, momentum, and moisture at global scales while intimately linking large-scale circulation to regional-scale meteorology. Within High Mountain Asia (HMA), there is no mechanism that is more important in contributing to water supply during winter, and therefore it is important that we create a baseline climatology of these disturbances and further explore variability over time. Eulerian methods of investigating variance of fields related to WWD at synoptic scales are typically employed as a general measure of storm track activity. However, Eulerian statistics cannot convey important information regarding the specifics of individual systems, nor can the attributes of a cyclone be taken directly. Thus, a Lagrangian method of automatically tracking WWD, which can provide complementary information about individual systems and allows us to investigate track activity, is desired. Currently, there is no technique that adequately captures WWD, which often propagate along relatively low latitudes, encounter highly variable topography, exhibit strong tropical influences, and are highly asymmetric.

In this study, we utilize the atmospheric wave signature of WWD in upper-level geopotential height to identify individual systems responsible for HMA precipitation and track their life cycles. This provides a valuable link between the large-scale climate, transient disturbances, and hydrologic processes within HMA, and allows us to evaluate WWD on a per-case basis while considering all factors that relate these systems to precipitation in the mountains. This framework enables us to consider the relative contribution of dynamically forced orographic precipitation in HMA associated with cyclone intensity and wind speed, as well as the contribution of convective instability, which may facilitate heavy precipitation with weak mechanical forcing of air over topography. Our individual event perspective aids in understanding the non-linear relationship between the strength of an extra-tropical cyclone, the characteristics of interacting air-masses over the Indian subcontinent, and the amount of precipitation received in HMA.