Tropical Moisture Export, Precipitation and the Role of Upper Level Circulation

Moisture exports from Tropics frequently cause extreme precipitation events in subtropics and extra-tropical regions, and are important to the climate system. In this study, both Eularian and Lagrangian methods are applied to obtain a comprehensive climatology analysis of moisture exports from Tropics. Cases are selected based on the precipitation mount associated with these moisture export events. The focus of the study is to understand the circulation features related to these strong moisture exports and associated precipitation, especially the role of upper level circulation. Model experiments are designed to isolate the potential importance of stratosphere and upper troposphere. Model states of horizontal velocity above 300 hPa are relaxed to the reference state (ERA-Interim reanalysis) and allow the lower part of model run freely. Comparison with reanalysis shows that constraint on upper level state can introduce significant forecasting skill in the lower level moisture and hence the total column water field. Upper level eddies (PV anomaly) induced flow in the lower levels acting on the basic moisture gradient contributes most of constraint on moisture field. Potential improvement of precipitation forecast in the subtropics may be achieved from upper level atmosphere.