Exploring Dynamic and Thermodynamic Factors Contributing to Extreme Tropical Rain Events on Daily 2x2 Degree Scales

Anomalously high Eulerian accumulations of rainfall pose a great socio-economical hazard to those affected. The underlying dynamics governing these events remains to be well understood. Previous research suggests that a combination of dynamical and thermodynamical factors acting in unison are involved, producing extreme rainfall cases. On the 2x2 degree and daily scales selected here, features such as low-level jets, moisture tongues, and upper-level forcing due to potential vorticity (PV) structures are contributing to episodes of deep moist convection. This two-pronged investigation will aim to identify and estimate the importance of various factors that contribute to the development of convective systems leading to high-quantile precipitation (>100 mm; >99%).

We extract cases that are observed directly by satellite (TRMM 3B42 data) as well as well-analyzed in the precipitation produced by a global dynamical model (MERRA) which only assimilates wind and thermodynamic data. In order to narrow the scope of our investigation, we select case studies over the upper tropical latitudes of Asia. In each case (via selection off of a scatterplot of TRMM vs. MERRA rainfall), a standard, multivariate depiction of the atmosphere (centered on the event's spatial-temporal coordinates) is generated. Unidata’s Interactive Data Viewer (IDV) produces IDV "bundles" which fetch and render a set of thermodynamic and dynamical variables. We then individually examine these variables in IDV's interactive, more user-friendly interface. Case examples may also involve those when MERRA fails to analyze the TRMM-observed precipitation. However, future goals of simulation experiments will be best reserved for well-analyzed cases.

Preliminary findings show that favorable dynamics for large rainfall accumulations involve those that favor longevity and stationarity of storms (i.e. weak 500 hPa wind fields, terrain anchoring, high PWATs). While upper-level dynamical forcing is commonly found in mid-latitude systems, it is less frequent in cases of tropical convection. However, tropical cyclones are commonly included in extreme rainfall cases observed on these scales. In future work, we will classify related cases, average their fields into composites in order to discover systematic factors over cases whilst removing any case-specific anomalies. WRF simulations with modified initial conditions will then be utilized to test hypotheses that determine which are the more important factors in extreme rainfall accumulations.