Identifying and Attributing Changes in the Ratio of Synoptically and Locally ForcedMidlatitude Precipitation Events

The unpredictability of precipitation over land is a major forecasting challenge, particularly when it is predominantly driven by local processes. In this study, we build a classification scheme for precipitation events based on the synoptic scale forcing for ascent. The ultimate goal of this approach is to study land-atmosphere coupling in regimes of weak synoptic forcing. Our classification scheme is based on the quasigeostrophic (QG) Omega equation, which approximates the forcing for ascent associated with the synoptic-scale flow.

Using NASA’s Modern-Era Retrospective Research and Application Version 2 (MERRA-2) reanalysis, we identify regions in which local precipitation (precipitation that occurs under weak synoptic forcing for ascent) frequency is relatively high and/or has exhibited large variability. We then test our hypothesis that changes in a region’s local precipitation regime are a proxy for changes in the region’s land-atmosphere coupling regime. The analysis hinges on the following points and relationships. First, the applicability of QG Omega as a metric to identify locally forced and synoptically forced precipitation events using the meteorological variables from the MERRA-2 dataset. Second, the correlation between precipitation type frequency and land-atmosphere coupling strength.

Initial results suggest a strong seasonality of the local and synoptically driven precipitation over CONUS, synoptically driven precipitation being dominant in the winter. In contrast, local precipitation dominates in summer, especially in the southern half of CONUS. We plan to analyze the long-term changes to the seasonality of local and synoptic precipitation events and their relative contribution to total precipitation in global hot spots of land-atmosphere interaction.