WCD-06. Tropical Dynamics Diagnostics for Numerical Weather Prediction

Precipitation and moisture variability in the Tropics covers a vast collection of scales from a few to several thousands of kilometers and from hours to weeks. Current operational numerical weather prediction (NWP) models struggle with representing the full range of scales and phenomena in the Tropics. It has been shown that in particular skill in the representation of larger scale phenomena such as convectively coupled equatorial waves (CCEWs) can be important for reducing error propagation from the Tropics to the Mid-latitudes at longer lead times. Novel diagnostics are therefore needed to assess NWP forecast skill of large spatial, long time scale phenomena such as CCEWs as well as to assess the representation of physical processes that are important for CCEW initiation and maintenance. Here we apply diagnostics from a tropical variability diagnostics toolbox currently under development at NOAA PSL to model output from two recent versions of the Unified Forecast System (UFS), operational V15 forecasts and experimental retrospective V16 forecasts from December 2019 through March 2020. The diagnostics include space-time coherence spectra to identify preferred scales of dynamics-precipitation coupling, pattern correlation of precipitation Hovmoeller diagrams to assess model skill in zonal propagation and an EOF based CCEW skill assessment. Two more diagnostics that look at the coevolution of moisture and convection in the Tropics can be used by model developers to assess whether changes in the parameterizations lead to improvements of this relationship. These diagnostics are in the process of being added to the Model Diagnostics Task Force (MDTF) code repository and the upcoming METplus python release (METcalcpy and METplotpy). Results show that the V16 forecasts are more realistic in reproducing the statistical relationship between precipitation and column moisture, and slightly more skillful in their coherence between precipitation and model dynamics at CCEW scales. However, this does not necessarily translate to a significant improvement in traditional precipitation skill scores such as ETS or MSESS. This highlights the utility of these process and phenomena focused diagnostics as the goal is to allow for better understanding of NWP model performance regarding coupling between moist convective processes and synoptic to planetary scale tropical circulations.