WCD-06. Tropical Dynamics Diagnostics for Numerical Weather Prediction
Abstract
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.