Tropical Dynamics Diagnostics for Numerical Weather Prediction

Convectively coupled equatorial waves (CCEWs) are important modes of tropical variability. They have long time and large spatial scales and are thought to be central to improving tropical weather forecasts. However, current operational numerical weather prediction (NWP) models struggle with predicting CCEWs past a few days lead time. Traditional methods of identifying CCEWs and their activity in observations or climate models rely on long time series (on the order of a few decades) to assess space-time characteristics of CCEWs. These methods are not easily applicable to NWP forecasts because when developing or improving weather models it is rare that long time series are available. This makes it difficult to assess the models' performance related to CCEWs and other phenomena with time scales longer than a few days. Novel diagnostics of CCEWs are therefore needed, in particular those designed to identify long time scale phenomena from short time series.

Here we apply diagnostics from a tropical diagnostics toolbox currently under development to model output from the FV3GFS. Two methods in particular are highlighted that evaluate model behavior on particular time and length scales without the need for long model runs. The first uses Empirical Orthogonal Functions (EOFs) of CCEWs derived from observed precipitation data. Projection of the model output onto the observational EOFs allows the instantaneous evaluation of CCEW activity. The second method evaluates model predictions based on their spectral coherence with observational fields of CCEWs. These analyses allow for better understanding of NWP model performance regarding coupling between moist convective processes and synoptic to planetary scale tropical circulation.