Representation of tropical convection in a near-global convection permitting seasonal simulation with WRF

Precipitation observations between March to May 2015 show several coherent propagating systems in an area between 10°N and 10°S with a lifetime of 3-4 weeks demonstrating the importance of simulations beyond a month. The eastward propagation speed is typically 1100 km day^-1. The main origins of significant amounts of precipitation along this belt are the tropical warm pools in the Western Pacific around 158-174°E and the eastern Indian Ocean around 90°E as well as the tropical rainforest over South America around 69°W.

We investigated the lifetime and propagation of tropical precipitating systems based on observations and a near-global convection permitting seasonal simulation with the Weather Research and Forecasting (WRF). The latitude-belt simulation covers an area between 57°S to 65°N with a grid increment of 0.03° over a period of 5 months forced by sea surface temperature (SST) observations.

Results of this simulation with respect to tropical convection were investigated by means of comparison with satellite-based cloud and precipitation observations and ECMWF operational analysis. Wavenumber-frequency spectra of the tropical convection and the detection of various wave pattern were derived from the 3-h outgoing longwave radiation at the top of the atmosphere (TOA OLR) fields and revealed by Wheeler-Kiladis diagrams. The simulation shows the observed spectral signatures of eastward propagating EIGs and Kelvin waves.

The EOF decomposition of the monthly averaged sea level pressure fields demonstrates that 65 % of the sea surface pressure fluctuations in the ECMWF analyses can be explained by the correlation pattern shown in the 1^st EOF. The agreement with the 1^st EOF of the WRF simulation is excellent despite a slight underestimation of the strength of the correlations. The spatial structure is very similar and 61 % of the variance are contained in first EOF. The EOF analyses provided strong evidence that the seasonal simulation with a convection permitting horizontal resolution captures the representation of the teleconnection pattern.