Articles | Volume 8, issue 9
Development and technical paper
22 Sep 2015
Development and technical paper |  | 22 Sep 2015

Improved simulation of precipitation in the tropics using a modified BMJ scheme in the WRF model

R. M. Fonseca, T. Zhang, and K.-T. Yong

Abstract. The successful modelling of the observed precipitation, a very important variable for a wide range of climate applications, continues to be one of the major challenges that climate scientists face today. When the Weather Research and Forecasting (WRF) model is used to dynamically downscale the Climate Forecast System Reanalysis (CFSR) over the Indo-Pacific region, with analysis (grid-point) nudging, it is found that the cumulus scheme used, Betts–Miller–Janjić (BMJ), produces excessive rainfall suggesting that it has to be modified for this region. Experimentation has shown that the cumulus precipitation is not very sensitive to changes in the cloud efficiency but varies greatly in response to modifications of the temperature and humidity reference profiles. A new version of the scheme, denoted "modified BMJ" scheme, where the humidity reference profile is more moist, was developed. In tropical belt simulations it was found to give a better estimate of the observed precipitation as given by the Tropical Rainfall Measuring Mission (TRMM) 3B42 data set than the default BMJ scheme for the whole tropics and both monsoon seasons. In fact, in some regions the model even outperforms CFSR. The advantage of modifying the BMJ scheme to produce better rainfall estimates lies in the final dynamical consistency of the rainfall with other dynamical and thermodynamical variables of the atmosphere.

Short summary
In this study the BMJ scheme, a convective adjustment scheme where temperature and humidity are relaxed towards reference profiles, as implemented in the WRF model version 3.3.1, is modified so that the precipitation produced by the model is in better agreement with that observed as given by the Tropical Rainfall Measuring Mission (TRMM) 3B42 data set. The "modified BMJ" scheme gives a better representation of the observed rainfall for the whole tropics in both winter and summer seasons.