SENSITIVITY OF FIVE CUMULUS SCHEMES IN SIMULATING WEST AFRICAN MESOSCALE CONVECTIVE SYSTEMS’ CHARACTERISTICS

Abstract

This research work evaluated the performances of five cumulus parametrization schemes (CPSs) in Weather Research and Forecasting (WRF) model for simulating the initiation, propagation and rainfall delivery capacity of mesoscale convective systems (MCSs) over West Africa. Five simulations were conducted using five different cumulus schemes, for the whole of September, 2012 with the initial and lateral boundary conditions from ERA-Interim reanalysis data. The five cumulus parameterization schemes used are the Kain-Fritsch (KF), Grell- 3Dimension ensemble (Grell 3D), Tiedtke (TKE), Grell-Freitas (GF), and New Simplified Arakawa-Schubert (NSAS). Infra-red brightness temperature from the satellite imagery was used for the performance evaluation while TRMM rain rate were used as observed to compare rain rate of the five CPSs. All the schemes successfully simulated spatial feature of MCS in the case 1, although there was 2 hours delay/lag for Grell 3D, GF and NSAS schemes before they initiated MCS. During initiation, TKE scheme performed better than the other schemes in reproducing initiation of MCS in all the cases. The monitored MCS in case 1, case 2 and case 4 propagated westward while it moved north-west in case 3. This may be caused by cold air-mass meeting with MCS around Democratic Republic of Congo. Furthermore, the NSAS scheme gave better average of daily rain-rate than the other schemes in all the cases, except Case 4 where KF performed better than the other schemes with their smallest mean bias. It was discovered that the scheme which performed better for average aggregate of daily rain-rate was almost the worst for hourly maximum value of rain-rate. This may be due to local forces. However, based on the subjective verification and objective evaluation, it can be concluded that the performance of the five schemes is highly case-and local-dependent, and the performance of the better TKE and NSAS schemes cannot be generalised over West Africa.