Sensitivity Experiments with Betts-Miller Scheme Parameters in the Eta Model

.Numerical atmospheric models have deficiencies in the representation of rains over Northeast of Brazil. This is because the precipitating cloud in that region are not very deep but hold high content of water. Numerical models use convection parametrization schemes that are adjusted for cumulus clouds from other regions. The Regional Eta Model, operational at CPTEC/INPE, uses the Betts-Miller scheme. The objective of this work is to adjust the Betts-Miller scheme through sensitivity experiments of the parameters: saturation depressions at cloud base, freezing level and cloud top (Sb, Sf and St); stability coefficient (Wt) and convective adjustment time (t). A case of underestimated precipitation forecast over Northeast of Brazil was chosen for this study. The experiments are evaluated through the comparison between model predicted and observed surface data. The skill of each experiment is compared with operational forecast skill. The results of the experiments indicated that the variation of the Sx parameters affect the intensity and area covered by precipitation. Experiments with Wt parameter resulted in intensification of the precipitation. Variation of the t parameter had effects directly in timing of precipitation. A combined set of S, W and t parameters were chosen to produce better forecasts of the Northeast Brazil rains