Moist Potential Vorticity Vector for Diagnosis of Heavy Rainfall Events in Tanzania

Abstract

In this paper, we modify the convective vorticity vector ( CVV ) defined as a cross product of absolute vorticity and gradient of equivalent potential temperature to moist potential vorticity vector (MPVV) defined as a cross product of absolute vorticity (ζa ) and the gradient of the moist-air entropy potential temperature (θs ). The patterns of (MPVV ) are compared with the patterns of heavy rainfall events that occurred over different regions in Tanzania on 20th to 22nd December, 2011 and on 5th to 8th May, 2015. Moreover, the article aimed at assessing the relative contribu- tions of the magnitude, horizontal and vertical components of (MPVV ) detecting on the observed patterns of rainfall events. Dynamic and thermodynamic variables: wind speed, temperature, atmospheric pressure and relative humidity from numeri- cal output generated by the Weather Research and Forecasting (WRF) model run- ning at Tanzania Meteorological Agency (TMA) were used to compute MPVV. It is found that MPVV provide accurate tracking of locations received heavy rainfall, suggesting its potential use as a dynamic tracer for heavy rainfall events in Tanzania. Finally it is found that the first and second components of MPVV contribute al- most equally in tracing locations received heavy rainfall events. The magnitude of MPVV described the locations received heavy rainfall events better than the com- ponents.