Simulating the characteristics of cut-off low rainfall over the Western Cape using WRF

Abstract

Cut-off lows (COLs) cause hazardous weather but also play a crucial role in the annual rainfall of the Western Cape, especially during a drought year. However, there is a dearth of information on the capability of atmospheric models to reproduce the characteristics of COLs over this region. This study evaluates the capability of a regional climate model (WRF) in simulating COLs over the Western Cape, with emphasis on the drought periods. Observation, reanalysis and simulations datasets were analysed for the study. The simulated climatology and inter-annual variability of COL with the associated patterns were compared to the reanalysis and observation results. The Self Organising Map (SOM) was used to group the observed and simulated COL rainfall to similar patterns. The results of the analysis show that WRF captures the seasonal and annual climatology of COL and the associated rainfall, but the model struggles to simulate the inter-annual variability of the systems. The model reproduces all the COL rainfall patterns well, though it under-estimates the frequency of dry COLs. However, WRF simulation agrees with the reanalysis that wetter COLs over the Western Cape are associated with more transport of warm, moist air from the tropics. The results of the study have application in improving weather and seasonal forecasting of COLs over the Western Cape.