Observational and Numerical Modeling Analysis of Nocturnal Convection over Tropical Lakes

High resolution lightning climatologies from the NASA Tropical Measurement Mission (TRMM) Lightning Imaging Sensor (LIS) show occurrence of nocturnal maxima in lightning flash rates over 20 lakes in the tropics.  Whereas, the lightning is associated with nocturnal convection forced by lake induced mesoscale circulation, timing and location of convective activity varies depending upon the lake characteristics and the nature of terrain.  Lake Maracaibo in Venezuela, with global maximum in lightning flash rates, is located adjacent to the complex terrain of the Andes.  Similarly, Lake Victoria, where local maxima in lightning and nocturnal convection have caused thousands of fatalities, is located adjacent to complex topography in the Rift Valley region of Central Africa. 

We conducted process studies using Weather Research and Forecasting (WRF) Model to investigate the role of lake characteristics and terrain settings on nocturnal convective initiation.  Eight nocturnal convection events were simulated for Lake Maracaibo and Lake Victoria and were compared to Global Precipitation Mission (GPM) satellite observations.  Simulations show that WRF has higher skill in capturing timing, location and intensity of nocturnal convection over Lake Maracaibo.  A subset of the simulations that best capture the GPM observed rainfall intensity and location were used to examine the environmental conditions and processes that are critical to nocturnal convection over the lakes.  These include: 1) high Convective Available Potential Energy (CAPE) over the lakes; 2) lake/land- breeze circulations, enhanced by complex terrain that converge over the lakes and trigger convection in the high CAPE environment and; 3) weak upper level winds that result in slow propagation and thus long-duration events over the lake.  Case study days where WRF failed to simulate GPM observed nocturnal convection were associated with weaker lake/land-breeze circulations or the formation of a strong inversion over the lake. 

The WRF model showed better skill at simulating nocturnal convection over Lake Maracaibo compared to Lake Victoria.  The nature of complex terrain surrounding these lakes is different; specifically, the topographical gradient is higher and more symmetrical around Lake Maracaibo compared to Lake Victoria.  Correspondingly, the lake/land-breeze circulation over Lake Victoria shows a higher degree of asymmetry compared to Lake Maracaibo and may exhibit higher sensitivity to surface and atmospheric initial conditions.  Results of ongoing research on such sensitivity and implications to forecasting of nocturnal convection over lakes will be discussed.