We know that enormous amounts of heat are released in tropical thunderstorm clouds and that this heating plays an important role in maintaining global circulation patterns. But tropical thunderstorms, and the associated heat release, are strongly influenced by myriad factors, including steep mountains, coastlines, time of day, and large, mostly unseen atmospheric waves that move slowly around the planet. Therefore, we developed a climatology of variations in heating associated with tropical thunderstorms over the particularly interesting tropical island of Sumatra. We used a fine-scale numerical model that could accurately represent physical cloud processes over the rugged mountains.

Our model covered a large enough part of the planet to incorporate planetary-scale variability, at a fine-enough scale to accurately represent cloud physics. We simulated conditions over 10 Austral Summer seasons (December-February). Heating associated with the changes between water vapour, liquid water and ice in a cloud is hard to observe. Therefore, the model gave us a unique window into variations in cloud processes at all scales.

We discovered that there is a large variation in the type of rain that falls in the tropics as planetary-scale waves move through the region. This creates differences in the type of cloud heating that occurs. These slow changes in the type of heating might even affect the way the planetary-scale waves themselves develop.

Our research demonstrates how cloud processes, steep mountains, tropical coastlines, the daily changes in solar insolation and planetary-scale waves work together to cause large variations in the tropical heating that drives global circulation patterns. Many of these effects are under-represented in global climate models. The next step is to compare our fine-scale simulations with global climate models, to find out exactly what is missing in a global climate model and how we might go about fixing it.

 

Paper: Vincent, C.L. and T.P. Lane2018Mesoscale Variation in Diabatic Heating around Sumatra, and Its Modulation with the Madden–Julian Oscillation. Mon. Wea. Rev., 146,2599–2614, https://doi.org/10.1175/MWR-D-17-0392.1