REHIPRE

SIMULATING REALISTIC HIGH-RESOLUTION PRECIPITATION IN THE TROPICS

Changes in rainfall characteristics over the Maritime Continent affect a rapidly increasing population of 275 million people through effects on agriculture, forest fires, floods and vector-borne diseases. However, current atmospheric models fail to represent most features of precipitation in the region, such as its distinctive strong diurnal cycle. Therefore, our knowledge of the physical processes initiating and propagating convective systems that generate precipitation in the Maritime Continent is very limited. In addition, the Maritime Continent is at a centre of interactions across scales, hence global model errors in this region propagate through the entire earth system and affect model estimates in remote regions too. This project is aimed at exploring our ability to simulate realistic precipitation by incorporating explicit convection and fine-scale ocean-atmosphere interactions in regional models. We will quantify the interplay between sea breeze circulation, ocean-atmosphere processes and convective systems to produce precipitation in the Maritime Continent and how it could change in the future. As such, this project will also advance our understanding of sea breeze and convective processes that are of major importance to a breadth of locations globally.

This project funded by the European Union's Horizon 2020 research and innovation programme is aimed at improving our understanding of the physical mechanisms driving tropical convection and how we incorporate them in mesoscale models. Since tropical precipitation is predominantly generated by deep convection,  this project will lead to a more realistic simulation of rainfall in the tropics and will help us determine present and future characteristics of precipitation in the Maritime Continent. 

A realistic simulation of precipitation processes in the Maritime Continent, and more broadly in the tropics, is vital to advance our knowledge of the climate system and increase population resilience to posisble future changes.

The Maritime Continent is the largest archipelago in the world, it is surrounded by one of the warmest oceans in the Earth (Indo-Pacific warm pool) and it is a major convective centre. It is a region that has sparked great interest because most atmospheric models fail to represent many of its precipitation features, such as the diurnal cycle. In addition, it is home of multiple interactions across scales, hence global model errors in this area propagate through the entire Earth system and may affect model estimates in remote regions.

Two key examples of the interactions across scales that occur in the Maritime Continent are the Madden-Julian Oscillation (MJO) and the Walker Circulation.

Precipitation in the Maritime Continent has a marked diurnal cycle that is tightly connected to temperature land-sea contrasts and sea breeze circulation. Sea breeze combined with very complex topograhy and plenty of moisture available are key ingredients in the triggering and development of deep convection, which is the primary rainfall-generating process in the region.

Thus, precipitation errors in the tropics, and specifically in the Maritime Continent are often attributed to the misrepresentaiton of the rainfall diurnal cycle and how we represent convection in models. 

Climate models have incresed their resolution in the last decade with the aim to explicitly resolve physical processes, such as convection in regional models, that otherwise need to be parameterized. By parameterizing, we mean introducing the process into the equations through semi-empirical approximations so that their role is taken into account even if the grid space is too coarse to resolve them. In the case of convection, the idea is to better represent the complex response of vertical motions and precipitation to the environmetal conditions. But we are reaching model resolutions where this is no longer required and recent research has shown that there is huge potential for improved precipitation simulation when convection is explicitly resolved (i.e. convection-permitting models). 

Also, if land-sea thermal contrasts are determinant in the generation of convection through local ciruclation patterns such as the sea breeze, it is also necessary to accurately represent mesoscale interactions between the air and the ocean.

Therefore, including both explicit convection and ocean-atmosphere interactions in models is an exciting step forward with impacts that this project will help quantify. 

The main objectives of REHIPRE are:

1. Evaluation of the model resolution effect on sea-breeze, convective processes and precipitation. Quantifying the improvement of simulated precipitation characteristics obtained from increased detail and explicit convection in an atmospheric model.
2. Identification of key mechanisms required to correctly represent sea-breeze and convection. Determining how convection-permitting models and ocean-atmosphere-land coupled systems represent these mechanisms. Quantifying the importance of these features on the improvement of convective precipitation estimates.
3. Establishing implications of future climate change for sea-breeze and convective processes. Understanding the potential contribution of thermodynamical changes from greenhouse gas emissions to mesoscale circulation and tropical convection.