Understanding the diurnal rainfall processes over tropical islands to improve subseasonal to seasonal precipitation forecasts

The main objectives of the proposed project are to understand key processes of the diurnal cycle of rainfall over the islands of the Maritime Continent and to assess the relationship between subseasonal to seasonal (S2S) models’ representation of the diurnal cycle and their forecast skill of tropical precipitation. Convective activity over the Maritime Continent influences a wide range of global weather and climate phenomena. One of the key phenomena influencing convective activity of the region on S2S timescales is the Madden-Julian Oscillation (MJO). However, current global models exhibit significant biases when predicting the evolution of the MJO over the Maritime Continent, which are argued to stem from inaccurate representation of the diurnal cycle and its intraseasonal variability. The diurnal cycle explains a substantial fraction of precipitation variability over the Maritime Continent, and hence it is crucial to address model deficiencies related to the diurnal cycle. Therefore, it is essential to first understand how errors in the representation of the diurnal cycle impact subsequent S2S skill in operational models. To advance S2S predictions, we then need to identify the sources of error to accurately represent convective processes and simulate diurnal rainfall over the Maritime Continent.
The proposed study will address the objectives through three major tasks. The first task will evaluate the model representation of the diurnal cycle over the Maritime Continent using S2S operational forecast datasets. We will identify relationships between short-range diurnal cycle errors and subsequent MJO prediction. This task will also quantify how the erroneous diurnal cycle relates to large-scale environmental variables, which will help us understand how the erroneous diurnal cycle impacts S2S predictions. After identifying how and when S2S models struggle to predict the diurnal cycle in the first task, the second task will use observations and process-oriented experiments using a cloud-resolving model during the Years of Maritime Continent (YMC) to understand key processes of the diurnal cycle. The second task will focus on identifying key processes for the development and offshore propagation of diurnal rainfall over the islands that global models often struggle to represent. To then understand the two-way feedback of the diurnal cycle and the MJO, the third task will examine the sensitivity of the diurnal processes to large-scale conditions using observations and process-oriented experiments using a regional cloud-permitting model.