Modelling Convection In The Maritime Continent

By: Steve Woolnough

The Maritime Continent, the archipelago, including Malaysia, Indonesia, the Philippines and Papua New Guinea is made up of hundreds of islands of varying shapes and sizes. It lies in some of the warmest waters on Earth and consequently is a major centre for tropical atmospheric convection, with most of the region receiving more than 2000mm of rainfall a year and some parts over 3500mm of rainfall. The latent heat released by the condensation of water vapour into rain in the clouds drives the tropical circulation and is collocated with the ascending branch of the Walker Circulation. On interannual timescales the rainfall over the region is modulated by El-Nino, and on sub-seasonal (2-4 week) timescales it’s modulated by the Madden-Julian Oscillation (the MJO, see Simon Peatman’s blog from 2018). The variations in heating associated with El Nino, the MJO and other modes of variability drive changes in the global circulation including influences over the North Atlantic and Europe (see Robert Lee’s blog).

Figure 1: Animations of one day of precipitation over the Maritime continent from: GPM-IMERG observations (top panel), a 2km model with explicit convection (middle panel) and a 12km model with convective parametrization (bottom panel).

Given the importance of this region for the tropical and global circulation it’s critical that the models we use for weather and climate predictions are able to represent the processes that control the variation in precipitation in the region. Precipitation is organized on a range of spatial and temporal scales from meso-scale convective systems (with scales of a few hundred kilometres) to synoptic scale systems like the Borneo Vortex, Equatorial Waves and Tropical Cyclones, and is strongly tied to the diurnal cycle. The top panel of Figure 1 shows an animation of one day of precipitation as observed from the Global Precipitation Measurement Mission (Huffman et al., 2019). It’s clear that precipitation is organized into clusters with regions of very intense precipitation. The bottom panel shows the precipitation simulated by model with Met Office Unified Model at 12km horizontal resolution, with parametrized convection typical of global weather forecast models. Whilst the model is able to capture some semblance of organization the simulation is dominated by weak to moderate precipitation over a large proportion of the domain.

As reported by Emma Howard, the TerraMaris project aims to improve our understanding of the processes that organize convection in the region and in particular their interaction with the diurnal cycle. We had planned a field campaign in Indonesia to observe the convection over Java and Christmas Island, along with a series of high resolution simulations as described by Emma, but the COVID-19 pandemic has finally put paid to the field campaign so we’re now relying on the high resolution model simulations. We have run 10 winter seasons of the Met Office Unified Model model at 2km horizontal resolution with no convective parametrization where the convection is explicitly simulated. The middle panel of the animation shows 1 day from these simulations. There is a clear difference between the representation of convection in the 2km model compared to the 12km model with small regions of intense convection, more similar to the observed precipitation, although the 2km model perhaps tends to produce precipitation structures which are two small.

Figure 2: Timing of the diurnal maximum precipitation in the 2km model simulations (left panel) and the 12km model simulations (middle panel). Precipitation anomaly composites in Phase 5 of the MJO in the 2km model (top right) and the 12km model (bottom right).

These differences in the representation of convection also lead to differences in the way variability is represented in the model. The left two panels of figure 2 shows the time of the diurnal maximum in precipitation, which typically occurs in the early afternoon/evening in the 12km model compared to late evening/early morning in the 2km model, much closer to observations. Notice that the 2km model also has a clear diurnal cycle of precipitation in the oceans surrounding the islands associated with offshore propagation of convective systems during the morning, which the 12km model largely doesn’t capture. The right-hand panel shows an example of the modulation of the precipitation by the MJO over the region, it’s clear that the 2km model shows a much larger impact of the MJO on the precipitation over the islands. During the next few years we hope to use the simulations to understand how large-scale variability associated with the MJO and El Nino modulates these meso-scale  convective systems, and the impact that has on the vertical structure of the heating over the region and it’s potential influence on the global circulation.

Reference:

Huffman, G.J., E.F. Stocker, D.T. Bolvin, E.J. Nelkin, Jackson Tan (2019), GPM IMERG Final Precipitation L3 Half Hourly 0.1 degree x 0.1 degree V06, Greenbelt, MD, Goddard Earth Sciences Data and Information Services Center (GES DISC), https://doi.org/10.5067/GPM/IMERGDF/DAY/06

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