Cold pools, Breezes, and Monsoons: Propagating Convection over New Guinea

TL;DR

This study tackles the offshore propagation of diurnal convection near New Guinea, identifying two distinct modes—ridge-to-coast and over-ocean—separated by a ~100 km gap. It combines 21 years of IMERG rainfall data with convection-permitting WRF simulations (2 km grid) and targeted SST-perturbation experiments to diagnose the role of multi-scale density currents, including sea- and land-breeze fronts and convective cold pools, in sustaining offshore convection and moist patches over warm ocean waters. The key findings show that boundary-layer density currents, modulated by nighttime radiative cooling and daytime sea-breeze dynamics, interact with cross-equatorial monsoon flows to enable long-range offshore propagation up to 200–600 km (and beyond 600 km under favorable conditions), and that modest SST increases amplify nocturnal convection and the second mode. These results advance understanding of diurnal tropical convection on the Maritime Continent and offer insights for improving rainfall forecasts and climate model performance in this region.

Abstract

The diurnal cycle of precipitation near New Guinea involves intricate land-ocean-atmosphere interactions, posing substantial challenges for tropical weather and climate simulations. Using over two decades of GPM satellite observations and convection-permitting WRF simulations, this study examines the physical mechanisms governing the pronounced offshore propagation of diurnal convection over New Guinea. We identify two distinct convective propagation modes: (1) a "ridge-to-coast" mode originated over elevated terrain and migrating toward the coastline, and (2) an "over-ocean" mode initiated near the coast, separated by a spatial gap of approximately 100 km. Our findings highlight the critical role of multi-scale density currents in shaping boundary layer dynamics over warm ocean waters. Specifically, the afternoon sea-breeze front advects cooler air onshore, stabilizing the lower atmosphere and interrupting the continuous propagation of the first mode. At night, the hybrid land breeze, enriched by cold pools, generates offshore moist patches that facilitate the convective regeneration and propagation of the second mode. These offshore convective systems interact with monsoonal background winds, sustaining precipitation well beyond 200-600 km from the coast. Sensitivity experiments indicate that even a modest increase in sea surface temperature can enhance convective intensity and extend offshore propagation. These results shed light on the mechanisms that enable diurnal offshore convection to persist overnight and propagate far from the coastline, highlighting the importance of moist-boundary-layer density currents and offering insights for improving precipitation forecasts and global model performance over the Maritime Continent.

Figures (14)

• Figure 1: Maps showing (a) topography height (m) and (b) climatological precipitation rate (mm hr$^{-1}$) in February using hourly GPM data within the WRF domain. Two coordinate systems are defined for Hovmöller diagrams. The x-axis is aligned with the mountain ridge (X'-Y') (a), and the northeast coastline of New Guinea (X-Y) (b). The y-axis extends offshore in the northeastern direction. The x-axis in the coastline-aligned coordinate system (b) is divided into negative and positive channels to account for the geographically varying influence of cross-equatorial monsoon flows.
• Figure 2: February (top) and August (bottom): (Left) Hovmöller diagrams showing the averaged diurnal cycle of precipitation rate (mm hr$^{-1}$), based on 21 years of GPM data and plotted in a topography-aligned coordinate system (X'-Y') (Figure \ref{['fig:2coors']}a). The red dashed line marks the ridge location. Deep orange arrows denote dominant daily offshore-propagating convection originating near the ridge (first mode), while pink dashed arrows indicate weaker onshore-propagating convection at the sea breeze front. Light orange arrows highlight further offshore extension following the decay of the main convective signal. (Right) Climatological SST (shading in K) and 950 hPa wind streamlines (colored by wind speed in m s$^{-1}$), derived from the 21-year averaged ERA5 dataset.
• Figure 3: Hovmöller diagrams of the averaged diurnal precipitation rate (mm hr$^{-1}$) for February, based on 21 years of GPM data. Panels show results for all x-channels (a, d), negative x-channels (b, e), and positive x-channels (c, f) using the coastline-aligned coordinate system (X-Y). The red dashed line represents the northeast coastline. Orange arrows indicate convective offshore propagation, where the circular arrowheads denote points of convective generation. Pink arrows represent convective onshore propagation. Solid arrows illustrate the movement of deep convection near the coastline (precipitation rates generally exceeding 0.8 mm hr$^{-1}$), while dashed arrows depict the movement of weaker, noisier convection farther offshore with lower precipitation rates. The purple dashed line marks the longest-lasting boundary of deep convection extending from the ridge to the coastline, with its duration (hours) labeled in purple text. Below each diagram, black text indicates the calculated speed of stable near-coast convection, and gray text indicates the speed of unstable offshore convection. Speeds in the top row correspond to offshore propagation, while those in the bottom row represent onshore propagation.
• Figure 4: Hovmöller diagrams of precipitation rate (mm hr$^{-1}$) from the case study in GPM (top), WRF control run (middle), and WRF SST (+0.5 K) experiment (bottom). Panels show results for negative (left) and positive (right) X channels, using the coastline-aligned coordinate system (X-Y).
• Figure 5: Hovmöller diagrams of vertical velocity at an altitude of 9.7 km (free atmosphere) from the case study in the control run (top) and the SST experiment (bottom). Panels show results for negative (left) and positive (right) X channels using the coastline-aligned coordinate system. Solid purple arrows represent gravity wave signals triggered by diurnal convection over the island, while dashed purple arrows represent gravity wave signals generated by the “over-ocean” second mode of convection. Dashed yellow arrows indicate the offshore propagation (offshore propagation) of deep convection, and dashed green arrows depict the "jump" between the first and second modes of convective offshore propagation. The estimated propagation speeds (m s$^{-1}$) of these signals are labeled next to the respective arrows.