Articles | Volume 16, issue 1
Atmos. Chem. Phys., 16, 161–175, 2016
Atmos. Chem. Phys., 16, 161–175, 2016

Research article 18 Jan 2016

Research article | 18 Jan 2016

The diurnal cycle of rainfall over New Guinea in convection-permitting WRF simulations

M. E. E. Hassim1,a, T. P. Lane1, and W. W. Grabowski2 M. E. E. Hassim et al.
  • 1School of Earth Sciences and ARC Centre of Excellence for Climate System Science, The University of Melbourne, Melbourne, Victoria, 3010, Australia
  • 2National Center for Atmospheric Research, Boulder, Colorado, USA
  • anow at: Centre for Climate Research Singapore, Meteorological Service Singapore, Singapore

Abstract. In this study, we examine the diurnal cycle of rainfall over New Guinea using a series of convection-permitting numerical simulations with the Weather Research and Forecasting (WRF) model. We focus our simulations on a period of suppressed regional-scale conditions (February 2010) during which local diurnal forcings are maximised. Additionally, we focus our study on the occurrence and dynamics of offshore-propagating convective systems that contribute to the observed early-morning rainfall maximum north-east of New Guinea.

In general, modelled diurnal precipitation shows good agreement with satellite-observed rainfall, albeit with some timing and intensity differences. The simulations also reproduce the occurrence and variability of overnight convection that propagate offshore as organised squall lines north-east of New Guinea. The occurrence of these offshore systems is largely controlled by background conditions. Days with offshore-propagating convection have more middle tropospheric moisture, larger convective available potential energy, and greater low-level moisture convergence. Convection has similar characteristics over the terrain on days with and without offshore propagation.

The offshore-propagating convection manifests via a multi-stage evolutionary process. First, scattered convection over land, which is remnant of the daytime maximum, moves towards the coast and becomes reorganised near the region of coastal convergence associated with the land breeze. The convection then moves offshore in the form of a squall line at  ∼ 5 ms−1. In addition, cool anomalies associated with gravity waves generated by precipitating land convection propagate offshore at a dry hydrostatic gravity wave speed (of  ∼ 15 ms−1) and act to destabilise the coastal/offshore environment prior to the arrival of the squall line. Although the gravity wave does not appear to initiate the convection or control its propagation, it should contribute to its longevity and maintenance. The results highlight the importance of terrain and coastal effects along with gravity waves in contributing to the diurnal cycle over the Maritime Continent, especially the offshore precipitation maxima adjacent to quasi-linear coastlines.

Short summary
Gravity waves from deep convection along with terrain and coastal effects control the development and movement of squall lines that affect the diurnal cycle of rainfall over New Guinea and its northern coast. Days with offshore propagating systems are governed by background conditions (more mid-tropospheric moisture, CAPE, and low-level convergence) as opposed to days without offshore propagation. Our results shed some light on the physics and dynamics of Maritime Continent organised convection
Final-revised paper