Physical Mechanisms of Offshore Propagation of Convection over Sumatra and their Dependence on Large-Scale Forcing

Abstract:

The diurnal cycle dominates convection over the Maritime Continent (MC), and is the primary control on the vertical structure of the atmosphere and heating profiles in the region. However, the physical processes governing the diurnal cycle are poorly observed and understood. In particular, the scale interactions of offshore-propagating convection with both the land-sea breeze and large-scale phenomena, require further investigation.

Observations from the pre-Years of the Maritime Continent (YMC; 2015) and YMC (2017) field campaigns in Sumatra and 2.2 km convection-permitting reforecasts using the Met Office Unified Model are used to investigate these processes. A cluster algorithm is used to categorize the land-sea breeze on each day, with clusters representing strong onshore, weak onshore and moderate offshore wind. Each cluster is associated with a different regime of offshore propagation. It is shown that the wind regimes are correlated with MJO phase but are chiefly forced by equatorial waves propagating through the MC.

It is often assumed that gravity waves are responsible for nocturnal offshore propagation of convection from MC islands, but hard evidence for this remains elusive. The land-sea breeze circulation and cold pool outflow from diurnally forced land-based storms also play roles in the propagation mechanism. Gravity waves associated with nocturnal cooling over land appear to be a pre-requisite for offshore propagation of convection, as they precondition the stability of the atmosphere, but the propagation of the convection itself is much slower than the waves' speed. Possible dynamical and thermodynamical mechanisms of the propagation are explored.