Episodes of Warm-Air Advection Causing Cloud-Surface Decoupling During the MARCUS

Abstract

It has been known for decades that advection of a cloud-topped marine boundary layer (CTBL) over warmer sea surface causes the stratification (or decoupling) of the CTBL via the entrainment feedback, a mechanism commonly known as “deepening-warming” decoupling that is typical in subtropics. This study focuses on the opposite direction of advection, that is, low-level warm air advection (LLWAA), and its impacts on the decoupling degree of a CTBL. Our hypothesis is that LLWAA stabilizes a CTBL, causing a decoupling of the CTBL. It is tested for three LLWAA episodes observed during the Measurements of Aerosols, Radiation, and CloUds over the Southern Ocean (MARCUS) field campaign between the Hobart (43°S, 147°E), Australia, and several Antarctic coast stations. By synthesizing the shipborne measurements of CTBL structure, Himawari-8 satellite imagery of cloud fields, and reanalysis of meteorological field, four common characteristics of CTBLs under the LLWAA are found: (1) CTBLs are highly stratified to the extent that penetrations of cumulus into main temperature inversions, which are common for subtropical decoupled CTBLs, do not exist; (2) sea surface temperature is 1–2 K lower than the near-surface air temperature; (3) clouds manifest stratiform with lifetime as long as several tens of hours; and (4) they locate in warm sectors of middle-latitude cyclones. Possible mechanisms for the maintenance of decoupled clouds under LLWAA are discussed in terms of dynamic and thermodynamic factors. Lapse rates of the decoupled CTBLs are markedly lower than those commonly used for passive satellite estimation of cloud top heights.