Mammatus Clouds as a Response to Cloud-Base Radiative Heating

Abstract

Mammatus clouds are the pouchlike lobes seen hanging from mid-to high-level clouds. They can look quite dramatic, but they are also interesting because they provide clues to what controls anvil cirrus dynamic evolution. Thus far, the most commonly accepted explanation for observed subsidence of mammatus lobes is that they are driven by evaporative cooling of precipitation, accelerated by mixing with dry subcloud air.Here, an alternative explanation is proposed: radiative temperature contrasts between cloud base and the lower troposphere destabilize cloudy air to create a rapidly deepening mixed layer, which creates positively buoyant intrusions of dry air into the cloud interior; mammatus lobes are just the descending branch of the resulting circulations. In this regard, mammatus cloud fields might be considered an upside-down analog to the radiatively driven formation of ''cloud holes'' seen at the tops of stratocumulus layers.

Satellite Observations of an Unusual Cloud Formation near the Tropopause

“…Mammatocumulus are simply the cloudy manifestation of the return circulation. Such radiatively driven entraining circulations have been reproduced numerically, both at cloud bottom and cloud top, and for similar atmospheric conditions to those described here (Garrett et al 2010;Schmidt and Garrett 2013). Garrett et al (2010) and Schmidt and Garrett (2013) showed how this problem can be viewed theoretically.…”

“…Such radiatively driven entraining circulations have been reproduced numerically, both at cloud bottom and cloud top, and for similar atmospheric conditions to those described here (Garrett et al 2010;Schmidt and Garrett 2013). Garrett et al (2010) and Schmidt and Garrett (2013) showed how this problem can be viewed theoretically. Calculation of a dimensionless ''spreading number'' helps to predict the dynamic response of a cloud with finite lateral dimensions when it is heated or cooled at its boundaries:…”

“…Any evaporative cooling of hydrometeors would propel the cloudy lobes in the wrong direction. Rather, we suggest that the mammatocumulus clouds that we observe might develop instead as a consequence of an instability that is driven by local radiative flux divergence (Garrett et al 2010;Schmidt and Garrett 2013), much as radiative cooling is seen as a driving force for turbulence at the tops of stratocumulus decks (Lilly 1968;Moeng et al 1995Moeng et al , 1996. In the case of the large cloud frontal system seen here, the instability is created by longwave radiative cooling to space from the cloud top.…”

“…Nonetheless, numerical simulations described in Garrett et al (2010) suggest that this value of S is more than sufficient to be associated with a rapidly deepening mixed layer and mammatus cloud formation. Equation (5) suggests that it should take only about 2 h to develop a 2-km mixed layer similar to that which is observed, at which point, from Eq.…”

“…(9.2.9), which addresses the response of a mixed layer to heating at a boundary. In Garrett et al (2010), it was shown in numerical simulations how mammatus lobes are the visible, cloudy portion of the mixed-layer circulations. They develop from an initially quiescent cloud only if values of S are greater than about 1000.…”