The Interaction of Trade-Wind Clouds with the Large-Scale Flow in Observations and Models

Abstract:

Most of the (sub)tropical oceans within the Hadley circulation experience either moderate subsidence or weak ascent. In these regions shallow trade-wind clouds prevail, whose vertical and spatial distribution have emerged as key factors determining the sensitivity of our climate in global climate models. A large unknown is how large the effect of these clouds should be. For instance, how sensitive is the radiation budget to variations in the distribution of trade-wind cloudiness in nature? How variable is trade-wind cloudiness in the first place? And do we understand the role of the large-scale flow in that variability?

In this talk we present how space-borne remote sensing and reanalysis products combined with ground-based remote sensing and high resolution modeling at a representative location start to answer these questions and help validate climate models.

We show that across regimes or seasons with moderate subsidence and weak ascent the cloud radiative effect and low-level cloudiness vary remarkably little. A negative feedback mechanism of convection on cloudiness near the lifting condensation level is used to explain this insensitivity. The main difference across regimes is a moderate change in cloudiness in the upper cloud layer, whereby the presence of a trade-wind inversion and strong winds appear a prerequisite for larger cloudiness. However, most variance in cloudiness at that level takes place on shorter time scales, with an important role for the deepening of individual clouds and local patterns in vertical motion induced by convection itself, which can significantly alter the trade-wind layer structure.

Trade-wind cloudiness in climate models in turn is overly sensitive to changes in the large-scale flow, because relationships that separate cloudiness across regimes in long-term climatologies, which have inspired parameterizations, also act on shorter time scales. We discuss how these findings relate to recent explanations for the spread in modeled climate sensitivity. Furthermore, we hypothesize what the relatively insensitivity of trade-wind cloudiness in nature implies about its role in the Hadley circulation.