Observed and Modeled Patterns of Co-Variabiiolty between Low-Level Cloudiness and the Structure of the Trade-Wind Layer
Patterns of covariability between low-level cloudiness and the trade-wind boundary layer structure are presented using measurements from the Barbados Cloud Observatory. We compare these with ECMWF's Integrated Forecast System and 10 CMIP5 models. Using time-step output at a single location, we find that models can produce a realistic trade-wind layer structure on average, but with unrealistic variability at shorter-time scales.
The unrealistic variability in modeled cloudiness near the lifting condensation level (LCL) is due to stronger than observed relationships with mixed-layer relative humidity (RH) and temperature stratification at the mixed-layer top. Those relationships are weak in observations, or even of opposite sign, which can be explained by a negative feedback of convection on cloudiness. Cloudiness near the trade-wind inversion varies more in observations on monthly time scales: cloudier conditions are related to stronger wind and stronger inversions. However, these parameters appear to be a prerequisite, rather than strong controlling factors on cloudiness, because they do not explain submonthly variations in cloudiness. Models underestimate the strength of these relationships and diverge in their responses to large-scale vertical motion. No model stands out by reproducing the observed behavior in all respects.
These findings suggest that climate models do not realistically represent the physical processes that underlie the coupling between trade-wind clouds and their environments in present-day climate, which is relevant for how we interpret modeled cloud feedbacks.