Abstract
A description of the daily cycle of oceanic shallow cumulus for undisturbed boreal winter conditions in the North Atlantic trades is presented. Modern investigation tools are used, including storm‐resolving and large‐eddy simulations, runover large domains in realistic configurations, and observations from in situ measurements and satellite‐based retrievals. Models and observations clearly show pronounced diurnal variations in cloudiness, both near cloud base and below the trade inversion. The daily cycle reflects the evolution of two cloud populations: (i) a population of nonprecipitating small cumuli with weak vertical extent, which grows during the day and maximizes around sunset, and (ii) a population o deeper precipitating clouds with a stratiform cloud layer below the trade inversion, which grows during the night and maximizes just before sunrise. Previous studies have reported that cloudiness near cloud base undergoes weak variations on time scales longer than a day. However, here we find that it can vary strongly at the diurnal time scale. This daily cycle could serve as a critical test of the models' representation of the physical processes controlling cloudiness near cloud base, which is thought to be key for the determination of the Earth's climate response to warming.
Highlights
There is increasing evidence that oceanic fair‐weather cumulus clouds, such as those prevailing in the trade wind regions, play an important role in the climate system
The daily cycle reflects the evolution of two cloud populations: (i) a population of nonprecipitating small cumuli with weak vertical extent, which grows during the day and maximizes around sunset, and (ii) a population of deeper precipitating clouds with a stratiform cloud layer below the trade inversion, which grows during the night and maximizes just before sunrise
Here we find that it can vary strongly at the diurnal time scale. This daily cycle could serve as a critical test of the models' representation of the physical processes controlling cloudiness near cloud base, which is thought to be key for the determination of the Earth's climate response to warming
Summary
There is increasing evidence that oceanic fair‐weather cumulus clouds, such as those prevailing in the trade wind regions, play an important role in the climate system. Observations from the (Dynamics of the Madden‐Julian Oscillation field experiment have revealed that during the suppressed phases of the MJO, shallow cumulus clouds prevail (with tops below 3 km) over the Indian Ocean of the warm pool region and exhibit a pronounced daily cycle with an afternoon increase in cloud depth (Ruppert & Johnson, 2016) This cumulus convection yields a day‐to‐day heating and moistening that precondition the atmosphere for the transition from a shallow to a deep convective regime (Ruppert, 2016; Ruppert & Johnson, 2016).
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