Abstract
ABSTRACTClouds play an important role in weather and climate. Therefore, it is important to quantify the dominant processes that influence cloud formation and dissolution. In this study, diagnostics of the relative humidity tendency in the ECHAM6 GCM are used to quantify the contribution of different atmospheric processes to the change in relative humidity and thus to quantify their impact on clouds. In the model, we find that the dominant processes are stratiform cloud microphysics, large-scale adiabatic horizontal advection and vertical motion, and cumulus convection. Tendencies calculated based on monthly mean fields approximate the monthly averages of instantaneous tendencies to within 50% in the mid-latitudes and 25% elsewhere. The correlation between the relative humidity tendencies and mid-tropospheric vertical velocity ω500 is analysed. The most important processes for cloud formation are tightly correlated with ω500; the monthly mean vertical velocity in most cases appears qualitatively useful to characterise the cloud-forming and cloud-dissipating processes.
Highlights
Clouds are of central interest in climate research because of their large impact on the energy and water cycles
Because the stratiform cloud parameterisation covers many subprocesses, it is divided into the following subdiagnostics, which are defined by the microphysical parameterisation in the
The positive correlation stems from the correlation between ω500 and cloud microphysical processes in storm-track depressions, which is averaged out on longer than synoptic timescales; the monthly mean ω500 is not well correlated with the monthly mean CLOUDdep diagnostics
Summary
Clouds are of central interest in climate research because of their large impact on the energy and water cycles. They are useful to understand how clouds and precipitation may respond to climate change (Miller, 1997; Bony et al, 2004, 2013), to evaluate GCMs and numerical weather prediction models (Medeiros and Stevens, 2011; Mülmenstädt et al, 2012; Nam and Quaas, 2013), and to assess and develop cloud parameterisations in GCMs (Davies et al, 2013; Govekar et al, 2014) In such studies, the lower tropospheric stability (LTS), defined as the difference in potential temperature between the 700 hPa and 1000 hPa pressure levels (Klein and Hartmann, 1993), or the estimated inversion strength.
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