The Physics of Post-cold-frontal Clouds Over the ARM East North Atlantic Site

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Description

The current study uses observations and numerical modeling to investigate the behavior of post-cold-frontal (PCF) low-level marine clouds at the ARM East North Atlantic Site and their representation in general circulation models. The passage of a cold front initiates a rapid modification of the boundary layer (BL): ahead of the cold front the BL is stable and behind the front the BL is strongly unstable. This rapid change in conditions may impart specific behavior to the clouds in the PCF region, therefore, our work seeks to determine the relative importance of BL mixing and convection in generating PCF clouds. As a first step in addressing this question, our current study explores the sensitivity of the PCF clouds to different physical parameterizations and their interactions through the use of satellite data, observations from the ARM-ENA site and Weather Research Forecast (WRF) model perturbed physics ensemble. The WRF simulations were performed at a coarse resolution to ensure that the convection is not resolved as in general circulation models. Regardless of the physics module used in each simulation, the WRF model ensemble results indicate that the detection of the cold front passage is not sensitive to the horizontal resolution and the physics of the model. The modeled cloud properties obtained from combinations of three distinct microphysics schemes, three distinct boundary layer schemes and four distinct convection schemes are examined. The examination of clouds response to changes in physics revealed significant variations in cloud cover and liquid water path with different physics configurations, from 0.1 to 0.9 and from 0.01 kg.m-2 to 0.2 kg.m-2 respectively. This wide range is found to be mainly driven by the convection schemes followed by the boundary layer and finally the microphysics schemes. To further explore this disparity in cloud cover, we are investigating its relationship with different environmental variables, for example the vertical velocity at the cloud base, the heat tendency or the estimated inversion strength.