Large-Eddy Observation of Post-Cold-Frontal Continental Stratocumulus

Abstract

More studies on the dynamics of marine stratus and stratocumulus clouds have been performed than comparable studies on continental stratocumulus. Therefore, to increase the number of observations of continental stratocumulus and to compare marine and continental stratocumulus to each other, the approach of large-eddy observation (LEO) was applied to a case of nocturnal continental stratocumulus observed over the Atmospheric Radiation Measurement Program (ARM) Climate Research Facility (ACRF) in the central United States on 8 April 2006. The stratocumulus occurred in cold-air and dry-air advection behind a surface cold front. LEOs were obtained from millimeter-wavelength cloud radar and micropulse lidar, whereas traditional meteorological observations described the synoptic environment. This study focuses on a 9-h period of a predominantly nonprecipitating stratocumulus layer 250-400 m thick. A slight thinning of the cloud layer over time is consistent with dry-air advection. A deep layer of descent overlaid a shallower layer of ascent from the surface up to 800 mb, providing a mechanism for strengthening the inversion at cloud top. Time series of Doppler velocity indicate vertically coherent structures identifiable throughout much of the cloud layer. The magnitude of turbulence, as indicated by the variance of the vertical velocity, was weak relative to typical marine stratocumulus and to the one other case of continental stratocumulus in the literature. Conditional sampling of the eddy structures indicate that strong downdrafts were more prevalent than strong updrafts, and negative skewness of vertical velocity in the cloud implies an in-cloud circulation driven by longwave cooling at cloud top, similar to that in marine stratocumulus.

Sensitivity of a continental night‐time stratocumulus‐topped boundary layer to varying environmental conditions

“…In our simulations, the surface latent heat flux is fixed, but the surface layer was also decoupled from the sub‐cloud layer so that changes in the surface fluxes had limited influence on the stratocumulus cloud properties (simulations LH0 and LH30). These results were similar to those found in Mechem et al (). However, the surface latent heat flux was an important parameter for the lower fog layer.…”

“…A surface-based inversion up to 250 m was initially present, below a capping inversion between 950 and 1200 m. The ice/liquid potential temperature jump across the upper inversion was about 5 K. The layer between the two inversions was well mixed. Note that the initial total moisture profile does not display a clear 'moisture jump' across the cloudtop inversion, which is consistent with previous studies of continental stratocumulus (Mechem et al, 2010;Fang et al, 2014;Ghate et al, 2014) but different from the environment of typical marine stratocumulus clouds (Wood, 2012). Marine stratocumulus clouds are often found in strong subsidence regions associated with the descending branch of the Hadley cell circulation.…”

Coupled Aerosol-Cloud Systems over Northern Vietnam during 7-SEAS/BASELInE: A Radar and Modeling Perspective

“…Previous investigations of aerosol impacts on low-level Sc have focused overwhelmingly on marine cloud systems, which are generally controlled by very different dynamical processes compared to systems over land (Mechem et al, 2010;Wood, 2012). The preliminary results from aerosol sensitivity simulations presented herein revealed the familiar response of increased numbers and smaller sizes of cloud droplets with increasing aerosol concentrations, whereas increases in LWP and cloud optical depth were mostly opposite to findings from previous studies on marine Sc.…”

Interactions between biomass-burning aerosols and clouds over Southeast Asia: Current status, challenges, and perspectives

“…For example, Mechem et al (2010a) observed that cloud layer circulations in post-frontal Sc over land are driven by cloud top radiative cooling similar to marine Sc, yet the former exhibited weaker turbulence structures than in their marine counterparts. Additionally, simulations of post-frontal Sc over land using a LES model revealed advection of temperature and moisture were governing factors over system maintenance and decay as opposed to entrainment or surface fluxes as in marine Sc (Mechem et al, 2010b).…”