Modeling springtime shallow frontal clouds with cloud‐resolving and single‐column models

This modeling study compares the performance of eight single-column models (SCMs) and four cloud-resolving models (CRMs) in simulating shallow frontal cloud systems observed during a subperiod of the March 2000 ARM intensive operational period (IOP). Except for the passage of a cold front at the beginning of this subperiod, frontal cloud systems are driven by a persistent frontogenesis over the Southern Great Plains and moisture transport from northwestern part of the Gulf of Mexico. This study emphasizes quantitative comparisons among model simulations and with data, focusing on a 27 h period when only shallow frontal clouds were observed. All CRMs and SCMs qualitatively simulated clouds in the observed shallow cloud layer. Significantly different cloud amounts and cloud microphysical properties are, however, found in the model simulations. All CRMs do not produce any clouds above the observed shallow cloud layer, but most SCMs produce clouds in the middle and upper troposphere. Possible causes are discussed in this study. One is the strong decoupling between cloud condensate and cloud fraction in nearly all SCM parameterizations. Clouds are produced whenever the relative humidity (RH) reaches its threshold although the cloud water content is almost nonexistent. The other is the weak upper tropospheric subsidencemore » that has been averaged over both descending and ascending regions, i.e., the unresolved upper-level dynamical forcings. The intermodel differences are also analyzed and related to the detailed formulations of cloud microphysical processes and fractional cloud parameterizations in the SCMs and possibly to the dynamical framework in the CRMs. The underestimate of cloud liquid water content (LWC) in some SCMs is related to the Sundqvist-type formulation of the autoconversion process or the small threshold value for autoconversion, based upon the comparison between the observed and simulated LWCs. Although two of the CRMs with anelastic dynamics simulate the shallow frontal cloud much better than the SCMs, the CRMs do not necessarily perform much better than the SCMs for the entire subperiod when deep frontal clouds are presents. For the simulations of the entire subperiod, the CRM results show a vertical phase tilting in the cloud fractions while the SCM results show a vertical phase tilting in the LWCs. The observed cloud property profiles do not have any vertical phase tilting. The peak magnitudes of the simulated cloud properties are underestimated in most models and the life cycles of the simulated frontal cloud systems are much longer than those observed. These results suggest the importance of the horizontal advection of hydrometeors, which are currently not available, to drive model simulations and to adequately evaluate the performance of the models in the future.« less