PNNL

Abstract

The ACE-2 data set along with ECMWF reanalysis meteorological fields provided the basis for the single column model (SCM) simulations, which were performed as part of the PACE (Parameterization of the Aerosol Indirect Climatic Effect) project. Six different SCMs were used to simulate ACE-2 case studies of clean and polluted cloudy boundary layers, with the objective being to identify limitations of the aerosol/cloud/radiation interaction schemes within the range of uncertainty in in situ, reanalysis and satellite retrieved data that were used to constrain model results. The exercise proceeds in three steps. First, SCMs are configured with the same fine vertical resolution as the ACE-2 in situ data base to evaluate the numerical schemes for the prediction of aerosol activation, radiative transfer and precipitation formation. Second, the same test is performed at the coarser vertical resolution of GCMs to evaluate its impact on the performance of the parameterizations. Finally, SCMs are run for a 24 to 48 hr period to examine predictions of boundary layer clouds when initialized with large-scale meteorological fields. Diagnostic and prognostic schemes have been tested for the prediction of droplet number concentration. Physically based activation schemes using vertical velocity still show noticeable discrepancies compared to the simpler empirical schemes due to biases in the diagnosed vertical velocity at cloud base. Prognostic schemes exhibit a larger variability than the diagnostic ones, due to a coupling between aerosol activation and drizzle scavenging in the calculation of droplet concentration. When SCMs are initialized at a fine vertical resolution with the locally observed vertical profiles of liquid water content, the predictions of optical thickness stay within the standard deviation of the observed values. The predictions however are degraded at coarser vertical resolution and are more sensitive to the mean liquid water path than to its spatial heterogeneity. The predicted precipitation fluxes are also severely underestimated and improve when accounting for sub-grid liquid water variability. Results from the 24 hr runs suggest that most models have problems in simulating boundary layer cloud morphology, mainly due to the fact that large scale initialization fields extracted from the ECMWF reanalysis do not accurately reproduce the meteorological conditions in the area of the ACE-2 experiment. As a result, liquid water path, optical thickness and broadband albedo are significantly overestimated by the models. Although problems exist when using the large-scale forcing fields, subgrid scale cloud effects do occur in nature and improved cloud morphologies were obtained by models that account for subgrid inversions and a subgrid cloud thickness scheme. This may be a result of representing subgrid scale effects though we do not rule out the possibility that better large-forcing data may also improve cloud morphology predictions.