What is CASS?

Purpose

The Continenal Active Surface-forced Shallow-cumulus (CASS) is a composite case for modeling studies such as large eddy simulation (LES) and single-column version of global climate models (SCM). CASS is built upon the long-term comprehensive diurnal-cycle observations by U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Climate Facility at Southern Great Plains (SGP) site. Particularly CASS represents a typical average behavior of 76 golden days of fair-weather shallow cumulus clouds selected in 13-yr summertime at SGP. The formation and dissipation of these shallow cumulus clouds are tightly coupled with the atmospheric boundary layer development which are strongly forced by land surface conditions and local atmospheric environmental factors.

CASS not only consists of forcing data that drives LES and SCM, but also provides a set of observed cloud statistics for model validation. CASS can be used in LES to investigate factors and mechanisms controlling the development of shallow cumulus cloud and aims to serve as a test bed for SCM studies on the development and improvement of traditional convection parameterizations.

(Left) GOES image of SGP region at 1315 Local Time on 2001/05/14, one of the shallow cumulus days selected in Zhang and Klein (2010, 2013); (Middle) Diurnal cycle of vertical cloud fraction (%) for our composite case based on the vertically pointing radar-lidar-ceilometer combined value added data product (ARSCL); (Right) Diurnal cycle of vertical cloud fraction (%) for our composite case from Large Eddy Simulation (LES).

Model Description and the Setup of Control Run

The System for Atmospheric Modeling (SAM, Khairoutdinov and Randall, 2003) is widely used in cloud studies. SAM has a non-hydrostatic anelastic dynamical core and is configured for large-eddy simulation with periodic boundary conditions. In this study, SAM is adopted with a 1.5-order closure based on a prognostic equation for the sub-grid scale turbulent kinetic energy. Advection of all scalar prognostic variables is done using a monotonic and positive-definite advection scheme in flux form. A Newtonian damping layer is implemented in the upper third of the domain to reduce gravity wave reflection and buildup. Longwave and shortwave radiation are calculated using the Rapid Radiative Transfer Model (RRTMG, Mlawer et al.,1997; Clough et al. 2005). Two cloud and precipitation microphysical packages are used: one is the default one-moment diagnostic bulk microphysics of precipitation and cloud water (Khairoutdinov and Randall, 2003); the other is a size-resolved spectrum bin microphysics (Khain et al., 2004; Fan et al., 2009). With the bin microphysics, cloud droplet concentration is prognostic, and droplet nucleation is calculated from the predicted supersaturation and specified aerosol size distribution according to the Kohler theory. Aerosol number concentration is set to 600/cm^3 representing a continental clean aerosol condition typical of the SGP.

Our simulations start at 0530 Local Time and end at 1730 Local Time and have a domain 29 km by 29 km in the horizontal and 16 km in vertical. Horizontal resolution is 50m and vertical resolution is 20 meters under 5 km with a stretched grid above. This resolution is similar to the cloud radar observed volume. Radar retrievals are provided for each vertical range gate of 45 m, and with a wind speed around 7 m/s, the horizontal distance sampled by the radar is about 70 m for each 10-second retrieval. SAM is run with a 1-second time step with RRTMG being called every minute. For the calculation of solar radiation, the day is set up to be July 24th (the 205th day in a year) at the central facility of the ARM SGP site, 36.5N, 97.5W. This represents the average solar insolation of the active shallow cumulus days from May to August in our case library. For radiation calculations above 16km, the vertical profiles of radiatively important trace gases, water vapor and temperature are assumed to be a mid-latitude summertime climatological mean value. The LES is forced with the large-scale horizontal advective tendencies for temperature and water vapor and a large-scale subsidence rate derived from long-term continuous forcing data. Temperature and humidity above 5 km are nudged towards the composite profile for the purpose of radiation calculations with a nudging time-scale of 1hour. Horizontal winds are nudged towards the composite winds derived from the continuous forcing with a nudging time-scale of 1hour. The surface roughness length was set to 0.035 m, a characteristic value for the ARM SGP site suggested by the ARM97 case (Brown et al., 2002). Turbulence was initiated by imposing random temperature perturbations at each grid point in the lowest 200m with a maximum amplitude at any model level decreasing linearly from 0.1K at the surface to zero at 200 m following the ARM97 case (Brown et al., 2002).