Use of Cloud and Radiation Testbed Measurements to Evaluate Cloud Cover and Convective Parameterizations

We have used temperature and humidity soundings and radiation measurements from the Atmospheric Radiation Measurement (ARM) Cloud and Radiation Testbed (CART) site in northern Oklahoma to evaluate an improved cloud cover algorithm. We have also used a new single-column model cumulus parameterization to estimate convective heating and moistening tendencies at the CART site. Our earlier analysis of cloud cover showed that relatively dry atmospheres contain small cloud amounts. We have found numerous periods during 1993 where maximum relative humidities within any layer of the atmosphere over the CART site are well below 60-80%, yet clouds are clearly reducing shortwave irradiance measured by a rotating shadowband radiometer. These ARM measurements support our earlier findings that most current climate models probably underestimate cloud coverage when relative humidities fall below the threshold humidities where clear skies are assumed. We have applied a “detraining-plume” model of cumulus convection to the June 1993 intensive observation period (16-25 June 1993). This model was previously verified with GARP(a) Atlantic Tropical Experiment (GATE) measurements. During the June intensive observing period (IOP), relative humidities over the CART site are typically 20% less than tropical Atlantic GATE relative humidities. Our convective model calculates that evaporation of convectively induced cloud and rainwater plays a much more important role in the heating and moistening convective tendencies at the drier CART location. In particular, we predict that considerable cooling and moistening in the lower troposphere should occur due to the evaporation of convectively initiated precipitation. Cloud Cover and Radiation at CART Clouds significantly perturb radiative processes in the atmosphere, yet they occupy areas considerably smaller than the grid size of most climate models. Therefore, most atmospheric columns within climate models are only partially covered by clouds. During climate simulations, fractional cloud coverage is usually diagnosed from modelcalculated relative humidity. Figure 1 shows a summary of several algorithms for calculating cloud cover. In a recent investigation of cloudiness, Walcek (1994) showed that small cloud amounts are present at nearly all relative humidities, especially in the middle troposphere. In contrast, most climate models employ “critical” relative humidities ranging from 60-80% below which totally clear skies are specified. The broader shaded area on Figure 1 shows a compilation of cloud cover and relative humidity measurements Walcek compiled at a particular atmospheric level during a midlatitude cyclone over the eastern United States. The most significant difference between climate model formulations and these measurements is the persistence of relatively small cloud amounts at humidities well below the “cutoff” humidities used by climate models. During most of 1993, routine measurements of relative humidity have been performed at the ARM CART site in northern Oklahoma. Additionally, a rotating shadowband radiometer has been recording solar irradiance, which is strongly modulated as clouds pass over the central facility. These radiometer measurements can be used as a crude indicator of partial cloudiness. We have found numerous periods when radiosondes launched from the central facility report humidities well below the “cutoff” humidities used by climate models throughout the depth of the atmosphere, yet downwelling shortwave irradiance is obviously perturbed by clouds. (a) GARP Global Atmospheric Research Program.