Observed microphysical and radiative structure of mid-level, mixed-phase clouds

Abstract

Airborne measurements of six mid-level clouds observed over the Great Plains of the United States in late 1999 and early 2000 are analyzed extensively. All cloud fields are associated with a 500-mb low-pressure center or a potential vorticity maximum, with additional lift provided by upper-level jet streams. Data show that these innocuous looking clouds display complicated microphysical and thermodynamic structures. Five of six cases include mixed-phase conditions in temperatures ranging from near freezing to -31° C, at altitudes of 2400 to 7200 m. Four of the cases consist of a single cloud layer, while the other two are multi-layered systems. Of particular note, in single-layered clouds, there is an increase of liquid water content with height versus a decrease in ice water content over the same depth. This is in contrast to multi-layered systems, where the liquid water content has the same basic shape, but the ice water content is distributed more uniformly throughout all layers. We attribute these structural differences to a seeder-feeder mechanism operating in the multi-layered systems. A lack of temperature inversions in these mid-level clouds is a major difference from the thermodynamic structure of most stratocumulus systems. We found the virtual potential temperature to be the best discriminator of cloud interfaces for mid-level clouds, with 1-2° C differences between ambient and cloud air. A noteworthy contribution to this observational study was the use of the Cloud Particle Imager (CPI) instrument for the qualitative analysis of the particle sizes, shapes, habits, and distributions through the cloud. An analysis of the liquid water budget of a Lagrangian cloud sample revealed that large-scale subsidence was the main mechanism responsible for its dissipation. Heating rates and fluxes are computed for each cloud using a single-column radiative transfer model. Sensitivity studies included the radiative effects of doubling and halving liquid and ice water content, which changed the radiative cooling and heating rates by 25 to 30%. Incorrect parameterizations of cloud water phase resulted in vertical net radiative heating rate errors of 400%. Microphysical data collected from these mid-level, mixed-phase clouds provide the observational base needed to increase our understanding of how mid-level clouds are generated, maintained, and dissipated, thus allowing for the development of better parameterizations in large-scale numerical models and improved methods for retrieving cloud properties with remote sensing instruments.