RADIATIVE CHARACTERISTICS OF THE ARCTIC ATMOSPHERE DURING SPRING AS INFERRED FROM GROUND-BASED MEASUREMENTS

The radiative characteristics of low-level clouds over the Arctic ice pack in spring are inferred from ground-based broadband flux measureme nts using a radiative transfer model. An informal comparison of severa l radiative transfer models is performed for clear-sky conditions obse rved in April during LEADEX. The broadband longwave and shortwave surf ace fluxes obtained with Streamer and the broadband longwave surface f lux obtained with the Rapid Radiative Transfer Model (RRTM) correlate well with the observations. The Streamer code is chosen to determine c loud optical and microphysical properties based on its performance und er clear-sky conditions, its sophisticated treatment of clouds, and it s option for calculating fluxes using a discrete ordinate technique. T he radiative properties of the Arctic atmosphere are complicated by th e presence of clouds. Low clouds occurred 30% of the time during LEADE X. The longwave and shortwave optical depths of clouds based below 2 k m are inferred from surface-based measurements of cloud base height an d broadband radiative fluxes by matching the observed fluxes with thos e obtained with the four-stream radiative transfer code available as p art of Streamer. The retrieved broadband shortwave and longwave optica l depths for low clouds observed during LEADEX ranged between 0.2 and 3. An attempt is made to infer cloud phase and cloud particle size fro m the ratio of the retrieved broadband shortwave and longwave cloud op tical depths. This ratio ranges from 1.33 to 1.75 for ice, from 0.85 t o 1.9 for liquid, and from 0.85 to 1.9 for mixed-phase clouds. Althoug h there is much overlap between the ranges, it is shown that liquid-wa ter clouds are characterized by optical depth ratios that are generall y smaller than those found in ice clouds. This method for inferring cl oud microphysics from surface-based measurements of broadband radiativ e fluxes and cloud base height may prove useful where more sophisticat ed observations of cloud microphysics are lacking.