A comparison of cloud properties at Barrow and SHEBA during the summer of 1998

During the SHEBA field campaign, a number of instruments were deployed at the SHEBA site that permitted the inference of cloud properties for the summer months of June through September, 1998. These instruments include the multi-filter rotating shadowband radiometer (MFRSR), the microwave radiometer (MWR), and a suite of instruments that measures shortwave broadband solar radiation. At the time of the SHEBA campaign a similar set of instruments was deployed (on a continuing basis) at the ARM site at Barrow, Alaska and from these instruments a similar set of cloud properties can be derived. Thus, a comparison of cloud properties between the two sites is possible. Cloud optical depths were derived from the MFRSR using the code of Min and Harrison (1996). Liquid water path (LWP) values from the microwave radiometer were also available, making it possible to calculate cloud droplet effective radius as well. Cloud fraction was calculated using the algorithm of Long et al. (1996).

Of particular importance for these calculations is the issue of data quality. Some data had to culled from the SHEBA data set because the twisting of the ship in the ice floe caused the MFRSR to become misaligned. Calibration of the MFRSR was checked using Langley regressions supplemented by a comparison between calculated broadband fluxes and measured broadband fluxes. The calculated broadband fluxes were obtained from the radiative transfer model SBDART (Ricchiazzi et al., 1998) using MFRSR data (aerosol optical depths) and MWR data (LWP) as input. MWR data had to be screened to eliminate data contaminated by the presence of water on the MWR window.

Once the data quality issues had been resolved, optical depths and effective radius were calculated at the two sites and compared. Some differences are evident that are significant for radiative transfer calculations in climate and numerical weather prediction models. It appears that SHEBA had a greater incidence of clouds and that their optical depths were greater; while the effective drop radii at SHEBA were smaller than at Barrow.

Min, Q. and L. Harrison, 1996: Cloud properties derived from surface MFRSR measurements and comparison with GOES results at the ARM SGP site. Geophys. Res. Lett., 23, 1641-1644.

Ricchiazzi, P., Shiren Yang, and Catherine Gautier, 1998: SBDART. Bulletin of the American Meteorological Society.

Long, C. N. and T. P. Ackerman, 2000: Identification of Clear Skies from Broadband Pyranometer Measurements and Calculation of Downwelling Shortwave Cloud Effects, JGR, 105, No. D12, 15609-15626.