Shortwave radiative closure for cloudy cases at the Cabauw BSRN site, the Netherlands

The shortwave radiative closure for the cloudy cases has been studies for Cabauw Basline Surface Radiaiton Network (BSRN) site (51.97°N, 4.93°E). The cloudy cases are carefully selected for the overcast, single layer water clouds. Further limits for the cases are no precipitation and no direct irradiance. According to this criteria we have total 630 cases in 9 days between May 2008 and May 2009. Same as the clear-sky closure, the Doubling-Adding KNMI (DAK) code is used to simulate the global irradiance. The cloud optical thickness, derived from cloud liquid water path from microwave radiometer (MWR) measurement and MODIS L2 cloud effective radius, MWR integrated water vapor column and the aerosol climatology are used as the input for the DAK model. The scattering phase matrix is calculated using Mie code with the Gamma distribution for the cloud particles. The cloud cases cover a large range of liquid water path, water vapor column and solar zenith angle. The closure was achieved for all the cases: the mean differences between model and BSRN measurement are 6 W/m2 (5%) for the global irradiance and the standard deviation is 15 W/m2. The robust of the closure was examined by analysis the effect of clouds, aerosol, water vapor on the global irradiance respectively. The sensitivity studies confirmed that the closure is well within the uncertainties of the clouds, water vapor and aerosol properties.