Cloudy sky shortwave radiative closure for a Baseline Surface Radiation Network site

Abstract

A shortwave radiative closure analysis for cloudy skies is presented for the Cabauw Baseline Surface Radiation Network (BSRN) site (51.97°N, 4.93°E). The cloudy cases are carefully selected to be overcast, single‐layer, homogeneous, nonprecipitating water clouds. We selected in total 639 cases on 9 days between May 2008 and May 2009 and on 30 January 2007. The Doubling‐Adding KNMI (DAK) code is used to simulate global irradiances. The cloud optical thickness is derived from the cloud liquid water path from microwave radiometer (MWR) measurements and the MODIS L2 cloud effective radius product. The scattering phase matrix of the cloud particles is calculated using a Mie code with the two‐parameter Gamma size distribution. The MWR integrated water vapor column and an aerosol climatology are also used in the simulations. The cloudy cases cover a large range of liquid water path (30–400 g/m2), water vapor column (0.7–3.1 cm), and solar zenith angle (41°–75°). The mean difference between simulated global irradiances and BSRN measurements is 6 W/m2 (5%), with a standard deviation of 14 W/m2 (13%). This difference is within the uncertainties of the model input parameters and measurement errors. The correlation coefficient between the measured and simulated global irradiances is 0.95. The good closure results demonstrate the high quality of the MODIS effective radius data and MWR liquid water path data and the accuracy of the DAK model for the selected water cloud cases. Furthermore, the effects of clouds, aerosols, water vapor, and surface albedo on the global irradiance have been analyzed carefully. The sensitivity study shows that in order to achieve the closure with an uncertainty of a few W/m2, more frequent effective radius data, simultaneous aerosol and cloud measurements, and surface albedo measurements are essential.

Evaluation of Model-Predicted Top-of-Atmosphere Radiation and Cloud Parameters over Africa with Observations from GERB and SEVIRI

“…1), especially since the deviations of the albedo observations from the DAK calculations can largely be explained by variations in observed r eff . Given the accuracy of the GERB observations ('0.01) and the RTM for cloudy atmospheres (see Wang et al 2011), we estimate the uncertainty of SEVIRI LWP for the stratocumulus region to be 20% or less. However, this uncertainty estimate cannot be transferred to other cloud regimes.…”

Sensitivity analyses of precipitable water vapor retrieval from the ground-based infrared measurements in clear sky conditions

“…The LOWTRAN aerosol data contain the aerosol profiles, single scattering albedos (SSA), asymmetry parameters (ASY), and extinction coefficients (EXT) at 47 wavelengths from 0.2 to 300 μm for several aerosol types (e.g., rural and urban type aerosols). 24,25 Rural aerosol is assumed to be composed of 70% water soluble material (ammonium, calcium sulfate, and organic compounds) and 30% dust-like aerosol. 24 The urban aerosol is taken to be a mixture of the rural aerosol (80%) with soot-like carbonaceous aerosol (20%).…”

Surface solar irradiance from SCIAMACHY measurements: algorithm and validation

“…where G clr is the clear-sky surface solar irradiance calculated using the MAGIC algorithm (Mueller et al, 2004(Mueller et al, , 2009 Wang et al (2011). The surface solar irradiance shows a weak dependency on surface albedo for clear-sky cases (Mueller et al, 2009), e.g.…”