Robert A. Roebeling

Validation of Cloud Liquid Water Path Retrievals from SEVIRI Using One Year of CloudNET Observations

Abstract The accuracy and precision are determined of cloud liquid water path (LWP) retrievals from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) on board Meteosat-8 using 1 yr of LWP retrievals from microwave radiometer (MWR) measurements of two CloudNET stations in northern Europe. The MWR retrievals of LWP have a precision that is superior to current satellite remote sensing techniques, which justifies their use as validation data. The Cloud Physical Properties (CPP) algorithm of the Satellite Application Facility on Climate Monitoring (CM-SAF) is used to retrieve LWP from SEVIRI reflectances at 0.6 and 1.6 μm. The results show large differences in the accuracy and precision of LWP retrievals from SEVIRI between summer and winter. During summer, the instantaneous LWP retrievals from SEVIRI agree well with those from the MWRs. The accuracy is better than 5 g m−2 and the precision is better than 30 g m−2, which is similar to the precision of LWP retrievals from MWR. The added value of the 15...

Evaluation of Cloud-Phase Retrieval Methods for SEVIRI on Meteosat-8 Using Ground-Based Lidar and Cloud Radar Data

Three cloud-phase determination algorithms from passive satellite imagers are explored to assess their suitability for climate monitoring purposes in midlatitude coastal climate zones. The algorithms are the Moderate Resolution Imaging Spectroradiometer (MODIS)-like thermal infrared cloud-phase method, the Satellite Application Facility on Climate Monitoring (CM-SAF) method, and an International Satellite Cloud Climatology Project (ISCCP)-like method. Using one year (May 2004–April 2005) of data from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) on the first Meteosat Second Generation satellite (Meteosat-8), retrievals of the methods are compared with collocated and synchronized ground-based cloud-phase retrievals obtained from cloud radar and lidar observations at Cabauw, Netherlands. Three aspects of the satellite retrievals are evaluated: 1) instantaneous cloud-phase retrievals, 2) monthlyaveraged water and ice cloud occurrence frequency, and 3) diurnal cycle of cloud phase for May–August 2004. For the instantaneous cases, all methods have a very small bias for thick water and ice cloud retrievals (5%). The ISCCP-like method has a larger bias for pure water clouds (10%), which is likely due to the 260-K threshold leading to misdetection of water clouds existing at lower temperatures. For the monthlyaveraged water and ice cloud occurrence, the CM-SAF method is best capable of reproducing the annual cycle, mainly for the water cloud occurrence frequency, for which an almost constant positive bias of 8% was found. The ISCCP- and MODIS-like methods have more problems in detecting the annual cycle, especially during the winter months. The difference in annual cycle detection among the three methods is most probably related to the use of visible/near-infrared reflectances that enable a more direct observation of cloud phase. The diurnal cycle in cloud phase is reproduced well by all methods. The MODIS-like method reproduces the diurnal cycle best, with correlations of 0.89 and 0.86 for water and ice cloud occurrence frequency, respectively.

Toward a Standard Procedure for Validation of Satellite-Derived Cloud Liquid Water Path: A Study with SEVIRI Data

Abstract Differences between satellite-derived and ground-based values of cloud liquid water path (LWPsat and LWPgr, respectively) in validation studies are partly associated with the validation itself, in particular with scale differences and parallax. This paper aims at establishing standards for validation procedures to minimize these contributions to the differences. To investigate this, LWP values were collected as computed from ground-based microwave radiometer (MWR) summer measurements made at two Cloudnet sites and from the spaceborne Spinning Enhanced Visible and Infrared Imager (SEVIRI) instrument. The large number of all-sky sample pairs (∼2500 after selection) formed an essential condition for the present study. The best validation method was determined by optimum statistical agreement between LWPsat and LWPgr. The method consists of (i) computation of LWPsat by averaging LWP over the pixels surrounding the ground station by means of a Gaussian weight function with a length scale defining the ...

On the Sensitivity of Satellite‐Derived Cloud Properties To Sensor Resolution and Broken Clouds

The sensitivity of cloud properties derived from meteorological imagers to sensor resolution is investigated by a comparison of results obtained from 1×1 km2 versus down‐sampled 3×3 km2 resolution MODIS reflectances. Focus is put on deviations caused by the plane‐parallel albedo bias for completely overcast, single‐phase pixels (67% of cloudy pixels), and by broken clouds (20% of cloudy pixels). For pixels corresponding to overcast water clouds, a low bias of −5.6% in cloud optical thickness and −3.8% in liqud water path is observed. A method to significantly reduce these biases is described, if information on unresolved variability in reflectance is available. For broken clouds, retrieval results are found to be rather unreliable. Again, a significant improvement in accuracy is achieved, if information on the sub‐pixel cloud fraction is used in the retrieval.