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 m. 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 Ϫ2 and the precision is better than 30 g m Ϫ2 , which is similar to the precision of LWP retrievals from MWR. The added value of the 15-min sampling frequency of Meteosat-8 becomes evident in the validation of the daily median and diurnal variations in LWP retrievals from SEVIRI. The daily median LWP values from SEVIRI and MWR are highly correlated (correlation Ͼ 0.95) and have a precision better than 15 g m Ϫ2 . In addition, SEVIRI and MWR reveal similar diurnal variations in retrieved LWP values. The peak LWP values occur around noon. During winter, SEVIRI generally overestimates the instantaneous LWP values from MWR, the accuracy drops to about 10 g m 2 , and the precision to about 30 g m Ϫ2 . The most likely reason for these lower accuracies is the shortcoming of CPP, and similar one-dimensional retrieval algorithms, to model inhomogeneous clouds. It is suggested that neglecting cloud inhomogeneities leads to a significant overestimation of LWP retrievals from SEVIRI over northern Europe during winter.

Operational climate monitoring from space: the EUMETSAT Satellite Application Facility on Climate Monitoring (CM-SAF)

“…First attempts to generate long-term data series of atmospheric quantities derived from satellite measurements go back to the early eighties when the International Satellite Cloud Climatology Project (ISCCP) started its work (Rossow and Garder, 1993). The cloud information from the ISCCP data set was successfully used to derive a climatology of the shortwave radiation budget .…”

A new algorithm for the downscaling of cloud fields

“…Due to the structure of the algorithm, it is possible to combine this downscaling algorithm with the original algorithm we use for the stochastic generation of surrogate clouds (Venema et al, 2006a) and with upcoming similar algorithms. For example, surrogate clouds based on the LWC height distribution and spatial correlations from aircraft microphysics data, such as presented in Schmidt et al (2007), can be combined with a coarse liquid water path field from satellite retrievals (Roebeling et al, 2008). The in situ information on the LWC height distribution will likely reduce the inaccuracies due to the fact that the current algorithm only estimates the subscale variance, but not the full subscale LWC distribution.…”

Characteristics of cloud liquid water path from SEVIRI onboard the Meteosat Second Generation 2 satellite for several cloud types

“…The retrieved particle size values are unreliable for optically thin clouds and so for clouds with cloud optical thickness COT < 8, the climatological value 8 µm is used, which is similar to values used by Rossow and Schiffer (1999). Roebeling et al (2008) validated the retrieved LWP values with CloudNET data from two measurement sites: Chilbolton and Palaiseau. At the two sites, measurements were taken with microwave radiometers (MWR).…”