Cloud Liquid Water Path Estimations Using Solar Reflected and Microwave Data
Presently, space-borne passive sensors are capable of observing the liquid water path (LWP) of clouds over land and water from two distinct methods. The first method makes use of solar reflectance measurements (SR)at nonabsorbing visible (0.5-0.7 um) and absorbing near-infrared (3.5-4.0 um) frequencies to determine the cloud visible optical depth and the effective radius of the cloud droplets. From these quantities the cloud LWP can be inferred. The second approach, which is applied mainly over water surfaces and has had some success over land as well , is more direct and uses microwave frequencies ((18-85 GHz) to retrieve cloud LWP, independent of the drop size . Because validating these methods is often very difficult due to a lack of in situ or ground measurements, another way to test these independent methods is to compare them directly. Indeed, identifying the conditions under which these methods agree or disagree is an essential step in understanding the uncertainties and limitations in the retrievals. This knowledge is crucial for future instrument platforms, such as the Earth Observing System (EOS), which will include a suite of multi-spectral sensors to make it possible to apply both methods for the global monitoring of cloud properties for climate studies . The SR retrievals of cloud LWP are obtained from measurements of the new generation Geostationary Operational Environmental Satellite (GOES)-8, which offers a unique opportunity to examine sub-FOV effects since cloud amount and cloud properties such as optical depth and effective radius can be determined within the SSM/I FOV. One of the main strengths of this study is the use of the GOES-8 measurements at full resolution, whenever possible, when collocating the measurements from the different sensors.
Satellite observations of cloud liquid water path (LWP) are compared from Special Sensor Microwave Imager (SSM/I) measurements and GOES-8 solar reflectance(SR) measurements to ascertain the impact of sub-FOV cloud effects on SSM/I retrievals at 37 GHz. Cloud droplet effective radii derived from the GOES-8 3.9 (m channel are also used. The comparisons consist of collocated and full-resolution measurements and are limited to nonprecipitating marine stratocumulus in the eastern Pacific for two case studies in October 1995. The independent methods agree well for overcast SSM/I FOVs, with rms differences of 0.029-0.037 kgm-2.For broken cloudiness within the FOV, the beam-filling effect causes low biases of about 30%, on average, in the microwave retrievals. A first-order linear correction is applied for microwave retrievals above the estimated retrieval noise level, which roughly corresponds to cloud amounts above 25% within the FOV, yielding rms differences of 0.041-0.49 kgm-2 when compared to the SR retrievals. The beam-filling effects reported here are significant and are expected to impact directly upon studies that use instantaneous SSM/I measurements of cloud LWP, such as cloud classification techniques and validation studies involving ground-based and in situ data.
Greenwald, T. J., S. A. Christopher, and J. Chou, 1996:SSM/I and GOES-8 Comparisons of Cloud Liquid Water Path overWater: Assessment of Sub-Field of View Effects in Microwave Retrievals. J. Geophys. Res.-Atmospheres, 102, D16, 19585-19596.
Greenwald, T. J., and S. A. Christopher, The GOES-IM Imagers: New Tools for studying the microphysical properties of boundary layers clouds. Bulletin of Amer. Meteorol. Soc, 81, 2607-2620, 2000. (pdf version)
Greenwald T.J., and S.A. Christopher, Daytime variation of Marine Stratocumulus Properties as observed from Geostationary Satellite, Geophys.Res. Lett., 26, 1723-1726, 1999. (pdf version)