"README" file to accompany the Special Sensor Microwave Imager (SSM/I) Derived Vertically Integrated Water Data Sets Vertically integrated water vapor and cloud liquid water data products (designated as version 1) were derived over the oceans from measurements of the Special Sensor Microwave/Imager on the DMSP F-8 satellite. The products are limited to water surfaces since ocean surface emissivities are low at microwave frequencies which provide a sufficiently cold background in which to sense water vapor and liquid water. Over land and ice surfaces, however, the retrieval of these quantities is more difficult because of the large and highly variable surface emissivities. These products were generated using the physical method of Greenwald et al. (1993). Briefly, the method consists of an inversion of a simple physical model at two frequencies (19.35 and 37 GHz) to yield the integrated water vapor and cloud liquid water simultaneously. The data are made up of monthly mean quantities on 1 degree by 1 degree grids from July, 1987 to December, 1991. All products are in units of kg per meter squared (or, equivalently, millimeters). The uncertainties in the cloud liquid water observations are estimated to range typically from about 25% to 40% depending on the surface and atmospheric conditions and on the amount of liquid water (Greenwald et al., 1993). A limited comparison to ground-based microwave radiometer measurements over the North Sea and at San Nicolas Island off the coast of California revealed rms differences of 0.036-0.046 mm. The water vapor observations, when compared to a limited set of radiosonde measurements, have estimated uncertainties of 5.9 mm for water vapor above about 30 mm and 1.9 mm for water vapor below 30 mm (Greenwald et al., 1993). Two different cloud liquid water data sets are given. The first, which is designated as "lwp", consists of the average of all the liquid water retrievals. That is, even under situations where there is an absence of clouds within the instrument's field of view (FOV), the retrievals are still included in the monthly average. This data set is appropriate for comparing observations of liquid water to, for example, GCM simulations of cloud liquid water. The second set of liquid water data, labeled as "lwpcld", was added to the list of products as an attempt to include only those situations where water clouds were present within the instrument's FOV. Thus, these data sets provide the "true" cloud liquid water since FOV's mostly free of water clouds are excluded from the monthly average. A threshold of 0.025 mm in the liquid water retrievals was used to distinguish between clear and cloudy FOV's. A more thorough discussion of this topic is presented by Lin and Rossow (1994). There are two important issues with regard to the quality control of the data products. First, every effort was taken to eliminate retrievals contaminated by land and sea ice. Unfortunately, sea ice contamination, which is somewhat more difficult to identify than land contamination, remains in some of the data sets, particularly during the summer in the northern hemisphere near the sea ice boundary. Second, retrievals of water vapor and cloud liquid water are less reliable when there is substantial precipitation (both water and ice phases) within the FOV of the instrument. As a result, a liquid water threshold of 0.5 mm was used to identify FOV's greatly contaminated by precipitation (see Greenwald et al, 1993). The water vapor retrievals were excluded from the analysis when the retrieved liquid water was greater than 0.5 mm. However, the liquid water retrievals were arbitrarily set to a constant value of 0.5 mm to avoid the potentially large biases that can occur in the monthly mean cloud liquid water fields. Version 2 of these data products will become available in the near future. The new data sets are an improvement over the version 1 products in several respects. For instance, an improved retrieval model is used (Greenwald et al., 1995) to correct deficiencies in the Greenwald et al. (1993) model for very moist atmospheric conditions. Measurements from the 22.235 GHz channel (which is centered on a weak water vapor absorption line) are also incorporated into the retrieval method to provide improved estimates of integrated water vapor for values under about 50 mm. There are also other improvements in specifying certain physical variables in the model, which are discussed by Greenwald et al. (1995). References: Greenwald, T. J., G. L. Stephens, T. H. Vonder Haar, and D. L. Jackson, 1993: A physical retrieval of cloud liquid water over the global oceans using Special Sensor Microwave/Imager (SSM/I) observations, J. Geophysical Research, 98, 18471-18488. Lin, B. and W. B. Rossow, 1994: Observations of cloud liquid water path over oceans: Optical and microwave remote sensing methods, J. Geophysical Research, 99, 10907-10,927. The following is a list of published studies that have used the version 1 data products: Fowler, L. D., D. A. Randall, S. A. Rutledge, 1995: Liquid water and ice cloud microphysics in the CSU general circulation model. Part I: Model description and simulated microphysical processes, J. Climate, in press. Stephens, G. L., A. Slingo, M. J. Webb, P. J. Minnett, P. H. Daum, L. Kleinman, I. Wittmeyer, D. A. Randall, 1994: Observations of the Earth's radiation budget in relation to atmospheric hydrology. 4. Atmospheric column radiative cooling over the world's oceans, J. Geophysical Research, 99, 18585-18604. Stephens, G. L., D. L. Jackson, J. J. Bates, 1994: A comparison of SSM/I and TOVS column water vapor data over the global oceans, Meteorol. Atmos. Phys., 54, 183-201. Stephens, G. L., A. Slingo and M. Webb, 1993: On measuring the greenhouse effects of Earth. High Spectral Resolution Infrared Remote Sensing for Earth's Weather and Climate Studies, Eds. A. Chedin and M. T. Chahine. NATO ASI Series. Webb, M. J., A. Slingo, G. L. Stephens, 1993: Seasonal variations of the clear-sky greenhouse effect: the role of changes in atmospheric temperatures and humidities, Climate Dynamics, 9, 117-129. Wittmeyer, I. L., and T. H. Vonder Haar, 1994: Analysis of the global ISCCP TOVS water vapor climatology, J. Climate, 7, 325-333. Published studies that have used the upcoming version 2 data products: Greenwald, T. J., G. L. Stephens, S. A. Christopher, and T. H. Vonder Haar, 1995: Observations of the global characteristics and regional radiative effects of marine cloud liquid water, J. Climate, in press. Jackson, D. L., and G. L. Stephens, 1995: A study of SSM/I derived columnar water vapor over the global oceans, J. Climate, in press.