Nephanalysis

Abstract: The U.S. Air Force Three-Dimensional Nephanalysis (3DNEPH) has been employed to analyze the vertical distribution of clouds and cloud overlap statistics during January 1979 over the north Atlantic Ocean (40° to 60°N). The 3DNEPH integrates both satellite and conventional observations and gives what is probably the best available information on the vertical distribution of clouds. This region was chosen because of the predominance of layered clouds and the high density of conventional observations. The grid size of the 3DNEPH is ∼45 km, and the data set was additionally averaged over three different grid sizes: 90, 221, and 442 km. Three cloud overlap assumptions that have been commonly used were tested, namely the maximum, minimum, and random overlap assumptions. For adjacent layers of the 3DNEPH that contained cloud the maximum overlap assumption performed the best, accurately determining the total cloud cover to within the roundoff error for the layered cloud amounts in 82% of the cases. For two or three cloud layers that were separated by clear interstices the random overlap assumption performed the best overall for all resolutions, although there were systematic biases in the predicted cloud fraction which depend on the cloud fraction of the layers and on grid size. For grid sizes ≥9O km, random overlap resulted in a systematic underestimation of total cloud cover, ∼5%. Random overlap performed worst when there were two layers with intermediate cloud fractions (30–70%). The results of this study indicate that while random overlap performs reasonably well on average, the systematic bias (which depends on grid resolution) and random discrepancies could result in significant errors when this approximation is used in general circulation modeling and cloud climatologies.

Abstract: Longwave emissivities and the vertical profile of cooling rates of tropical cirrus clouds are determined using broadband hemispheric irradiance data. Additionally, a broadband mass absorption coefficient is defined and used to relate emissivity to water content. The data used were collected by the National Center for Atmospheric Research (NCAR) Sabreliner during the GARP Atlantic Tropical Experiment (GATE) in the summer of 1974. Three case studies are analyzed showing that these tropical cirrus clouds approached an emissivity of 1.0 within a vertical distance of 1.0 km. Broadband mass absorption coefficients ranging from 0.076 to 0.096 sq m per g are derived. A comparison of these results with other work suggests that tropical cirrus cloud emissivities may be significantly larger than heretofore believed. Ice water content of the clouds were deduced from data collected by a one-dimensional particle spectrometer. Analyses of the ice water content and the observed particle size distributions are presented.

Abstract: In this paper we present an analysis of the integrated liquid water content and precipitation characteristics of stratiform clouds using data from the Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) for January 1979, over the North Atlantic Ocean (40°–60°N). Concurrent analysis of the SMMR data with the U.S. Air Force 3-Dimensional Nephanalysis (3DNEPH) allows the interpretation of the SMMR-derived liquid water paths and precipitation characteristics in terms of cloud type, cloud fraction, and cloud height. Combining the initialized analyses from the European Center for Medium-Range Weather Forecasting (ECMWF) with the 3DNEPH enables vertical temperature and humidity profiles to be incorporated into the retrievals. The interpretation and presentation of our results are guided by their implications for the parameterization of liquid water content of layer clouds in largescale atmospheric models. The average liquid water paths for middle and low clouds were determined to be 115 and 102 g m−2, respectively, with a maximum value of 1070 g m−2. A comparison of the SMMR-derived values of the liquid water path with the adiabatic liquid water path determined from the 3DNEPH cloud data and the ECMWF temperature profiles indicated that these clouds were for the most part substantially diluted by a combination of precipitation, freezing, and entrainment. Analysis of the liquid water path as a function of temperature showed that clouds with average temperature below 246 K had little liquid water and were inferred to be predominantly crystalline. There was little evidence that cloud liquid water path increases with temperature for cloudiness on a large scale, suggesting that cloud thickness plays the dominant role in determining cloud liquid water path. A total of 8.5% and 4.4% of the total middle and low clouds, respectively, were determined to be raining. Liquid water paths of 350 g m−2 and 500 g m−2 for middle and low clouds, respectively, were determined to be average thresholds for the onset of precipitation. Maximum rain rates for these clouds were determined to be 7 mm h−1. The autoconversion of cloud water to rain water was determined to occur at a rate of 0.001 s−1.

Abstract: : The Air Force Global Weather Central (AFGWC) automated cloud analysis program (3DNEPH) produces high resolution, three-dimensional analyses of clouds over the entire globe. Up to eight analyses are scheduled per day with additional limited area analyses available on request. Horizontal grid spacing is 25 nautical miles and the vertical grid consists of 15 layers of varying thickness from the earth's surface to 55,000 feet above mean sea level. The program is a string of individual modules which process and integrate meteorological cloud information from surface, pilot, and upper air reports. An additional capability to interpret and incorporate visual and infrared satellite imagery results in high resolution, worldwide coverage. Recent advances in the techniques and methodology for interpreting satellite imagery are described. A new manual data input processor, quality control procedures, and applications are also described. An appendix provides detailed information on the high resolution terrain, geography, temperature, and background brightness (albedo) fields used to support the model. A history of the model is included and detailed descriptions of the satellite processing algorithms are also provided. Samples of displayed data using several display methods are included.