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Donald P. Wylie

Abstract

The use of geostationary satellite data for estimating precipitation in non-tropical areas was explored with data taken in Montreal, Canada. The previous studies using geostationary images for rain estimation have concentrated primarily on tropical clouds (Griffith et al., 1978; Stout, et al., 1979). The intent of this study was to evaluate the applicability of using these data and techniques in other geographical areas. The Montreal area provided a wide range of weather situations common to midlatitudes for which the techniques could be tested. Because of the many variables in this area (different cloud types, moisture availability, temperature vertical structure and others) the rain rates of the cloud areas varied. Large differences in rain rates between the days studies in Montreal were found. The Montreal data also had rain rates that were considerably smaller than found in the tropical studies.

To explain these differences the environments of the clouds were investigated using sounding data. By applying a cumulus model (Simpson and Wiggert, 1969) to the soundings most of the daily differences in rain rates were explained. The large differences between the tropical studies and Montreal also were described by the model. It is proposed that future rain estimation schemes combine satellite image with sounding data through a cloud model to form a technique applicable to a wide variety of weather situations and geographical areas.

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Donald P. Wylie and Denise Laitsch

Abstract

Rain estimates for the Great Plains States were made for a one-month period, August 1979, using different combinations of satellite and other data. The data tested were as follows: 1) two satellite images per day without any other data, 2) four satellite images per day, 3) 24 images per day, 4) 24 images per day with hourly surface observations and two per day radiosonde soundings (excluding the 6 h raingage reports), 5) two images per day with the Service A 6 h raingage reports, 6) 24 images per day with the Service A raingage reports, and 7) an automatic rain estimate made from infrared temperatures without human intervention.

Each method was applied to the same geographic area by the same meteorologists. Estimates produced from the seven data combinations were compared to a withheld data set of 538 hourly recording raingages.

The rain estimates from all methods tested were very similar in their ability to locate rainfall and estimate the monthly patterns. The first two methods tested, using only satellite imagery at low-frequency sampling rates, gave slightly poorer skill scores than the more data-rich methods. Best scores were found for methods using the Service A raingage reports (Methods 5 and 6). The frequency of satellite imagery did not change the quality ofthe estimates when the raingages were included.

The rain estimates made without the judgment of a meteorologist (Method 7) scored surprisingly close to the other methods tested. The additional effort of a meteorologist improved the rain estimates in all cases, but the level of improvement was small beyond that produced by a simple automated scheme.

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Donald P. Wylie and Barry B. Hinton

Abstract

Cloud motions over the Indian Ocean for May–July 1979 were used to obtain spatial auto correlations of the deviations of the wind components from local means. Best correlations were associated with u′, low altitude clouds and alongwind displacements. Worst correlations arose from v′, high clouds and crosswind displacements. The crosswind anisotropy was ∼15%. All correlations were 0.49 or greater at 5° separation or less.

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Donald P. Wylie and Harold M. Woolf

Abstract

A comparison of diurnal cycles in high clouds (<440 hPa) measured by the Geostationary Operational Environmental Satellite Visible Infrared Spin Scan Radiometer (VISSR) Atmospheric Sounder (GOES-VAS) and the International Satellite Cloud Climatology Project (ISCCP) was made. The GOES-VAS longwave infrared data allow uniform detection of upper-tropospheric cirrus clouds from daylight into night without effects from solar reflections. It is sensitive to thin cirrus, which are difficult to detect. But it is not available globally because the sounder instrument is flown only on geostationary satellites operated by the United States. The ISCCP, however, is a global dataset using five to seven geostationary satellites.

Large diurnal cycles were found in the Rocky Mountains and along the northern coast of the Gulf of Mexico mainly in the summer season. In the Tropics substantial diurnal cycles also were found in central Brazil and the Atlantic ITCZ. In the winter over the continental United States, diurnal cycles were very small or nearly absent.

The ISCCP found similar diurnal cycles over land. The amplitudes of the cycles were about 1.5 times larger in the VAS data than the ISCCP (IR only) data over land because of the former's sensitivity to thin cirrus. The phase relationships were very similar. In the western tropical Atlantic ITCZ, the VAS found dual maxima in the diurnal cycle (morning and later afternoon), which the ISCCP could not detect. These changes in high cloud cover probably were driven by radiative cooling of the cloud tops over the ocean. Over land the obvious cause of diurnal cycles in high clouds is deep convection from solar surface heating.

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Robert A. Mack and Donald P. Wylie

Abstract

A technique was developed for estimating the condensation rates of convective storms using satellite measurements of cirrus anvil expansion rates and radiosonde measurements of environmental water vapor. Three cases of severe conviction in Oklahoma were studied and a diagnostic model was developed for integrating radiosonde data with satellite data.

Two methods were used to measure the anvil expansion rates–the expansion of isotherm contours on infrared image, and the divergent motions of small brightness anomalies tracked on the visible images. The differences between the two methods were large as the storms developed, but these differences became small in the latter stage of all three storms.

A comparison between the three storms indicated that the available moisture in the lowest levels greatly affected the rain rates of the storms. This was evident from both the measured rain rates of the storms and the condensation rates estimated by the model. The possibility of using this diagnostic model for estimating the intensities of convective storms also is discussed.

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Donald P. Wylie and Harold M. Woolf

Abstract

An analysis technique for Geostationary Operational Environmental Satellite-VISSR (Visible and Infrared Spin Scan Radiometer) Atmospheric Sounder (GOES-VAS) sounder data was developed to extract cloud and clear radiance information. This technique employed many of the concepts used in the International Satellite Cloud Climatology Project (ISCCP) such as spatial and time comparisons of neighboring satellite pixels. It improved upon the previous studies that used VAS data by using all available VAS data at full time and space resolution. The previous studies utilized <10% of the original data.

The GOES-VAS cloud and clear radiance statistics compared well with rawinsondes and the ISCCP cloud analysis. The best agreement between the ISCCP and this GOES-VAS cloud analysis was for upper-tropospheric clouds (<440 hPa) in both cloud frequency and infrared emissivity. The two cloud datasets agreed to within 2% for both parameters. A comparison of the GOES-VAS clear radiance data to National Weather Service (NWS) rawinsondes showed agreement within 1.7 K (blackbody radiances). The upper-tropospheric VAS channels were warmer than the rawinsondes. The VAS water vapor channels suggested that the NWS rawinsondes have a dry bias in the upper troposphere.

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Donald P. Wylie and W. Paul Menzel

Abstract

Over the last 8 yr frequency and location of cloud observations have been compiled using multispectral High Resolution Infrared Radiation Sounder (HIRS) data from the National Oceanic and Atmospheric Administration polar-orbiting satellites; this work is an extension of the 4-yr dataset reported by D. Wylie et al. The CO2 slicing algorithm applied to the HIRS data exhibits a higher sensitivity to semitransparent cirrus clouds than the cloud algorithm used by the International Satellite Cloud Climatology Project; the threshold for cloud detection appears to require visible optical depths (τ vis) greater than 0.1.

The geographical distributions of clouds in the 8-yr dataset are nearly the same as those reported from 4 yr of data. The detection of upper-tropospheric clouds occurs most often in the intertropical convergence zone and midlatitude storm belts with lower concentrations in subtropical deserts and oceanic subtropical highs. The areas of concentrated cloud cover exhibit latitudinal movement with the seasons as in other cloud datasets. HIRS finds clear sky in 25%, opaque cloud in 32%, and semitransparent cloud in 43% of all its observations. The effective emissivity of the all semitransparent clouds (τ vis < 6) ranges from 0.2 to 0.6 with an average value of about 0.5.

Time trends are reexamined in detail. A possible cirrus increase in 1991 reported by Wylie and coauthors in 1994 is found to be diminished upon reinspection. The revised 8-yr record has indications of an increase in high clouds in the northern midlatitudes (0.5% yr−1) but little change elsewhere. The seasonal cycle of cloud cover in the Southern Hemisphere becomes very noticeable in 1993.

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David O'C. Starr and Donald P. Wylie

Abstract

Detailed descriptions of the rawinsonde-resolved meteorological conditions (3-hourly soundings) associated with a succession of five distinct mesoscale cirrus cloud regimes, which were observed intensely over a 36-hour period, is given. The synoptic scale systems in which these features were embedded are described and a brief overview of the experiment are given. Regional analyses of the static stability structure and vertical motion are presented and interpreted with respect to the characteristics of the corresponding cloud fields as deduced from satellite and lidar observations. The cloud fields exhibited a high degree of persistent mesoscale organization on scales of 20–500 km reflecting corresponding scales of dynamic and thermodynamic structure/variability as on the synoptic scale. Cloud generation was usually confined to layers less than 1 km deep (typically 0.5 km in depth) and cellular organization was evident in most cases irrespective of the thermal stratification. Multilayered development was prevalent (2–3 layers) and was associated with vertical structure of the temperature and moisture fields resulting primarily from vertical gradients in horizontal advection. One convective generation layer was usually present. Destabilization resulted primarily from advective processes that also led to the formation of a transient stable layer above and/or below the convective layer. Though resembling elevated frontal surfaces, the stable layers were not extensions of surface features. Cloud processes, primarily ice particle fall-out and evaporation, but also including cloud top detrainment, contributed to generating the multilayered structure. Two cases of clouds spawned from an overlying cloud deck were seen where one involved natural seeding of an ice-saturated and conditionally unstable layer in which vigorous convective development was subsequently observed. Subvisual cirrus in the lower stratosphere were found to be associated with prior tropopause features (upwind) where denser cirrus existed. Inferences are drawn with respect to the parameterization of cirrus in large-scale models. In particular, vertical resolution on the order of 0.5 km will probably be required to adequately resolve the forcing required for implementation of a physically-based parameterization. Greater understanding of the nature and causes of the observed mesoscale structure is also needed.

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Donald P. Wylie and Chester F. Ropelewski

A tethered sonde, the Boundary Layer Instrument System (BLIS), was designed for use from shipboard platforms in the GARP Atlantic Tropical Experiment (GATE). This system was able to monitor the thermal and kinematic properties of the boundary layer from approximately 100 m to the level of cloud base (800–1000 m). Five levels were simultaneously sampled for periods up to 24 h in length. More detailed vertical structure measurements were obtained by raising and lowering the tethered balloon. The mechanical details of the system and its accuracy in monitoring boundary layer changes and vertical motions are described.

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Barry B. Hinton and Donald P. Wylie

Abstract

The errors in ship wind reports of light winds tend to significantly bias their mean. This occurs because wind speed is a scalar quantity that is constrained to zero or positive values. Therefore, observations tend to overestimate the light winds because of the one-sided distribution of errors, but the bias disappears under stronger winds. A method for removing this bias from ship data is presented. In particular, the method is applied to interpreting the ratios of wind speeds observed by ships to those obtained from tracking low level clouds. Contracted ratios allow low cloud speeds to serve as proxy data for surface based observations.

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