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Gary P. Ellrod

Abstract

A technique is described for the detection of fog and low clouds at night using multispectral infrared (IR) imagery from Geostationary Operational Environmental Satellites (GOES). The technique requires subtraction and enhancement of digital data from IR window channels at 3.9- and 10.7–11.2-µm wavelengths. Resulting images show stratiform clouds over any type of terrain for a wide range of surface temperature conditions. The bispectral difference images are a considerable improvement over the use of enhanced images for a single window IR channel. An image enhancement technique that displays the approximate depth of fog was developed based on the correlation of brightness differences in the two IR channels to cloud-top heights reported by aircraft. The improved resolution and greater frequency provided by the GOES I-M satellites will result in a substantial improvement in the ability to monitor areas of reduced ceilings and visibilities at night.

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Gary P. Ellrod and David I. Knapp

Abstract

An objective technique for forecasting clear-air turbulence (CAT) is described. An index is calculated based on the product of horizontal deformation and vertical wind shear derived from numerical model forecast winds aloft. The forecast technique has been evaluated and is now in operational use at two forecast centers with international aviation responsibilities: the National Meteorological Center (NMC) in Washington, D.C., and the Air Force Global Weather Central (AFGWC) in Omaha, Nebraska. The index is also an operational forecast tool at the Canadian Atmospheric Environment Service (AES), and the National Aviation Weather Advisory Unit (NAWAU) in Kansas City, Missouri, both responsible for domestic aviation forecasts. The AFGWC index also includes horizontal convergence in its calculation. Thresholds were selected empirically by comparing index values with the location and intensity of observed CAT. Verification indicates that the index is quite reliable. The probability of detection (POD) varied from 70%–84%. False-alarm ratios (FAR) ranged from a low of 22% for the NMC aviation model to more than 40% for the AFGWC global model. An average threat score of 0.17 was calculated for the aviation model 24-h forecast. The operational capabilities of the NMC and AFGWC indices are compared in two CAT episodes that differ in synoptic-scale conditions and times of the year.

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Gary P. Ellrod and Andrew A. Bailey

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A satellite product that displays regions of aircraft icing potential, along with corresponding cloud-top heights, has been developed using data from the Geostationary Operational Environmental Satellite (GOES) imager and sounder. The icing product, referred to as the Icing Enhanced Cloud-top Altitude Product (ICECAP), is created hourly for the continental United States and southern Canada, and is color coded to show cloud-top altitudes in 1.9-km (6000 ft) intervals. Experimental ICECAP images became routinely available on the Internet during the spring of 2004. Verification of separate ICECAP components (imager icing potential and sounder cloud-top heights) using aircraft pilot reports (PIREPs) indicates that the product provides useful guidance on the spatial coverage and maximum altitude of current icing conditions, but not icing intensity, stratification, or minimum altitude. The imager icing potential component of ICECAP was compared with the operational 40-km resolution National Weather Service (NWS) current icing potential and NWS Airman’s Meteorological Advisories via the NOAA Real-Time Verification System, while GOES cloud-top heights were compared with altitudes of moderate or greater icing from PIREPs. Benefits and deficiencies of the GOES icing product are discussed.

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Donald W. Hillger and Gary P. Ellrod

Abstract

The detection of dust, fire hot spots, and smoke from the Geostationary Operational Environmental Satellite (GOES) is made easier by employing the principal component image (PCI) technique. PCIs are created by an eigenvector transformation of spectral band images from the five-band GOES Imager. The transformation is a powerful tool that provides a new set of images that are linear combinations of the original spectral band images. This facilitates viewing the explained variance or signal in the available imagery, allowing both gross and more subtle features in the imagery to be seen. Whereas this multispectral technique is normally applied to high-spatial-resolution land remote sensing imagery, the application is herein made to lower-spatial-resolution weather satellite imagery for the purpose of feature detection and enhancement. Features used as examples include atmospheric dust as well as forest and range fire hot spots and their resulting smoke plumes. The applications of PCIs to GOES utilized the three infrared window images (bands 2, 4, and 5) in dust situations as well as the visible image (band 1) in smoke situations. Two conclusions of this study are 1) atmospheric and surface features are more easily identified in multiband PCIs than in the enhanced single-band images or even in some two-band difference images and 2) the elimination of certain bands can be made either directly by inspection of the PCIs, discarding bands that do not to contribute to the PCIs showing the desired features, or by including all available bands and letting the transformation process indicate the bands that are useful for detecting the desired features. This technique will be increasingly useful with the introduction of new and increased numbers of spectral bands with current and future satellite instrumentation.

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Kenneth L. Pryor and Gary P. Ellrod

Abstract

The downburst is defined as a strong downdraft produced by a deep convective storm that induces strong or damaging winds on or near the earth's surface. Because of the intense wind shear they produce, downbursts are a hazard to aircraft in flight, especially during takeoff and landing phases. Retrieved profiles of temperature and moisture obtained from the Geostationary Operational Environmental Satellite (GOES) sounders have been shown to be useful in assessing the potential for convective downbursts. Sounder-derived parameters examined in this paper include the wind index (WINDEX), used to estimate maximum wind gusts; a dry microburst index (DMI), used to estimate dry microburst potential; and maximum theta-e deficit (TeD), used to estimate wet microburst potential. Currently under development is a new wet microburst index that will summarize the physical processes of convective storm development and downburst generation to quantify the potential severity of convective wind gusts. The experimental indices are plotted on regional GOES images (visible, infrared, or water vapor) and are made available on the GOES microburst products Web page.

This paper briefly reviews the development of each of the GOES microburst products, describes recent improvements, provides updated validation data and a case study, and discusses future plans. Recent improvements in the processing of sounding data to generate the microburst products include a change in the first-guess numerical model, use of single field-of-view retrievals, a filter for removing high DMI values where convection is unlikely, and a change in the calculation of nighttime WINDEX values to reduce a nighttime low bias. Improvements to the display of the microburst products include animation of many sectorized products, color coding of TeD and WINDEX ranges for data plots, plotting of numerical values of WINDEX instead of color-coded boxes, and the plotting of Storm Prediction Center (SPC) severe weather reports. Validation continues by comparing product output values to preliminary severe weather reports from the SPC as well as surface observations. Mean absolute error was <2 kt (1 m s−1) for 43 daytime events during summer 2002, a significant improvement over a mean absolute error of 3 kt (1.5 m s−1) for the 2001 convective season. A marked reduction in mean error for nighttime events was noted, improving from>6 kt (3 m s−1) for the 2001 convective season to 4 kt (2 m s−1) for summer 2002. A case study is presented that discusses the improved performance of the WINDEX during a nighttime convection event in the central plains.

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Gary P. Ellrod and John D. Marwitz

Abstract

The subcloud inflow and outflow structures of two multicell thunderstorms were synthesized from data by a variety of observation systems. The systems included digitized radar, instrumented aircraft, radar chaff, rawinsondes and surface data. The thunderstorms occurred on two consecutive days in northeast Colorado. The data were particularly examined for evidence of horizontal accelerations of the inflow air and hence pressure perturbations in the inflow region and were also examined for indications of interactions between the inflow and outflow air.

From the horizontal accelerations it was inferred that widespread but weak mesolows (≤1 mb) existed in the inflow region of each storm. Most of the acceleration appeared to have occurred in the region between the aircraft and environmental rawinsonde observations. The thermal structures of the inflow and outflow regions of both storms are presented. The location of the strongest updrafts near cloud base was generally above and to the rear of the ground-level wind-shift line. This position suggested that the updrafts were enhanced by interaction with the cold air outflow. Possible explanations for the decay of the storms are offered.

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Gary P. Ellrod and John A. Knox

Abstract

An operational clear-air turbulence (CAT) diagnostic index has been modified to improve its performance. The Ellrod–Knapp turbulence index (TI) was developed in the early 1990s and is in use at many aviation forecasting facilities worldwide. It has been recognized, however, that TI often does not sufficiently account for situations where anticyclonic shear or curvature is present. The proposed modification to TI is based on the addition of a proxy term for divergence tendency, appropriate for both anticyclonic flow and gravity wave generation in cyclonic regions. Examples show how the modified index [referred to as the divergence-modified turbulence index (DTI)] leads to better anticipation of significant CAT for two scenarios where rapid divergence changes were occurring. Preliminary objective evaluation of the 6-h forecast DTI derived from the Rapid Update Cycle-2 (RUC-2) was completed for 2 months in 2007 (using more than 1100 pilot reports). Results showed significant improvements over TI, based on verification metrics such as the probability of detection of turbulence (PODy) and the true skill statistic (TSS). Further evaluation is planned using a larger database of pilot reports, as well as forecast data from additional state-of-the-art prediction models, altitude ranges, and forecast times.

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S. Jeffrey Underwood, Gary P. Ellrod, and Aaron L. Kuhnert

Abstract

Radiation fog in the Central Valley of California has received very little attention in terms of climatological research. This study uses the Geostationary Operational Environmental Satellite (GOES) nighttime fog product to develop a sequence of images and datasets that reveal patterns of nocturnal radiation-fog development in the Central Valley. Twenty long-lived, spatially extensive radiation-fog episodes, occurring from October through January, were selected for the period of 1997–2000. Mean hourly parameters for fog cover, fog development rate, and vertical development were calculated for the 20 episodes in the Central Valley. The study region is separated into five analysis divisions oriented from south to north for spatial comparisons within the valley. Large-scale radiation fog begins developing before 1800 LST, and rates of development vary widely from south to north. Radiation fog develops earlier and covers a larger area of the southern valley as compared with the central and northern portions of the valley. The horizontal extent of radiation fog is maximized at 0600 LST in the southern valley and near midnight in the central and northern parts of the valley. Vertical development reaches 300 m with regularity in the southern valley. Radiation-fog development of greater than 300 m occurs primarily in the early morning hours. Vertical development “bursts” are also observed in the southern valley during the morning hours. Climatologically important conditions for radiation-fog development in the Central Valley include cool 1600 LST surface temperatures, moisture availability as reflected by 1600 LST dewpoint temperatures, early evening surface cooling trends, the rapidity with which mean relative humidity reaches 90%, and the presence of cool, dry air aloft (700–500 hPa).

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Gary P. Ellrod, James P. Nelson III, Michael R. Witiw, Lynda Bottos, and William P. Roeder

Abstract

Several experimental products derived from Geostationary Operational Environmental Satellite (GOES) Sounder retrievals (vertical profiles of temperature and moisture) have been developed to assist weather forecasters in assessing the potential for convective downbursts. The product suite currently includes the wind index (WINDEX), a dry microburst index, and the maximum difference in equivalent potential temperature (θ e) from the surface to 300 hPa. The products are displayed as color-coded boxes or numerical values, superimposed on GOES visible, infrared, or water vapor imagery, and are available hourly, day and night, via the Internet. After two full summers of evaluation, the products have been shown to be useful in the assessment of atmospheric conditions that may lead to strong, gusty surface winds from thunderstorms. Two case studies are presented: 1) a severe downburst storm in southern Arizona that produced historic surface wind speeds and damage, and 2) multiple dry and wet downbursts in western Kansas that resulted in minor damage. Verification involved comparing the parameters with radiosonde data, numerical model first guess data, or surface wind reports from airports, mesonetworks, or storm spotters. Mean absolute WINDEX from the GOES retrievals differed from the mean surface wind gust reports by <2 kt (1 m s−1) for 82 events, but underestimated wind gusts for 7 nighttime events by 22 kt (11 m s−1). GOES WINDEX was also slightly better than that derived from the concurrent National Centers for Environmental Prediction’s Eta Model first guess. There are plans to incorporate these downburst parameters into a future upgrade of the National Weather Service’s Advanced Weather Interactive Processing System, with the option to derive them from either GOES Sounder data, radiosondes, or numerical model forecast data.

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Gary P. Ellrod, Rao V. Achutuni, Jaime M. Daniels, Elaine M. Prins, and James P. Nelson III

The Geostationary Operational Environmental Satellite-8 (GOES-8), the first in the GOES I–M series of advanced meteorological satellites was launched in April 1994 and became operational at 75 °W longitude the following year. GOES-8 features numerous improvements over prior GOES platforms such as 1) improved resolution in the infrared (IR) and water vapor bands, 2) reduced instrument noise, 3) 10-bit visible and IR digitization, 4) greater image frequency, 5) more spectral bands, and 6) an independent sounder. A qualitative and quantitative comparison of the imager data from GOES-8 and GOES-7 shows that imagery from the newer spacecraft is superior in most respects. Improvements in resolution and instrument noise on GOES-8 provide sharper, cleaner images that allow easier detection of significant meteorological or oceanographic features. Infrared temperature comparisons between GOES-8 and GOES-7 were within 0.5°–2.0°C for all IR bands, indicating consistency between the two spacecraft. Visible band albedos from GOES-8 were at least 50% greater than GOES-7 for a wide range of scenes, suggesting that output from the GOES-7 visible detectors had degraded since its launch in 1987. Products derived from GOES-8 imager data for observing fog at night, fire detection, heavy precipitation estimation, and upper-level winds based on cloud or water vapor motion have been shown to be superior to similar products from GOES-7. Early difficulties with image registration and IR striping were alleviated after the first year. Based on the performance of GOES-8, future spacecraft in the GOES I–M series can be expected to provide many years of useful service to meteorologists, oceanographers, and the environmental monitoring community.

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