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Carlyle H. Wash, Delain A. Edman, and John Zapotocny

Abstract

GOES observation of rapid dissipation of a 5–10 cm (2–4 inches) depth snowband over the central and upper Mississippi River valley on 15 April 1980 is presented. Differences in the local weather between the stations with and without melting snow are discussed.

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Robert C. Allen Jr., Philip A. Durkee, and Carlyle H. Wash

Abstract

An algorithm is developed and evaluated for discriminating between clouds, snow-covered land and snow-free land in satellite image data. The multispectral technique uses daytime images of NOAA AVHRR channels 1 (0.63 μm), 3 (3.7 μm), and 4 (11.0 μm). Reflectance is derived for channel 3 by using the channel 4 emission temperature to estimate and remove the channel 3 thermal emission. Separation of clouds from snow and land is based primarily on the derived channel 3 reflectance. Observed reflectance in channel 3 is 0.02 to 0.04 for snow, 0.03 to 0.10 for land, 0.02 to 0.27 for ice clouds and 0.08 to 0.36 for liquid clouds. These ranges overlap for thin cirrus and snow, so the routine attempts analysis of cirrus based on differences in transmission between channels 3 and 4. Six case were analyzed and the total cloud cover was verified against a total of 110 surface observations in the standard categories of clear, scattered, broken and overcast. One of the cases is presented in detail to illustrate the algorithm procedures and results. Analysis of cloud cover from the satellite algorithm matched surface observations at 55% of the stations and was one category different at 33% of the stations. The algorithm differed from the surface observations by two categories at 9% of the stations and by three categories at 4% of the stations. A major remaining problem is discrimination between ice clouds and snow cover.

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Carlyle H. Wash, Robert A. Hale, Paul H. Dobos, and Eric J. Wright

Abstract

Explosive cyclogenesis during the winter of the First Global GARP Experiment (January–February 1979) is analyzed using the revised European Centre for Medium Range Weather Forecasts (ECMWF) analyses. Explosive cyclogenesis is defined as a decrease in the sea level pressure at the rate of 1 mb h−1 for at least 12 h. Diagnostics for 23 explosively developing cases and 16 nonexplosive cases are evaluated. Parameters compared include the dry static stability, low-level relative vorticity, vorticity advection, upper-level divergence, kinematic vertical velocities, and the strength of the low-level baroclinity. These parameters are compared statistically at the initial, 12-, and 24-h time periods. Parameters for which the explosive and nonexplosive cyclone ensembles were statistically separable are the kinematic vertical velocity and the upper-level divergence and vorticity advection. The strong upper-level processes for the explosive cases at the initial time indicate the importance of upper-tropospheric features in producing the stronger vertical motions and more rapid cyclogenesis.

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Carlyle H. Wash, Stacey H. Heikkinen, Chi-Sann Liou, and Wendell A. Nuss

Abstract

A cyclone that developed explosively during Intensive Observation Period (IOP) 9 of the Genesis of Atlantic Lows Experiment (GALE) is studied. Detailed surface analysis is conducted based on operationally available data, late reporting ship observations and GALE special observations to determine the surface storm track and deepening rate. GALE dropsonde and rawinsonde data are used to supplement the normal upper-level database, and are analyzed by a research version of the Navy Operational Regional Analysis and Prediction System (NORAPS) using optimal interpolation analysis. These analyses reveal critical subsynoptic features important in the development. Two surface lows are present during the early coastal development period. The western center is coupled to a mobile 500 mb short-wave trough while the eastern center develops in a strong baroclinic zone offshore. The objective analyses also show a strengthening of a jet streak east of the mobile short wave. The divergent quadrant of this jet streak induces upward vertical motion over the eastern of the two coastal low systems.

The rapid development of the eastern center occurs due to the superposition of the upper-level forcing (jet streak) over the low-level perturbation with strong thermal adymion. Dropwindsonde data document the low static stability in the region. NORAPS operational and GALE data forecasts from 1200 UTC 24 February erroneously deepen the western center and result in track errors of 300 to 600 km. The GALE forecast from 0000 UTC 25 February deepens the correct center and makes the best track forecast. All forecasts fail to predict the full extent of the rapid development of this cyclone.

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Mark H. Pickett, Wenqing Tang, Leslie K. Rosenfeld, and Carlyle H. Wash

Abstract

To determine the accuracy of nearshore winds from the QuikSCAT satellite, winds from three satellite datasets (scientifically processed swath, gridded near-real-time, and gridded science datasets) were compared to those from 12 nearshore and 3 offshore U.S. West Coast buoys. Satellite observations from August 1999 to December 2000 that were within 25 km and 30 min of each buoy were used. Comparisons showed that satellite–buoy wind differences near shore were larger than those offshore. Editing the satellite data by discarding observations recorded in rain and those recorded in light winds improved the accuracy of all three datasets. After removing rain-flagged data and wind speeds less than 3 m s−1, root-mean-squared differences (satellite minus buoy) for swath data, the best of the three datasets, were 1.4 m s−1 and 37° based on 5741 nearshore comparisons. By removing winds less than 6 m s−1, these differences were reduced to 1.3 m s−1 and 26°. At the three offshore buoys, the root-mean-squared differences for the swath data, with both rain and winds less than 6 m s−1 removed, were 1.0 m s−1 and 15° based on 1920 comparisons. Although the satellite's scientifically processed swath data near shore do not match buoy observations as closely as those offshore, they are sufficiently accurate for many coastal studies.

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Carlyle H. Wash, Stacey H. Heikkinen, Chi-Sann Liou, and Wendell A. Nuss

Abstract

A cyclone that developed explosively during Intensive Observation Period (IOP) 9 of the Genesis of Atlantic Lows Experiment (GALE) is studied. Detailed surface analysis is conducted based on operationally available data, late reporting ship observations and GALE special observations to determine the surface storm track and deepening rate. GALE dropsonde and rawinsonde data are used to supplement the normal upper-level database, and are analyzed by a research version of the Navy Operational Regional Analysis and Prediction System (NORAPS) using optimal interpolation analysis. These analyses reveal critical subsynoptic features important in the development. Two surface lows are present during the early coastal development period. The western center is coupled to a mobile 500 mb short-wave trough while the eastern center develops in a strong baroclinic zone offshore. The objective analyses also show a strengthening of a jet streak east of the mobile short wave. The divergent quadrant of this jet streak induces upward vertical motion over the eastern of the two coastal low systems.

The rapid development of the eastern center occurs due to the superposition of the upper-level forcing (jet streak) over the low-level perturbation with strong thermal adymion. Dropwindsonde data document the low static stability in the region. NORAPS operational and GALE data forecasts from 1200 UTC 24 February erroneously deepen the western center and result in track errors of 300 to 600 km. The GALE forecast from 0000 UTC 25 February deepens the correct center and makes the best track forecast. All forecasts fail to predict the full extent of the rapid development of this cyclone.

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Chi-Sann Liou, Carlyle H. Wash, Stacey M. Heikkinen, and Russell L. Elsberry

Abstract

A research version of the Navy Operational Regional Atmospheric Prediction System (NORAPS) is used to study cyclogenesis events during the second Intensive Observation Period of the Genesis of Atlantic Lows Experiment (GALE). From 1200 UTC 26 January to 0000 UTC 29 January 1986, two cyclogeneses occurred over the East Coast of the United States. NORAPS analyses reveal that the first cyclone develops rapidly due to the superposition of upper-level jet streak forcing over a shallow surface system associated with a well-developed coastal front. Large latent heat release around the cyclone center is considered to be a contributing factor for the rapid deepening of the first cyclone between 0000UTC and 1200 UTC 27 January. Small static stability at low levels coupled with a new upper-level trough-jet system is considered to be an important factor for the formation of the secondary cyclone off the East Coast at 1800 UTC 27 January.

NORAPS predicted the two cyclogenesis events fairly well up to 0000 UTC 28 January. A prediction of too early and too weak of a cold surge is believed to be the main reason for poor forecasts during later periods.

Extra data available from GALE sounding and surface data tapes are added to the operationally received dataset to study the impact of those extra data on analyzing and predicting the two cyclogenesis events. The GALE data impact investigated in this study is concentrated in the increase of spatial resolution, but not temporal resolution, by GALE networks and dropwindsondes. Because the two cyclogenesis events were over land of close to the coast, the regular operational data coverage over the East Coast of the United States was sufficient for the NORAPAS Optimum Interpolation (OI)_ analysis to analyze the important features for the cyclone developments. As a result, the enhancement of data spatial resolution from GALE soundings and surface reports made only limited improvement on NORAPS analyses and forecasts of these two cyclogenesis cases.

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Paul A. Hirschberg, Matthew C. Parke, Carlyle H. Wash, Mark Mickelinc, Roy W. Spencer, and Eric Thaler

Abstract

A statistical analysis is performed on a 6-month global dataset consisting of satellite-derived channel 3 Microwave Sounding Unit (MSU3) brightness temperature and various conventionally derived fields to quantify the potential usefulness of MSU3 analyses in the nowcasting and forecasting of baroclinic waves. High positive spatial and temporal correlations are obtained between the MSU3 brightness temperature and 400–100-mb thickness fields over all wavelengths in the data. Slightly lesser positive correlations are found between the MSU3 and the 200-mb temperature. The MSU3–500-mb and MSU3–50-mb height correlation results indicate a scale dependence in the hydrostatic spreading of thickness anomalies in the vertical. Most significantly, relatively high negative MSU3–500-mb height correlations for the short (≤ synoptic scale) wavelength portion of the data suggest that upper-level thermal anomalies are reflected downward and that MSU3 analyses can be used to track midlevel synoptic-scale baroclinic waves. This conclusion is also supported by relatively high negative MSU3–500-mb vorticity and MSU3–dynamic tropopause correlations along the climatological storm tracks.

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Louis W. Uccellini, Paul J. Kocin, Ralph A. Petersen, Carlyle H. Wash, and Keith F. Brill

Abstract

The Presidents' Day cyclone of 18–19 February 1979 was an intense and rapidly developing storm which produced heavy snowfall along the East Coast of the United States. An analysis of the cyclone is presented which isolates three jet streaks that appear to have played important roles in the development of two separate areas of heavy snow. One area of heavy snow developed prior to cyclogenesis and is linked, in part, to an increasingly unbalanced subtropical jet streak (STJ) and a noticeably ageostrophic low-level jet. The second area of heavy snow developed in conjunction with the explosive cyclogenesis off the East Coast as a polar jet streak and midtropospheric trough propagated toward the coastal region from the north-central United States.

This paper examines the STJ in detail. The maximum wind speeds associated with the STJ increased by 15 to 20 m s-1 between 1200 GMT 17 and 1200 GMT 18 February 1979 as the jet propagated from the south-central toward the eastern United States. During the 24 h period, the flow in the STJ became increasingly supergeostrophic and apparently unbalanced. Ageostrophic wind speeds increased to greater than 30 m s-1, with a significant cross-contour component directed toward lower values of the Montgomery streamfunction, as the flow along the STJ became increasingly divergent with time. The increased wind speed, ageostrophic flow, and divergence along the axis of the STJ are linked to the increasing confluence in the entrance region of the jet streak and the decreasing wavelength of the trough-ridge system in which the jet streak was embedded. The upper level divergence and upward vertical motion near the axis of the STJ along with the moisture transport associated with the LLJ are found to be important factors in the development of the first area of heavy snow.

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Finbarr O'Sullivan, Carlyle H. Wash, Michael Stewart, and Craig E. Motell

Abstract

An automated statistical pattern recognition technique is presented that uses visible and IR satellite imagery to estimate instantaneous surface rainfall rates. The technique uses both brightness and textural statistics to estimate rainfall in 10 × 10 pixel arrays of satellite data. Each array is centered over one of 137 Service A weather stations scattered over southeastern United States. Surface reports from these stations obtained during a 30 day period in August of 1979 are used to ground truth the technique. The technique classifies each 10 × 10 array into one of three categories: no rain, light rain, moderate/heavy rain. Cross-validation is used to estimate classification errors; results of these estimates yielded an overall error rate of 35% when both visible and IR data are used. When only visible or IR data are used the overall error rates are 39% and 42%, respectively. In addition to the three class problem, the two class problem of classifying rain / no rain is studied. Overall error rates of 18% are achieved using a technique with 16 image statistics and both visible and IR data. A simpler technique that uses only the mean and standard deviation statistics, derived from the visible and IR data, achieved an overall error rate of 20%. We conclude that the visible and IR pattern recognition technique could be used successfully to estimate instantaneous rainfall in three classes: no rain, light rain, moderate/heavy rain. During the night and during hours of low sun attitude, IR data could be used but with a slight decrease in accuracy. We also conclude that a simpler pattern recognition technique, based upon the mean and standard deviation statistics, could be used to distinguish between rain and no rain classes.

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