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WILLIAM E. SHENK

Abstract

The origin of a small, tight cloud vortex located over the Caspian Sea as viewed by TIROS VII in late November 1963 was investigated. Charts at the mandatory levels from the surface to 500 mb. at 12-hr. intervals were examined. Possible causes of formation that were investigated included upper air circulation effects, mechanical effects induced by the Caucasus Mountains, surface heating from the sea, and association with a cold front. Evidence is presented that the cloud vortex was associated with a frontless circulation. A well-defined upper-level wind circulation is believed to be the major cause for the formation of the cloud spiral.

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William E. Shenk

Abstract

The difference between observed window radiation and radiation expected without clouds was related to various synoptic parameter. The but correlation, 0.72, was obtained with vertical motion computed by the adiabatic methods at 700 mb. Other variables tested were meridional flow and vertical motion at other level.

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WILLIAM E. SHENK

Abstract

During March 1962, the cloud changes associated with the predevelopment and development-periods of two secondary cyclones in the North Pacific Ocean were viewed on at least two successive days by the 8–12µ “window” channel of the TIROS IV meteorological satellite. Both secondary cyclones developed at the base of the occlusion. For the two cases, when the secondary circulations were first noticeable on the surface chart, the equivalent blackbody temperatures (TBB) as measured by the radiometer averaged 15°K colder than the day before in the northeast quadrant relative to the base of the occulusion. The average TBB was taken over an area of approximately 300,000 mi2. For two other cases, an average warming of 4°K occured from day to day over the same area relative to the base of the occlusion when no significant secondary development occurred and when the primary occluded cyclone was slowly weakening. These results suggest that, with the assistance of meteorological satellite radiation data, considerable cloud growth is noticeable prior to the generation of a secondary cyclone at the base of the occlusion.

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William E. Shenk

Abstract

Cloud top height variability of cloud domes above the cirrostratus anvil was investigated for two severe local storm occurrences over Texas on 12 and 13 May 1972. From a Lear Jet at 14 km, side-looking photographs were taken at distances of 50 to 200 km from the thunderstorm clusters every 30 sec for periods up to 1.5 hr. Ground based WSR-57 radar measurements were used to establish the positions of the cells and airplane positions were recorded every minute. Twenty-three domes were monitored and the average maximum height was 1.6 km above the anvil. It was estimated that a dome would take a maximum of about 6 min to grow from the anvil level to peak altitude based on measurements over the last 2 min of the growth period when the upward vertical velocity averaged 4 m sec−1. The extreme vertical growth rate was 27 m sec−1 for a 30-sec period. This vertical growth rate compared with a range of 11.4–19.4 m sec−1 determined from a method of estimating vertical motions based on laboratory measurements. Average dome collapse was at the same rate as the average ascent for the first 90 sec after the maximum altitude was reached. With a slightly smaller sample it was determined that the domes grew both horizontally and vertically until the maximum cloud top altitude was attained. Then, for the next 60 sec, horizontal growth continued while the vertical height was decreasing.

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William E. Shenk and William A. Hope

Abstract

The impact of time compositing on infrared profiling from geosynchronous orbit was evaluated for two convective outbreak cases. Time compositing is the accumulation of the data from several successive images taken at short intervals to provide a single field of measurements with the temporal resolution equal to the time to take all of the images. This is especially effective when the variability of the measurement is slow compared to the image interval. Time compositing should be able to reduce the interference of clouds for infrared measurements since clouds move and change.

The convective outbreak cases were on 4 and 21 May 1990 over the eastern Midwest and southeastern United States, respectively. Geostationary Operational Environmental Satellite imagery was used to outline clear areas at hourly intervals by two independent analysts. Time compositing was done every 3 h (1330–1530 UTC; 1630–1830 UTC) and over the full 5-h period. For both cases, a significant increase in coverage was measured with each 3-h compositing (about a factor of 2) and a further increase over the full period (approximately a factor of 3). The increase was especially useful in areas of broken cloud cover where large gaps between potential profiting areas on each image were reduced.

To provide information an measurement variability over local areas, the regions where the clear-area analyses were done were subdivided into 0.5° latitude-longitude boxes, and if some portion of each box was clear. It was assumed that at least one profile could he obtained within the box. In the largest clear areas, at least some portion was clear every hour. Even in the cloudier regions, multiple clear looks were possible during the entire period.

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William E. Shenk and Robert Holub
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William E. Shenk and Edward M. Brooks

Abstract

Over 50 cases of TIROS-viewed cloud vortices associated with extratropical cyclones over North America, Europe and Asia between April 1962 and November 1963 were examined to determine what meteorological information could be derived from the satellite pictures. Radiosonde stations were grouped in five principal classes according to their time-adjusted positions with reference to the vortex center and the major cloud bands. Meteorological parameters were statistically related to the age of the vortex, season, and geographical location as Well as to the principal sounding class for the position of the station.

The thermal results indicated that the means of the dew-point depression for each principal class were significantly different from the overall mean of the sample, according to the analysis of variance test. Relative tropopause heights were estimated from the means and variances of the vertical temperature differences. These results will be of value as input parameters for numerical prediction over data-silent areas.

The wind results showed that the wind directions within 333 km ahead of the major cloud band aids averaged 16° clockwise from the nearest band orientation, and behind the bands, 25° clockwise. The mean vertical wind shears within the major cloud bands and in the dry zones behind the bands were about 60° and 20°, respectively, clockwise from the band orientation. Most of the cloud bands were characterized by a speed convergence of the normal components and by a cyclonic shear and vorticity of the parallel components of the mean winds on the two sides of the bands.

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William E. Shenk and Vincent V. Salomonson

Abstract

Three channels of the Nimbus 2 five-channel Medium Resolution Infrared Radiometer (MRIR) have been employed in the development of a technique to determine sea surface temperature. Two of the channelsin the 0.2-4.0 and 6.4-6.9 μm spectral regions are used to indicate a cloud-free line of slight and the third, a high signal-to-noise window channel (10-11 μm), measures the equivalent blackbody temperature (TBB) which is a function of the thermal emission from the sea surface and the intervening atmosphere. Equivalent blackbody temperatures and normalized reflectance thresholds were established using frequency distributions from the 6.4-6.9 and 0.2-4.0 μm channels, respectively, to determine the existence of cloud-free conditions. The window TBB's were compared with ship ocean temperature measurements for a one-month period over the western North Atlantic. This comparison revealed a ±1.5K dispersion about the mean difference between the ship temperatures and window TBB's between 31-34N. An empirical method has been developed to correct for the atmospheric contribution to the observed window TBB's that considers the measurements from the other two channels and the viewing angle from the radiometer to the ocean surface.

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William E. Shenk and Edward B. Rodgers

Abstract

Three periods within the life cycle of Hurricane Camille (1969) are examined with radiometric and camera measurements from Nimbus 3 and camera information from ATS 3 in conjunction with conventional information. These periods are the deepening phase, the interaction of Camille with mid-latitude westerlies, and the excessive rain-producing period when the cyclone was over the central Appalachians.

Just prior to significant deepening, the Nimbus 3 Medium Resolution Infrared Radiometer (MRIR) window and water vapor channels showed a band of developing convection that extended to the cirrus level in the southeastern quadrant of the storm which originated from the ITCZ. Low-level wind fields were derived from conventional sources as well as from cumulus clouds tracked from a series of ATS 3 images. Within this band were low-level 30 kt winds that supplied Camille with strong inflow where the air passed over sea surface temperatures that were 1–3 standard deviations above normal.

At the beginning of the rapid deepening the MRIR radiometer measurements indicated a rapid contraction of the central dense overcast and then an expansion as the maximum deepening rate occurred. Simultaneously, the increase in the MRIR equivalent blackbody temperatures (TBB) indicated the development of large-scale subsidence throughout the troposphere northwest of the center. When Camille weakened as it moved over the lower Mississippi Valley, the cyclone acted as a partial obstruction to the synoptic-scale flow and increased the subsidence west and north of the cyclone center as indicated by the increase in water vapor TBB and verified by three-dimensional trajectories. Increased cloud-top elevations, approaching the levels reached when Camille was an intense cyclone over the Gulf of Mexico, were estimated from the Nimbus 3 High Resolution Infrared Radiometer (HRIR) measurements on 20 August 1969, when Camille produced rains of major flood proportions near the east slopes of the Appalachians in central Virginia.

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William E. Shenk and Vincent V. Salomonson

Abstract

The effect of sensor spatial resolution on estimating the amount of clouds covering the earth were investigated by simulating various cloud distributions and sizes, and measuring the known cloud amount with resolution of different sizes cloud-no cloud threshold technique often applied in automatic data processing. Cloud amount statistics have been tabulated for a three-orders-of-magnitude range in the ratio (R) of areal cloud size to areal resolution size for seven cloud amounts between 6 and 90%. Three different cloud patterns were used. These were 1) a regularly spaced pattern of homogeneous dots arranged in rows and columns (to simulate cloud streets), 2) a randomly spaced pattern of the same dots (to simulate randomly oriented cumulus clouds), and 3) a heterogeneous cloud size distribution irregularly spaced (to simulate a view of different cloud types and sizes). Two cloud amount estimation techniques were tested. Cloud amounts of 100% (method 1) and 50% (method 2) were assigned to partially filled resolution elements. Using criteria applicable to some studies carried out in the past, it is shown that cloud amount estimations can be in error by as much as 86% and 38%, respectively, for the two methods. Nomograms have been developed which substantially improve the estimate of the true cloud cover for R < 100 provided that R can be determined. Good agreement was found when a check was performed on whether or not the simulated cloud patterns were representative of real cloud patterns. The check was made by testing the cloud cover estimate nomograms constructed with the simulated data against similar curves prepared from cloud fields extracted from high-resolution photographs obtained from an Apollo flight.

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