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Marilyn M. Wolfson

Abstract

This report describes in detail the FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) automatic weather station network which is being used in the Terminal Doppler Weather Radar program to assess the radar detectability of wind shear and to help gain an understanding of microburst forcing mechanisms. The weather stations are descended from the PROBE stations originally operated by the Bureau of Reclamation. The current instrumentation has been modified slightly but is largely the same as that originally used as is the hardware structure, but the data collection platforms are entirely new. Each station in the 30-station network transmits 1 min averages of temperature, relative humidity, barometric pressure, wind speed, wind direction and precipitation amounts, as well as peak wind speed, on a single GOES satellite channel.

Performance results from the first 3 yr (1984–86) of mesonet operations are presented. During June and July 1986 the FLOWS network was collocated with the NCAR PAM-II network near Huntsville, Alabama to measure surface data on microbursts as part of the Cooperative Huntsville Meteorological Experiment (COHMEX). A preliminary assessment of the overall performance of the two networks suggests that they performed with comparable accuracy for those meteorological characteristics most important to the detection of microbursts. While differences and discrepancies were noted, none would preclude treating PAM-II and FLOWS data together as if they were generated by a single network.

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Marilyn M. Wolfson and T. Theodore Fujita

Abstract

The effects of obstructions on winds measured by the 30 station FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) mesonet and the 6 station FAA LLWAS (Low Level Wind Shear Alèrt System) near Memphis, TN in 1985 are analyzed. The slowing of surface winds by anemometer site obstructions is a continuing problem for scientific and operational wind shear measurement system This paper considers an improved version of the technique used by Fujita and Wakimoto for compensating the obstruction effects by the use of mathematical models relating the unobstructed wind speed to the measured wind speed and the observed obstructions at each site. Over eight million wind speed measurements gathered over 197 days (15 February–31 August) were used. The effects of obstructions at a particular site were evidenced by a strong negative correlation between the observed wind speed transmission factors and the obstruction angles as measured from panoramic photographs taken of the horizon around each station. The functional relationship between them was modeled as a decaying exponential plus a constant, and an iterative least squares regression technique was used on data from all of the stations at once in deriving the three parameters of the equation. It was found that the first 8° of obstruction have the greatest blockage effects, and that even a 2° or 3° high isolated clump of trees can have a pronounced effect on the measured wind speeds from that direction. The possibility that the transmission factors are scale dependent and time dependent is explored.

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Diana L. Klingle, David R. Smith, and Marilyn M. Wolfson

Abstract

Gust fronts produce low altitude wind shear that can be hazardous to aircraft operations, especially during takeoff and landing. Radar meteorologists have long been able to identify gust front signatures in Doppler radar data, but in order to use the radar efficiently, automatic detection of such hazards is essential.

In a study designed to accumulate statistics on the gust frontal signature in Doppler radar data, nine gust front cases were analyzed. Data were collected on those characteristics thought to be most important in developing rules for automatic gust-front detection such as gust front length and height, maximum and minimum values of reflectivity, velocity and spectrum width, and estimates of radial shear. To provide the reader with a concrete example, photographs of the Doppler radar displays of just two (in the interests of brevity) of the nine gust fronts are presented and discussed, as well as summary data for all cases. For these cases, outflows could be detected most reliably in the velocity field. Line features in the spectrum width and reflectivity fields associated with some of the gust fronts could also be identified, although somewhat less reliably than in the Doppler velocity.

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Wei-Chyung Wang, William B. Rossow, Mao-Sung Yao, and Marilyn Wolfson

Abstract

We illustrate the potential complexity of the feedback between global mean cloud amount and global mean surface temperature when variations of the vertical cloud distribution are included by studying the behavior of a one-dimensional radiative–convective model with two types of cloud variation: 1) variable cloud cover with constant optical thickness and 2) variable optical thickness with constant cloud cover. The variable parameter is calculated assuming a correlation between cloud amount and precipitation or the vertical flux convergence of latent heat. Since the vertical latent heat flux is taken to be a fraction of the total heat flux, modeled by convective adjustment, we examine the sensitivity of the results to two different critical lapse rates, a constant 6.5 K km−1 lapse rate and a temperature-dependent, moist adiabatic lapse rate. The effects of the vertical structure of climate perturbations on the nature of the cloud feedback are examined using two cases: a 2% increase in the solar constant and a doubling of the atmospheric carbon dioxide concentration. The model results show that changes in the vertical cloud distribution and mean cloud optical thickness can be as important to climate variations as are changes in the total cloud cover. Further the variety and complexity of the feedbacks exhibited even by this simple model suggest that proper determination of cloud feedbacks must include the effects of varying vertical distribution.

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