Search Results

You are looking at 1 - 4 of 4 items for :

  • Author or Editor: Earl E. Gossard x
  • Journal of Atmospheric and Oceanic Technology x
  • Refine by Access: All Content x
Clear All Modify Search
Earl E. Gossard

Abstract

The advent of Doppler clear-air radars for wind-height profiling opens the way for their use in a variety of other applications. This paper uses knowledge of the clear-air Doppler spectrum from a zenith-pointing radar together with the measured water droplet Doppler vertical velocity spectrum to calculate spectra of drop number density through clouds of droplets having substantial fall velocity. The method has been applied by Japanese scientist to measure drop-size distributions of precipitation particles from data acquired at the VHF MU radar facility. Here the method is applied to records obtained with a 915 MHz wind profiler located at Denver, Colorado, and the resulting spectra are presented and compared with the spectra that would have been obtained if the clear-air information were ignored. From the number density drop-size distribution, the corresponding liquid water distribution can be calculated. It is concluded that failure to take into account turbulence in the medium can result in order-of-magnitude errors in number density and liquid water. The requirements and limitations of a radar remote sensing drop spectrometer are discussed.

Full access
Earl E. Gossard

Abstract

A new technique is examined for using Doppler radars to extract information about the size spectrum of cloud droplets too small to have terminal velocities large enough to be resolvable by the radar. If the drops are very small, motions of the drops are dominated by turbulent fluctuations in the medium rather than their fall velocity. Their motion is then the convolution of the terminal velocity with the turbulent velocity probability density function, and size information about the population can be obtained only by deconvolving the spectra. Doppler radars can extract this velocity and size information, as well as cloud liquid and liquid flux, using a surprisingly simple and accurate technique assuming some functional form (e.g., gamma) for the drop number density spectrum. The method also allows Doppler radars to extract drop size information independent of up-/downdrafts in the medium in which they are embedded. Various gamma and lognormal functions are compared, and finally, a “Stokes range” of drop sizes is added and found to he important. Examples are shown and errors are discussed.

Full access
Earl E. Gossard
,
Daniel E. Wolfe
, and
B. Boba Stankov

Abstract

Bragg backscatter of radar waves from elevated turbulent layers is very highly correlated with the height profile of the gradient of radio refractive index through elevated turbulent layers, as has often been documented in past research. However, many users need profiles of radio refractive index or the associated humidity rather than profiles of their gradients. Simple integration of the gradients is usually not feasible because clutter and various noise sources often severely contaminate the lower-range gates. The authors show that if the total integrated humidity is independently available [for example, from the Global Positioning System (GPS)] and if the surface value of humidity is known, the profiles of humidity are retrievable with good accuracy. This method is demonstrated with data collected in Southern California, where 7 h of 449-MHz data were recorded along with GPS data. Three radiosonde balloons were launched during that period, and the profiles of humidity from the two sources are compared. Simulations are used to assess errors that result from factors such as lack of the sign of the humidity gradients. In conclusion, a humidity profile found by statistical retrieval is compared with one found by the technique proposed in this paper.

Full access
B. Boba Stankov
,
Earl E. Gossard
,
Bob L. Weber
,
Richard J. Lataitis
,
Allen B. White
,
Daniel E. Wolfe
,
David C. Welsh
, and
Richard G. Strauch

Abstract

An algorithm to compute the magnitude of humidity gradient profiles from the measurements of the zeroth, first, and second moments of wind profiling radar (WPR) Doppler spectra was developed and tested. The algorithm extends the National Oceanic and Atmospheric Administration (NOAA)/Environmental Technology Laboratory (ETL) Advanced Signal Processing System (SPS), which provides quality control of radar data in the spectral domain for wind profile retrievals, to include the retrieval of humidity gradient profiles. The algorithm uses a recently developed approximate formula for correcting Doppler spectral widths for the spatial and temporal filtering effects. Data collected by a 3-beam 915-MHz WPR onboard the NOAA research vessel Ronald H. Brown (RHB) and a 5-beam 449-MHz WPR developed at the ETL were used in this study. The two datasets cover vastly different atmospheric conditions, with the 915-MHz shipborne system probing the tropical ocean atmosphere and the 449-MHz WPR probing continental winter upslope icing storm in the Colorado Front Range. Vaisala radiosonde measurements of humidity and temperature profiles on board the RHB and the standard National Weather Service (NWS) radiosonde measurements at Stapleton, Colorado, were used for comparisons. The cases chosen represent typical atmospheric conditions and not special atmospheric situations. Results show that using SPS-obtained measurements of the zeroth, first, and second spectral moments provide radar-obtained humidity gradient profiles up to 3 km AGL.

Full access