Earl E. Gossard

Measuring Drop-Size Distributions in Clouds with a Clear-Air-Sensing Doppler Radar

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 ord...

Ground-Based Remote Sensing of the Atmospheric Boundary Layer: 25 Years of Progress

The role of ground-based remote sensors in boundary-layer research is reviewed, emphasizing the contributions of radars, sodars, and lidars. The review begins with a brief comparison of the state of remote sensors in boundary-layer research 25 years ago with its present-day status. Next, a summary of the current capabilities of remote sensors for boundary-layer studies demonstrates that for boundary-layer depth and for profiles of many mean quantities, remote sensors offer some of the most accurate measurements available. Similar accuracies are in general not found for most turbulence parameters. Important contributions of remote sensors to our understanding of the structure and dynamics of various boundary-layer phenomena or processes are then discussed, including the sea breeze, convergence boundaries, dispersion, and boundary-layer cloud systems. The review concludes with a discussion of the likely future role of remote sensors in boundary-layer research.

Observed generation of an atmospheric gravity wave by shear instability in the mean flow of the planetary boundary layer

Observations of a single boundary-layer event — the generation of an atmospheric gravity wave by an unstable shear flow at Haswell, Colorado on November 12, 1971 — are briefly described and discussed. The observations were made using: (a) an acoustic echo sounder, (b) anemometers mounted at two fixed levels on a 150-m tower, (c) an anemometer and a thermometer mounted on a movable carriage on the tower, and (d) a microbarograph array, including one microbarograph mounted atop the tower. The wave phase velocity (−3.5–4.0 m s−1) was found to equal the wind velocity in the middle of the shear flow, as assumed by other authors. The wave-associated vertical fluxes of momentum and energy measured just above the wave critical layer were estimated to be −5 dyn cm−2 and −800 erg cm−2 s−1, respectively. These are large values. The annual average vertical flux of momentum at temperate and high latitudes is −0.25 dyn cm−2, while the average kinetic energy dissipation rate in a unit column of atmosphere is −5 × 103 erg cm−2 s−1. If the region of wave generation was itself propagating horizontally, its propagation velocity was large compared with the horizontal phase speed of the small-scale waves generated. Wave generation appeared to occur over an area large compared with the size of the microbarograph array (i.e., ≫ 2 km).

Measurement of Cloud Droplet Size Spectra by Doppler Radar

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” ...

The Potential of 8-mm Radars for Remotely Sensing Cloud Drop Size Distributions

Abstract This paper describes the use of a vertically pointing 8.6-mm-wavelength Doppler radar for measuring drop size spectra in clouds. The data used were collected in the Atlantic Stratocumulus Transition Experiment in 1992. This paper uses the full Doppler velocity spectrum from the time series of Doppler radial velocities to extract information farther into the small-drop regime than previously attempted. The amount of liquid residing in the cloud regime is compared with that found in the precipitation regime where drop fall velocities are resolvable. Total liquid is compared with that measured with a collocated three-channel microwave radiometer. Examples of number density spectra, liquid water spectra, and flux spectra are shown and compared with what is known of these quantities from various in situ measurements by aircraft in similar clouds. Error estimates and uncertainties are discussed. It is concluded that 8-mm Doppler radars have the potential for broader use in cloud and precipitation studi...

Profiles of radio refractive index and humidity derived from radar wind profilers and the Global Positioning System

It has often been pointed out that the Bragg backscatter of radar waves from elevated turbulent layers is very highly correlated with the height gradient of radio refractive index (RI) through these layers. However, many users need the profiles of RI, or the associated humidity, rather than profiles of their gradients. Simple integration of the gradients is usually not feasible because of ground or sea clutter and because biological scatterers such as insects and birds often severely contaminate the lower range gates. We show that if the total height-integrated RI is independently available (say, from the Global Positioning System (GPS)), and if the surface value of RI is known, the profiles of RI are retrievable with good accuracy. For those profiler systems equipped with a radio acoustic sounding system to measure temperature, the humidity is also retrievable. The method is demonstrated with data collected in southern California, where 7 hours of profiler data were recorded at 449 MHZ along with GPS data. Three radiosonde balloons were launched during the period, and the profiles of RI from the balloon and the profiler are compared. The advantages of the system are its invulnerability to nonprecipitating clouds (at frequencies of 449 MHZ or lower) and that it uses only facilities that will soon be deployed globally. Simulations are used to assess errors from various factors such as loss of sign of the gradient of the potential RI (important especially during some frontal events) and the presence of biological contaminants in some geographical areas (such as coastal zones and some agricultural areas at night).

Cloud Layers, Particle Identification, and Rain-Rate Profiles from ZRVf Measurements by Clear-Air Doppler Radars

Abstract Networks of radars that point almost vertically and continuously measure the vertical profile of the horizontal wind will, in the future, be operated at many locations around the world. Although such radars are designed to measure the Doppler-sensed movement of clear-air refractive-index inhomogeneities, they are an exceptional tool for sensing precipitating ice and water particles in clouds. Because of the low detection threshold and long averaging time of these radars water-particle-size distributions can be measured down to 100-µm diameter and mean vertical fall velocities Vf as small as 0.2 m s−1 can be accurately measured. In this paper, data are presented from two events in which clouds form, intensify, and finally produce precipitation. Height profiles are analyzed in terms of ZRVf plots versus height, where Z is the radar reflectivity factor and R is liquid flux (rainfall rate). The observations provide new insight into drop-growth and breakup processes. Special attention is given to the ...

Effect of wind shear on atmospheric wave instabilities revealed by FM/CW radar observations

An FM/CW radar sounding system designed and built by one of us (Richter, 1969) reveals atmospheric wave structure in unparalleled detail.The most outstanding features evident in the record are; internal gravity waves; features resembling Kelvin/Helmholtz instability structures; and multiple layering, often with lamina only a few meters thick.This paper shows a variety of atmospheric structural patterns and compares them with several hypothetical models of internal waves to obtain more insight into the atmospheric processes at work. Special attention is given to the distribution of the Richardson number in trapped and untrapped gravity waves. It is proposed that the multiple layers result from untrapped internal gravity waves whose propagation vector is directed nearly vertically within very stable height regions. It is argued that the layers are caused by dynamic instability resulting from reduction in the Richardson number due to wave induced shear and to some background wind shear when the amplitude-to-wavelength ratio grows during propagation into thermally stable height regions of the atmosphere.

Relationship of the Variances of Temperature and Velocity to Atmospheric Static Stability—Application to Radar and Acoustic Sounding

Abstract The relationship between the variances of temperature and vertical velocity fluctuations is examined experimentally and theoretically. Comparison of the variance data and the mean gradient data recorded on the 300 m tower at the Boulder Atmospheric Observatory leads to the conclusion that the remotely sensed ratio of the temperature and velocity variances offers hope of measuring gradients of temperature and radar refractive index from ground-based acoustic or radar clear-air sounders. Relationships in which temperature gradient depends only on the ratio of the variances of temperature and vertical velocity are found both from the flux equation and from the energy budget/temperature variance equations. From the two independent relations, a theoretical expression for Prandtl number versus Richardson number is found for a limited range of Richardson numbers. Finally, the character and magnitude of the influence of the stress and conductivity terms are estimated from the linearized problem, and solu...

Measurement of Humidity Profiles in the Atmosphere by the Global Positioning System and Radar Wind Profilers

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 ...