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A. S. Frisch and S. F. Clifford

Abstract

Simultaneous atmospheric acoustic echo sounding and Doppler radar measurements were used to study some of the characteristics of an unstable boundary layer capped by an inversion. The resultant set of observations shows that the turbulent energy dissipation rate was constant with height up through the inversion layer. Since production from buoyancy and shear were found to be negligible in the upper part of the convective layer, the turbulent energy must be supplied by energy flux convergence. The stress at the top of the momentum layer was estimated to be approximately 2.5 dyn cm−2, assuming the energy flux divergence, term was small above the inversion. In addition, the height where the downward heat flux above the inversion went to zero and the height where the shearing stress went to zero were different. For the case analyzed in this paper, the apparent height at which the heat flux went to zero was 0.6 times the height where the shearing stress became zero.

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A. S. Frisch and B. L. Weber

Abstract

The distribution of backscattered power was computed for three wind profilers in the Colorado network that operated at 50,405, and 915 MHz. Since the backscattered power is a function of fluctuations in the refractivity index, this power distribution also gives the relative distribution of C 2 n. Similar distributions were found for all three frequencies in the lower troposphere where the atmosphere is often well mixed. But near and above the tropopause the distributions for the three frequencies different, probably because they responded to different processes in the atmosphere.

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A. S. Frisch and G. R. Ochs

Abstract

Aircraft measurements of C T2 in an unstable marine boundary layer suggest a modification of the surface layer free-convection model. This modification is given by a function of z/zi, where z is the observation height and zi the height to the inversion base. This variation of C T2 with height may be expressed as z −4/3[1 + 0.84(z/zi) + 4.13(z/zi)2] for 0 ≤ z/zi ≤ 0.8.

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A. S. Frisch and R. G. Strauch

Abstract

In this paper we report the results of measurements of turbulent kinetic energy dissipation rates within a convective storm. The measurements were obtained with two Doppler radars, one scanning the storm from a distance at low elevation angles and the other pointing vertically with the storm passing overhead. With the scanning radar we measured the wind shear in the radial velocity field and the turbulent kinetic energy dissipation rates within the storm. These dissipation rates showed good agreement with those measured by the zenith-pointing radar data; dissipation rates ranged from 30 cm2 s−3 to greater than 3500 cm2 s−3 in the region between the updraft and downdraft.

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Graham Feingold, W. R. Cotton, Bjorn Stevens, and A. S. Frisch

Abstract

This paper considers the production of drizzle in statocumulus clouds in relation to the boundary-layer turbulent kinetic energy and in-cloud residence times. It is shown that drizzle production in statocumulus of the order of 400 m in depth is intimately related to the vertical velocity structure of the cloud eddies. In a series of two dimensional numerical experiments with fixed cloud condensation nucleus concentrations, the effect on drizzle production of enhanced or diminished vertical velocities is simulated. Rather than do this by simulating clouds exhibiting more or less energy, we modify drop terminal velocities in a manner that conserves the fall velocity relative to the air motions and allows droplet growth to occur in a similar dynamical environment. The results suggest that more vigorous clouds produce more drizzle because they enable longer in-cloud dwell times and therefore prolonged collision-coalescence. In weaker clouds, droplets tend to fall out of the cloud before they have achieved significant size, resulting in smaller amounts of drizzle. In another series of experiments, we investigate the effects of the feedback of drizzle on the boundary-layer dynamics. Results show that when significant amounts of drizzle reach the surface, the subcloud layer is stabilized, circulations are weaker, and the boundary layer is not well mixed. When only small amounts of drizzle are produced, cooling tends to be confined to the region just below cloud base, resulting in destabilization, more vigorous circulations, and a better mixed boundary layer. The results strongly suggest that a characteristic time associated with collision-coalescence be incorporated into drizzle parameterizations.

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A. S. Frisch, C. W. Fairall, and J. B. Snider

Abstract

Data are used from a Kα-band radar and microwave radiometer along with a droplet model to determine the droplet parameters of drizzle and clouds. Drizzle droplet parameters are determined from the zeroth, first, and second moments of the Doppler spectrum. Cloud droplet parameters are determined from the zeroth moment of the Doppler spectrum and the measured integrated liquid water. Measurements of stratus clouds were made during the Atlantic Stratocumulus Transition Experiment (ASTEX) on the island of Porto Santo in the Madeira Islands, Portugal. Potential applications of this technique would be in the long-term monitoring of stratus clouds and in determining the vertical profiles of cloud liquid water, number of cloud droplets, and modal radius.

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A. S. Frisch, D. H. Lenschow, C. W. Fairall, W. H. Schubert, and J. S. Gibson

Abstract

A cloud-sensing Doppler radar is used with a vertically pointing antenna to measure the vertical air motion in clouds during the Atlantic Stratocumulus Transition Experiment. The droplet fall velocity contamination was made negligible by using only measurements during the time the reflectivity was below − 17 dBZ. During one day of measurements, the daytime character of the vertical velocity variance is different than that of the nighttime case. In the upper part of the cloud, the variance had a distinct maximum for both day and night; however, the nighttime maximum was about twice as large as the daytime case. Lower down in the cloud, there was a second maximum, with the daytime variance larger than the nighttime case. The skewness of the vertical velocity was negative near cloud top in both the day and night cases, changing to positive skewness in the lower part of the cloud. This behavior near cloud top indicates that the upper part of the cloud is behaving like an upside-down convective boundary layer, with the downdrafts smaller in area and more intense than the updrafts. In the lower part of the cloud, the behavior of the motion is more like a conventional convective boundary layer, with the updrafts smaller and more intense than the downdrafts. The upside-down convective forcing in the upper part of the cloud is due to radiative cooling, with the daytime forcing less because of shortwave warming.

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A. S. Frisch, B. L. Weber, R. G. Strauch, D. A. Merritt, and K. P. Moran

Abstract

The maximum height performance of the 50, 405 nd 915 MHz Colorado wind profiles is computed from the wind profiler database. Results show that even though the 50 MHz profiler has the largest seasonal variation in the maximum height coverage, it also has the greatest height coverage. In addition, it also has a greater increase in height for the same increase in sensitivity. On the basis of thew measurements we predict the height coverage of the 405 MHz wind profiler for the proposed wind profiler network.

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A. S. Frisch, B. L. Weber, D. B. Wuertz, R. G. Strauch, and D. A. Merritt

Abstract

We computed the monthly average backscattered power over a five-year period for the Fleming 50 MHz wind profiler, which is proportional to CN 2. We found that in addition to seasonal cycle in CN 2 below the tropopause, there was a year-to-year variation as well. Above the tropopause, the seasonal variations were almost gone; however, there were significant changes with periods longer than one year. We examined a shorter back-scattered power record from the Stapleton wind profiler and found similar longer-term trends. These long-term trends will affect the performance of wind profilers.

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Bruce A. Albrecht, Christopher S. Bretherton, Doug Johnson, Wayne H. Scubert, and A. Shelby Frisch

The Atlantic Stratocumulus Experiment (ASTEX) was conducted over the northeast Atlantic Ocean during June 1992 with substantial international collaboration. The main goal of ASTEX was to study the climatologically important transition between solid stratocumulus and subtropical trade cumulus cloud regimes using island, aircraft, ship, and satellite measurements. Typically, the boundary layer was found to support cumulus clouds detraining into a patchy and fairly thin upper-stratocumulus layer. The substantial microphysical variability between clean marine and polluted continental air masses observed during ASTEX affected both drizzle and cloud properties. Highlights of the ASTEX research strategy included use of the ECMWF operational forecast model for assimilation of ASTEX soundings to obtain improved regional meteorological analyses; “Lagrangian” measurements of boundary-layer evolution following an air mass using aircraft and balloons, extensive coordinated use of surface, airborne, and satellite platforms; and an extensive suite of island-based remote sensing systems including millimeter-wavelength radars. A summary of ASTEX is presented and some initial results are presented.

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