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Jeffry Rothermel
and
Ernest M. Agee

Abstract

The phenomenon of selective scaling in shallow atmospheric convection is examined with the use of a two-dimensional, fine-resolution numerical model with a large domain aspect ratio. No extra model physics (such as latent heat release, eddy anisotropy, large-scale sinking motion, or radiative-entrainment effects) is incorporated in order to show that the preferred convective mode can be determined through the action of the nonlinear terms. The governing equations have the same form as that for Benard-Rayleigh convection, with the Rayleigh number being varied over approximately three orders of magnitude times supercritical.

The scale of the convection in the steady state solutions is found to increase with Rayleigh number. At the largest Rayleigh numbers considered, the scale increases with time from initial modes with aspect ratio of roughly unity, to modes with aspect ratio larger by nearly one order of magnitude. The results, supported by radar and aircraft observations of boundary layer clear-air convection and mesoscale cellular convection, corroborate the evidence to suggest a mesoscale organization of small-scale convective elements into a system with a broad horizontal scale. Such a process may be attributable to the nonlinearity inherent in the shallow convection problem as demonstrated in these model simulations.

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Jeffry Rothermel
and
Ernest M. Agee

Abstract

The 1 s data set collected by the NCAR Electra research aircraft in the presence of closed mesoscale cellular convection (MCC) has been examined for the purpose of determining the convective wind field and horizontal profiles of temperature and specific humidity. Two flight legs during AMTEX (16 February 1975) were selected for study: one at the 100 m level (case 1) and the other at the 970 m level (case 2), 30 m below cloud base

Two closed cells with diameters of 39,0 and 33.0 km were traversed in case 1. Filtered virtual temperature data showed a double-cycle variation, with an average difference between warm and cool regions of 0.32 K. The difference in specific humidity between moist cell center and dry cell wag was ∼1.0 g kg−1. The vertical velocity power spectrum showed no MCC-scale energy. Horizontal wind velocity data indicated convergence toward cell center with mesoscale wind velocity components ∼15 m s−1. The data for case 2 encompassed two closed cells, with traverse lengths ∼32 km each. The difference between dry cell wall and moist cell center was ∼0.7 g kg −1 Filtered vertical velocity showed downward motion in cell walls and upward motion near cell center, the difference being ∼1.0 m s−1 Filtered virtual temperature data indicated a weak single-cycle variation. Horizontal convective velocities were 0.75 m s−1

A physical explanation of the double-cycle temperature profile at the 100 m level is offered based on the combined effects of 1) warm air entrainment from an overlying inversion layer; 2) radiative cooling at cloud-top level, side-wall mixing and evaporative cooling, and subsequent (partial) moist adiabatic descent of air; and 3) sensible heating at cell center due to the warm sea surface.

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Jeffry Rothermel
,
Cathy Kessinger
, and
Darien L. Davis

Abstract

Two 10.6 μm coherent Doppler lidars participated in the Joint Airport Weather Studies (JAWS) Project field experiment, conducted in summer 1982 near Denver's Stapleton International Airport. One was operated by NOAA/ERL, Wave Propagation Laboratory (WPL), the other by NASA, Marshall Space Flight Center (MSFC). Periodic coordinated scans were made with the two lidars spared 15 km apart. This permitted the calculation of Cartesian winds. This paper presents 1) a brief comparison of radar and lidar system and performance characteristics, 2) results of the first dual-Doppler analyses to be based upon lidar measurements and 3) a comparison of radial wind estimates between the MSFC lidar and a 5.5 cm Doppler radar operated by the National Center for Atmospheric Research (NCAR).

Dual-Doppler analyses were made for the flow behind gust fronts, with the desired flow fields consistent with both surface winds measured by the NCAR Portable Automated Mesonet (PAM) and models derived from previous studies of Great Plains thunderstorm outflows. A comparison of low elevation scans made by the MSFC lidar and the NCAR CP-4 Doppler radar revealed distinct differences which could be explained by a bias in the radar estimates (toward weaker velocities) due to ground clutter contamination. Root-mean-square (rms) difference between radar- and lidar-measured radial velocities was 3.1 m s−1 which could be explained by other causes; however, the mean of the radar data set was 1–2 m s−1 lower than that of the lidar. These findings are consistent with a recent previous study comparing the WPL lidar with the NCAR CP-3 5.5 cm radar.

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Jeffry Rothermel
,
Dean R. Cutten
,
R. Michael Hardesty
,
Robert T. Menzies
,
James N. Howell
,
Steven C. Johnson
,
David M. Tratt
,
Lisa D. Olivier
, and
Robert M. Banta

In 1992 the atmospheric lidar remote sensing groups of the National Aeronautics and Space Administration Marshall Space Flight Center, the National Oceanic and Atmospheric Administration/Environmental Technology Laboratory (NOAA/ETL), and the Jet Propulsion Laboratory began a joint collaboration to develop an airborne high-energy Doppler laser radar (lidar) system for atmospheric research and satellite validation and simulation studies. The result is the Multicenter Airborne Coherent Atmospheric Wind Sensor (MACAWS), which has the capability to remotely sense the distribution of wind and absolute aerosol backscatter in three-dimensional volumes in the troposphere and lower stratosphere.

A factor critical to the programmatic feasibility and technical success of this collaboration has been the utilization of existing components and expertise that were developed for previous atmospheric research by the respective institutions. For example, the laser transmitter is that of the mobile ground-based Doppler lidar system developed and used in atmospheric research for more than a decade at NOAA/ETL.

The motivation for MACAWS is threefold: 1) to obtain fundamental measurements of subsynoptic-scale processes and features to improve subgrid-scale parameterizations in large-scale models, 2) to obtain datasets in order to improve the understanding of and predictive capabilities for meteorological systems on subsynoptic scales, and 3) to validate (simulate) the performance of existing (planned) satellite-borne sensors.

Initial flight tests were made in September 1995; subsequent flights were made in June 1996 following system improvements. This paper describes the MACAWS instrument, principles of operation, examples of measurements over the eastern Pacific Ocean and western United States, and future applications.

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