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Matthew T. Vaughan
and
Robert G. Fovell

Abstract

Subgrid-scale turbulence in numerical weather prediction models is typically handled by a PBL parameterization. These schemes attempt to represent turbulent mixing processes occurring below the resolvable scale of the model grid in the vertical direction, and they act upon temperature, moisture, and momentum within the boundary layer. This study varies the PBL mixing strength within 4-km WRF simulations of a 26–29 January 2015 snowstorm to assess the sensitivity of baroclinic cyclones to eddy diffusivity intensity. The bulk critical Richardson number for unstable regimes is varied between 0.0 and 0.25 within the YSU PBL scheme as a way of directly altering the depth and magnitude of subgrid-scale turbulent mixing. Results suggest that varying the bulk critical Richardson number is similar to selecting a different PBL parameterization. Differences in boundary layer moisture availability, arising from reduced entrainment of dry, free tropospheric air, lead to variations in the magnitude of latent heat release above the warm frontal region, producing stronger upper-tropospheric downstream ridging in simulations with less PBL mixing. The more amplified flow pattern impedes the northeastward propagation of the surface cyclone and results in a westward shift of precipitation. In addition, trajectory analysis indicates that ascending parcels in the less-mixing simulations condense more water vapor and terminate at a higher potential temperature level than do ascending parcels in the more-mixing simulations, suggesting stronger latent heat release when PBL mixing is reduced. These results suggest that spread within ensemble forecast systems may be improved by perturbing PBL mixing parameters that are not well constrained.

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C. G. Mohr
and
R. L. Vaughan

Abstract

A fast and efficient procedure has been developed which allows the systematic interpolation of digital reflectivity data from radar space into Cartesian space. The algorithm is designed so that only one ordered pass through the original PPI scan data is necessary to complete the interpolation process. As a result, 100 cross sections may be interpolated and displayed for approximately five times the cost of producing PPI plots for the same volume. Computer-generated displays produced by the system include contoured and gray-scale plots of orthogonal sections and perspective images of two- and three-dimensional reflectivity surfaces.

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C. Dearden
,
G. Vaughan
,
T. Tsai
, and
J.-P. Chen

Abstract

Numerical simulations are performed with the Weather Research and Forecasting Model to elucidate the diabatic effects of ice phase microphysical processes on the dynamics of two slow-moving summer cyclones that affected the United Kingdom during the summer of 2012. The first case is representative of a typical midlatitude storm for the time of year, while the second case is unusually deep. Sensitivity tests are performed with 5-km horizontal grid spacing and at lead times between 1 and 2 days using three different microphysics schemes, one of which is a new scheme whose development was informed by the latest in situ observations of midlatitude weather systems. The effects of latent heating and cooling associated with deposition growth, sublimation, and melting of ice are assessed in terms of the impact on both the synoptic scale and the frontal scale. The results show that, of these diabatic processes, deposition growth was the most important in both cases, affecting the depth and position of each of the low pressure systems and influencing the spatial distribution of the frontal precipitation. Cooling associated with sublimation and melting also played a role in determining the cyclone depth, but mainly in the more intense cyclone case. The effects of ice crystal habit and secondary ice production are also explored in the simulations, based on insight from in situ observations. However in these two cases, the ability to predict changes in crystal habit did not significantly impact the storm evolution, and the authors found no obvious need to parameterize secondary ice crystal production at the model resolutions considered.

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T. G. Owe Berg
,
Thomas A. Gaukler
, and
Urte Vaughan

Abstract

Collision efficiencies E have been determined from particle trajectories for the case of a 1-mm glass sphere and 6–20 μ spherical glass particles falling in still air. An empirical formula for the dependence of E upon scavenger size, scavenger velocity, and particle terminal velocity has been derived.

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T. G. Owe Berg
,
Robert J. Trainor Jr.
, and
Urte Vaughan

Abstract

Experiments with charged water droplets show the existence of the metastable states predicted by Cahn as well as the well-known unstable state predicted by Lord Rayleigh. The charge lost at metastability is completely recovered with time, whereas the charge lost at instability is only partially recovered. The recovery of charge may be a space-charge effect, or it may be a result of electrification that accompanies the exchange of water vapor.

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T. G. Owe Berg
,
Thomas A. Gaukler
, and
Urte Vaughan

Abstract

The collision of a falling drop With a small particle has been studied by high-speed photography. The trajectory of the particle relative to the center of the drop and relative to a fixed point has been determined under various conditions. The effect of electrostatic charges on drop and particle has been studied.

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J. C. Fankhauser
,
C. J. Biter
,
C. G. Mohr
, and
R. L. Vaughan

Abstract

Objective numerical techniques are applied in analyzing constant altitude aircraft measurements obtained from coordinated research flights in thunderstorm inflow regions. The approach combines meteorological and flight track data from dual or single aircraft missions in a common frame of reference and transforms the observations from original analogue format to horizontal two-dimensional Cartesian coordinates. Operational procedures guiding the data collection, intercomparison techniques for refining instrument calibrations and corrections for aircraft navigation errors are all considered.

Results of the interpolations are judged in the context of the storms' associated radar echo features. Primary applications include calculation of water vapor influx in cloud base updrafts. Evidence indicates that the fullest exploitation of the inflow mapping will come through combining kinematic fields observed concurrently by aircraft and multiple Doppler radars.

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Carl G. Mohr
,
L. Jay Miller
,
Robin L. Vaughan
, and
Harold W. Frank

Abstract

During the 1981 summer season within a 70 000 km2 area surrounding Miles City, Montana, researchers from approximately twenty institutions participated in the Cooperative Convective Precipitation Experiment (CCOPE). The measurements collected during this project comprise one of the most comprehensive datasets ever acquired in and around individual convective storms on the high plains of North America. Principal data systems utilized during CCOPE included 8 ground-based radar (7 of which had Doppler capability), 12 instrumented research aircraft, and a network of 123 surface stations.

A major data processing goal has been to combine these independently acquired mesoscale measurements into a numerical description of observed atmospheric conditions at any point in time. Using the CCOPE data archive as an example, this paper describes the procedures used to reduce these high resolution observations to a common spatial and temporal framework. The final product is a digital description of the environment similar to that employed by most modelers—a three-dimensional Cartesian coordinate system containing fields that represent the instantaneous state of the atmosphere at discrete times across the period of interest. A software package designed to facilitate the construction and analysis of these composite data structures will also be discussed.

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Steven B. Alejandro
,
George G. Koenig
,
J. Michael Vaughan
, and
P. Hywel Davies

The plans for an atmospheric aerosol field measurement experiment are described. The South Atlantic Backscatter Lidar Experiment (SABLE) is an important step in determining the feasibility of obtaining space-based lidar measurements for the determination of atmospheric winds, and the spatial and temporal variation of aerosol backscatter. Two field campaigns operating from Ascension Island (a British colony of St. Helena) have been completed. The first field campaign obtained lidar backscatter measurements utilizing an airborne platform during a 3-week period from mid-October through mid-November 1988. The second SABLE program was expanded to include measurements utilizing airborne particle-measurement probes, ground-based lidars, radiosondes, and supporting meteorological equipment. This program was conducted during a 4-week period from mid-June through mid-July 1989. This is the first time that such a comprehensive, aerosol-measurement program has been undertaken in the Southern Hemisphere. SABLE-type measurements are not only pertinent to the development of space-based sensors, but also have direct utility in the study of aerosols and their effect on climate.

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H. Chepfer
,
G. Cesana
,
D. Winker
,
B. Getzewich
,
M. Vaughan
, and
Z. Liu

Abstract

Two different cloud climatologies have been derived from the same NASA–Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)-measured attenuated backscattered profile (level 1, version 3 dataset). The first climatology, named Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations–Science Team (CALIPSO-ST), is based on the standard CALIOP cloud mask (level 2 product, version 3), with the aim to document clouds with the highest possible spatiotemporal resolution, taking full advantage of the CALIOP capabilities and sensitivity for a wide range of cloud scientific studies. The second climatology, named GCM-Oriented CALIPSO Cloud Product (CALIPSO-GOCCP), is aimed at a single goal: evaluating GCM prediction of cloudiness. For this specific purpose, it has been designed to be fully consistent with the CALIPSO simulator included in the Cloud Feedback Model Intercomparison Project (CFMIP) Observation Simulator Package (COSP) used within version 2 of the CFMIP (CFMIP-2) experiment and phase 5 of the Coupled Model Intercomparison Project (CMIP5).

The differences between the two datasets in the global cloud cover maps—total, low level (P > 680 hPa), midlevel (680 < P < 440 hPa), and high level (P < 440 hPa)—are frequently larger than 10% and vary with region.

The two climatologies show significant differences in the zonal cloud fraction profile (which differ by a factor of almost 2 in some regions), which are due to the differences in the horizontal and vertical averaging of the measured attenuated backscattered profile CALIOP profile before the cloud detection and to the threshold used to detect clouds (this threshold depends on the resolution and the signal-to-noise ratio).

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