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J. R. Wang
,
J. Zhan
, and
P. Racette

Abstract

Radiometric measurements were made by a millimeter-wave imaging radiometer (MIR) at the frequencies of 89, 150, 183.3 ± 1, 183.3 ± 3, 183.3 ± 7, and 220 GHz aboard the NASA ER-2 aircraft at an altitude of about 20 km over two rainstorms: one in the western Pacific Ocean on 19 January 1993 and another in southern Florida on 5 October 1993. These measurements were complemented by nearly simultaneous observations by other sensors aboard the same aircraft and another aircraft flying along the same path. Analysis of data from these measurements, aided by radiative transfer and radar reflectivity calculations of hydrometeor profiles, which are generated by a general cloud ensemble model, demonstrates the utility of these frequencies for studying the structure of frozen hydrometeors associated with storms. Particular emphasis is placed on the three water vapor channels near 183.3 GHz. Results show that the radiometric signatures measured by these channels over the storm-associated scattering media bear a certain resemblance to those previously observed over a clear and fairly dry atmosphere with a cold ocean background. Both of these atmospheric conditions are characterized by a small amount of water vapor above a cold background. Radiative transfer calculations were made at these water vapor channels for a number of relative humidity profiles characterizing dry atmospheres over an ocean surface. The results are compared with the measurements to infer some characteristics of the environment near the scattering media. Furthermore, radiometric signatures from these channels display unique features for towering deep convective cells that could be used to identify the presence of such cells in storms.

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J. R. Wang
and
L. A. Chang

Abstract

Upwelling radiometric measurements at 90 GHz and three side bands near 183 GHz are used to retrieve water vapor profiles over the ocean surface. An algorithm incorporating a new technique of handling moderate cloud cover is illustrated for the profiling of both relative humidity and water vapor burden. It is shown that the retrieved relative humidity profiles reflect gross features of the corresponding profiles recorded by the radiosondes. However, the retrieval generally cannot produce fine details of the observed profiles at altitudes where a rapid change in relative humidity occurs. For this reason, comparison of retrieved and observed values at a given altitude often yields an appreciable rms error. Profiling of water vapor burden, a parameter equivalent to total integrated water vapor above a certain altitude, results in much better agreement, as expected. The rms error obtained from the results of the retrieval at the surface is comparable to that derived from the combination of measurements at 18 GHz and 21 GHz channels of the Scanning Multichannel Microwave Radiometer aboard the Nimbus 7 satellite.

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J. L. Lions
,
O. P. Manley
,
R. Temam
, and
S. Wang

Abstract

In a series of recent papers, some of the authors have addressed with mathematical rigor some aspects of the primitive equations governing the large-scale atmospheric motion. Among other results, they derived without evaluating it an expression for the dimension of the attractor for those equations.

It is known that the long-term behavior of the motion and states of the atmosphere can be described by the global attractor. Namely, starting with a given initial value, the solution will tend to the attractor as t goes to infinity. The dimension estimate of the global attractor is evaluated in this article, showing that this global attractor possesses a finite but large number of degrees of freedom. Using some arguments based on the known physical dissipation mechanisms, the bound on the dimension of the attractor in terms of the observable quantities governing the heating and energy dissipation accompanying the motion of the atmosphere is made immediately transparent. Consequently, to the extent that the resolution needed in numerical simulations of the long-term atmospheric motion is related to the dimension of the attractor, the result in this article suggests that the required resolution is quite sensitive to the magnitude of the effective (or eddy) viscosity, while it appears to be less sensitive to the details of the way that the atmosphere is heated.

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J. M. Toole
,
R. C. Millard
,
Z. Wang
, and
S. Pu

Abstract

Hydrographic surveys were conducted off the Philippine coast in September 1987 and April 1988 as part of the United States/People's Republic of China cooperative research program. These cruises sampled the western Pacific Ocean where the North Equatorial Current (NEC) meets the western boundary and divides into the Kuroshio and Mindanao Currents. The requirement for mass conservation within a region enclosed by stations is utilized here to obtain absolute circulation fields for the two surveys. In both realizations, the surface flow of the NEC was observed to bifurcate near latitude 13°N; NEC flow poleward of this latitude turned north as the Kuroshio while flow to the south fed the Mindanao Current. Most striking was a twofold increase in the strength of the current system in spring 1988 as compared with fall 1987. We note that the observations in fall 1987 were obtained during the height of the 1986/87 El Niño, while those in spring 1988 were during a cold phase of the El Niño/Southern Oscillation. It is not clear how the observed current changes relate to the evolution of this event. The potential vorticity (Q) distributions of the surface waters were examined to explore the dynamics of the bifurcation. Within the NEC, Q was nearly constant (layer thickness change balanced meridional planetary vorticity variation). Within the Kuroshio and Mindanao currents, near constant Q (with magnitude comparable to that in the NEC) was also found with a balance between relative vorticity variation and layer depth change as would be expected for inertia] boundary currents.

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S. J. Martin
,
P. K. Wang
, and
H. R. Pruppacher

Abstract

Two theoretical models are presented which allow computing the efficiency with which aerosol particles of radius 0.001 ≤ r ≤ 10 μm are collected by simple ice crystal plates of radius 50 ≤ ac ≤ 640 μm, in air of various relative humidity, temperature and pressure. Particle capture due to Brownian diffusion, thermophoresis, diffusiophoresis and inertial impaction is considered. It is shown that, analogous to water drops, ice crystal plates exhibit a minimum collection efficiency within a specific size interval of aerosol particles. This minimum is strongly affected by the relative humidity of the ambient air. The collection efficiency of particles with r > 1 μm is controlled by the flow field around the ice crystal, while the collection efficiency of particles with r < 0.01 μm is controlled by convective Brownian diffusion. Trajectory analysis predicts that aerosol particles are preferentially captured by the ice crystal rim. Our theoretical results are found to agree satisfactorily with laboratory studies presently available.

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H. Wang
,
R. J. Barthelmie
,
A. Clifton
, and
S. C. Pryor

Abstract

Defining optimal scanning geometries for scanning lidars for wind energy applications remains an active field of research. This paper evaluates uncertainties associated with arc scan geometries and presents recommendations regarding optimal configurations in the atmospheric boundary layer. The analysis is based on arc scan data from a Doppler wind lidar with one elevation angle and seven azimuth angles spanning 30° and focuses on an estimation of 10-min mean wind speed and direction. When flow is horizontally uniform, this approach can provide accurate wind measurements required for wind resource assessments in part because of its high resampling rate. Retrieved wind velocities at a single range gate exhibit good correlation to data from a sonic anemometer on a nearby meteorological tower, and vertical profiles of horizontal wind speed, though derived from range gates located on a conical surface, match those measured by mast-mounted cup anemometers. Uncertainties in the retrieved wind velocity are related to high turbulent wind fluctuation and an inhomogeneous horizontal wind field. The radial velocity variance is found to be a robust measure of the uncertainty of the retrieved wind speed because of its relationship to turbulence properties. It is further shown that the standard error of wind speed estimates can be minimized by increasing the azimuthal range beyond 30° and using five to seven azimuth angles.

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J. R. Wang
,
T. T. Wilheit
, and
L. A. Chang

Abstract

The strong water vapor absorption line at 183 GHz is explored in this paper for retrieval of total precipitable water in the atmosphere. This strong line has generally been utilized in the past for the profiling of the atmospheric water vapor. It is shown from radiative transfer calculations that, under very dry atmospheric conditions, the radiometric response near this frequency behaves much like that near the 22 GHz absorption line but, with the advantages of an increase in sensitivity and potentially an improvement in spatial resolution. Total precipitable water can be retrieval almost independent of atmospheric temperature profiles under these conditions. The technique is demonstrated with the airborne Advanced Microwave Moisture Sounder (AMMS) which has four channels, three of them centered around 183 GHz (183 ± 2 GHz, 183 ± 5 GHz, and 183 ± 9 GHz) and another at 92 GHz. The calculated sensitivities of radiometric response to total precipitable water are approximately 410, 230, and 130 K (cm)2 g−1 for total precipitable water less than 0.2, 0.3, and 0.5 g (cm)−2 at 183 ± 2 GHz, 183 ± 5 GHz, and 183 ± 9 GHz respectively. The inclusion of the 92 GHz channel extends the range of the retrieval in excess of 1 g (cm)−2 total precipitable water. However, the effect of cloud cover proves to be strong at this frequency and the retrieval has to be applied with care. Two AMMS observations of dry atmosphere following the cold air outbreaks are analyzed to demonstrate the technique.

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J. Y. Wang
,
C. R. Claysmith
, and
M. Griggs

Abstract

A ground-based infrared spectroradiometer has been used to measure the vertical temperature profile of the lower atmosphere from 0 to 6 km. Eight measurements in the 15-μm carbon dioxide band have been used for the inversion in addition to three measurements in the 18-μm water vapor band for the water vapor corrections. One additional observation in the 11-μm window region is used to determine the presence of cloud. Twenty-one sets of clear sky data obtained in the summer of 1971 are used to verify the inversion technique. The resultant profiles have an accuracy comparable to that of radiosondes with an overall rms error of 1.58°C.

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J. J. Martin
,
P. K. Wang
,
H. R. Pruppacher
, and
R. L. Pitter

Abstract

A theoretical model is presented which allows determination of the efficiency with which electrically charged, simple planar ice crystals collide with electrically charged supercooled cloud drops. The calculations are carried out for ice crystal plates of diameter between 100 and 1300 μm colliding with cloud drops of diameters between 2 and 170 μm. The electric charges Q (esu) residing on the drops and ice crystals were assumed to vary with the radius a (cm) of the drop or crystal according to Q=qa 2, with 0≤q≤2.0. Our results show that the efficiency with which supercooled drops the collected by simple Planar ice crystals is enhanced by electric charges, in particular, if q>0.8, where q=0.8 represents an electric charge still considerably below thunderstorm charge.

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J. Wang
,
M. R. Hjelmfelt
,
W. J. Capehart
, and
R. D. Farley

Abstract

Numerical simulations of two snowfall events over the Black Hills of South Dakota are made to demonstrate the use and potential of a coupled atmospheric and land surface model. The Coupled Atmospheric–Hydrologic Model System was used to simulate a moderate topographic snowfall event of 10–11 April 1999 and a blizzard event of 18–23 April 2000. These two cases were chosen to provide a contrast of snowfall amounts, locations, and storm dynamics. The model configuration utilized a nested grid with an outer grid of 16-km spacing driven by numerical forecast model data and an inner grid of 4 km centered over the Black Hills region. Simulations for the first case were made with the atmospheric model, the Advanced Regional Prediction System (ARPS) alone, and with ARPS coupled with the National Center for Atmospheric Research Land Surface Model (LSM). Results indicated that the main features of the precipitation pattern were captured by ARPS alone. However, precipitation amounts were greatly overpredicted. ARPS coupled with LSM produced a very similar precipitation pattern, but with precipitation amounts much closer to those observed. The coupled model also permits simulation of the resulting snow cover and snowmelt. Simulated percentage snow melting occurred somewhat more rapidly than that of the observed. Snow–rain discrimination may be taken from the precipitation type falling out of the atmospheric model based on the microphysical parameterization, or by the use of a surface temperature criteria, as used in most large-scale models. The resulting snow accumulation patterns and amounts were nearly identical. The coupled model configuration was used to simulate the second case. In this case the simulated precipitation and snow depth maximum over the eastern Black Hills were biased to the east and north by about 24 km. The resulting spatial correlation of the simulated snowfall and observations was only 0.37. If this bias is removed, the shifted pattern over the Black Hills region has a correlation of 0.68. Snow-melting patterns for 21 and 22 April appeared reasonable, given the spatial bias in the snowfall simulation.

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