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J. C. Alpert and S. K. Avery

Abstract

A steady-state, linear, quasi-geostrophic model of stationary waves on a sphere is employed to study the lower boundary forcing of airflow over topography and the internal forcing that results from the geographical distribution of diabatic heating. The lower boundary vertical motions forced by airflow over topography are shown to depend on the following: 1) whether or not consideration is made of the horizontal deflection of airflow around topographic features; 2) the level of the wind profile at which flow over topography is assumed to take place; and 3) the topographic data set that was used in the forcing formulation. Different methods of calculating the lower boundary vertical motions give rise to sizeable differences in the calculated planetary waves. Large uncertainties are also found in the modeled results depending on the choices made as to the vertical distribution of the forcing by diabatic heating. Given these uncertainties, the relative roles of topographic forcing and diabatic heating in forcing stationary planetary waves are explored in an alternative manner. The lower boundary forcing is taken to be given by the observed stationary planetary wave in lower boundary (900 mb) geopotential height, and the internal forcing is computed using the planetary wave propagation equation on the observed wave structure. Using this method, it is found that the lower boundary forcing generally accounts for the phase structure of the stationary planetary waves, and the response to the internal forcing generally acts to destructively interfere with the response from the lower boundary forcing. This interference is larger for wavenumber 2 in the stratosphere than for wavenumber 1.

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W. B. Maguire II and S. K. Avery

Abstract

The behavior of precipitation is of great importance in obtaining a better understanding of heat transport estimates and global processes in the atmosphere. This paper discusses improvements in an earlier raindrop size distribution model that utilizes two Doppler wind profilers to obtain accurate measurements of rainfall. A UHF Doppler wind profiler provides a precipitation return while a VHF Doppler wind profiler provides vertical clear-air velocities. The model is tested using simulated data. Sensitivity tests examine the model's ability to estimate rainfall characteristics given typical (noisy) Doppler spectra, and the model's sensitivity to atmospheric parameters. Because it is necessary to estimate pressure, temperature, raindrop temperature, and water vapor pressure at the location of the radar volume, the sensitivity tests examine the effects of errors in the estimates of these parameters. Sensitivity testing of the raindrop size distribution model with simulated data indicates model accuracy and reliability and is necessary in the interpretation of results using real data. Testing shows that the model is very capable at estimating even multiple-peaked drop size distributions from noisy UHF spectra. Sensitivity analyses of atmospheric parameters reveal that model results are quite robust, yielding good accuracy for a wide range of atmospheric parameter estimation errors.

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G. A. Jones and S. K. Avery

Abstract

The effects of the zonal mean circulation and planetary-wave winds on the distribution of nitric oxide in the 55–120 km height region is investigated. A time-dependent numerical model is used to investigate the interaction between planetary waves and the zonal mean circulation, and the effect of the circulation on the nitric oxide distribution is determined. The initial nitric oxide (NO) distribution is obtained by using a simple source/sink chemistry, vertical eddy diffusion, and advective transport by the zonal mean circulation. Changes in the initial NO distribution which result from the addition of planetary-wave winds are described. Planetary waves are found to induce a wave-like structure in the nitric oxide distribution which resembles that derived from observational data. Planetary waves can affect the nitric oxide concentration in two ways: first,through the wave-induced changes in the mean meridional circulation, and second, through the nitric oxide perturbation induced by wave winds themselves. The changes in total nitric oxide are due primarily to the zonal asymmetries in nitric oxide induced by the planetary waves. Implications of this result for explaining the winter anomaly are discussed.

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Wayne M. Angevine, S. K. Avery, and G. L. Kok

Abstract

Measurements of the turbulent virtual heat flux in the convective atmospheric boundary layer made with a 915-MHz boundary-layer wind profiler-radio acoustic sounding system (RASS) are compared to flux measurements from a King Air aircraft. The profiler-RASS flux was calculated by a refined eddy correlation technique. The measurements were made during the Rural Oxidants in the Southern Environment II experiment in June 1992. The area over which the measurements were made is primarily pine forest, and the dominant weather conditions were hot with light winds. The profiler-RASS measurements and the aircraft measurements agree well. Even under these light wind conditions, a 2-h-average profiler-RASS measurement may be sufficiently accurate to be useful. The instrumental error is estimated to be less than the uncertainty due to sampling of the turbulence.

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Robert Schafer, Susan K. Avery, and Kenneth S. Gage

Abstract

VHF wind profiler measurements of zonal and meridional winds are compared with the NCEP–NCAR reanalysis at sites in the tropical Pacific. By December 1999 the profilers at Darwin, Australia, and Biak, Indonesia, in the western Pacific; Christmas Island, Kiribati, in the central Pacific; and Piura Peru, in the eastern Pacific had collected between 8 and 13 yr of nearly continuous data. While these profilers routinely observe winds up to about 20 km, only winds at Christmas Island are assimilated into the reanalysis. The long period of profiler operation provides an opportunity to study differences between the profiler and reanalysis winds in the equatorial Pacific, a region with geographically sparse observations. Mean and seasonal mean zonal and meridional winds are used to identify differences in the profiler and reanalysis winds. Two potential causes for the discrepancy between profiler and reanalysis winds are identified. The first of these is related to different spatial and temporal characteristics of the reanalysis and profiler data. The second cause is the geographical sparseness of rawinsonde data, and not assimilating wind profiler observations. The closest agreement between the mean and seasonal mean zonal winds was found at Christmas Island, a site at which profiler winds are assimilated. A good agreement between reanalysis and profiler meridional and zonal winds is also shown at Darwin, where nearby rawinsonde observations are available. The poorest agreement was found at Piura (where profiler winds are not assimilated), the closest rawinsonde is almost 2000 km from the profiler site, and topography is not adequately resolved in the reanalysis.

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Robert Schafer, Susan K. Avery, Kenneth S. Gage, and George N. Kiladis

Abstract

UHF (boundary layer) and VHF (troposphere–stratosphere) wind profilers have operated at Christmas Island (2°N, 157°W) in the central equatorial Pacific from 1986 to 2002. Observed profiles of winds are sparse over the tropical oceans, but these are critical for understanding convective organization and the interaction of convection and waves. While the zonal winds below about 10 km have previously shown good agreement with the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis (RI), significant differences were found above a height of 10 km that were attributed to the low detectability of the wind signal in the profiler observations. Meridional winds at all levels show less agreement, with differences attributed to errors of representativeness and the sparseness of observations in the region. This paper builds on previous work using the Christmas Island wind profilers and presents the results of reprocessing the 17-yr profiler record with techniques that enhance the detectability of the signal at upper heights. The results are compared with nearby rawinsonde soundings obtained during a special campaign at Christmas Island and the RI, NCEP–Department of Energy (DOE) reanalysis (RII), and the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). The newly processed profiler zonal and meridional wind observations show good agreement with rawinsonde observations from 0.5 to 19 km above sea level, with difference statistics similar to other studies. There is also significant improvement in the agreement of RI and RII reanalysis and profiler upper-level zonal and meridional winds from previous studies. A comparison of RII and ERA-40 reanalysis shows that difference statistics between the reanalyses are similar in magnitude to differences between the profiler and the individual reanalyses.

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Robert Schafer, Susan K. Avery, Kenneth S. Gage, Paul E. Johnston, and D. A. Carter

Abstract

A method is presented that increases the detectability of weak clear-air signals by averaging Doppler spectra from coplanar wind profiler beams. The method, called coplanar spectral averaging (CSA), is applied to both simulated wind profiler spectra and to 1 yr of archived spectra from a UHF profiler at Christmas Island (1 October 1999–30 September 2000). A collocated 50-MHz wind profiler provides a truth for evaluating the CSA technique.

In the absence of precipitation, it was found that CSA, when combined with a fuzzy logic quality control, increases the height coverage of the 1-hourly averaged UHF profiler winds by over 600 m (two range gates). CSA also increased the number of good wind estimates at each observation range by about 10%–25% over the standard consensus method.

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Wayne M. Angevine, S. K. Avery, W. L. Ecklund, and D. A. Carter

Abstract

A 915-MHz boundary-layer wind profiler radar with radio acoustic sounding system (RASS) capability has been used to measure the turbulent fluxes of heat and momentum in the convective boundary layer by eddy correlation. The diurnal variation of the heat flux at several heights between 160 and 500 m above ground level and values of the momentum flux for 2-h periods in midday from 160 to 1000 m are presented, as well as wind and temperature data. The momentum flux is calculated both from the clear-air velocities and from the RASS velocities, and the two results are compared.

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Robert Cifelli, Christopher R. Williams, Deepak K. Rajopadhyaya, Susan K. Avery, Kenneth S. Gage, and P. T. May

Abstract

Drop-size distribution characteristics were retrieved in eight tropical mesoscale convective systems (MCS) using a dual-frequency (UHF and VHF) wind profiler technique. The MCSs occurred near Darwin, Australia, during the 1993/94 wet season and were representative of the monsoon (oceanic) regime. The retrieved drop-size parameters were compared with corresponding rain gauge and disdrometer data, and it was found that there was good agreement between the measurements, lending credence to the profiler retrievals of drop-size distribution parameters. The profiler data for each MCS were partitioned into a three-tier classification scheme (i.e., convective, mixed convective–stratiform, and stratiform) based on a modified version of to isolate the salient microphysical characteristics in different precipitation types. The resulting analysis allowed for an examination of the drop-size distribution parameters in each category for a height range of about 2.1 km in each MCS.

In general, the distributions of all of the retrieved parameters showed the most variability in convection and the least in stratiform, with the mixed convective–stratiform category usually displaying intermediate characteristics. Although there was significant overlap in the range of many of the parameter distributions, the mean profiles were distinct. In the stratiform region, there was minimal vertical structure for all of the drop-size distribution parameters. This result suggests an equilibrium between depletion (e.g., evaporation) and growth (e.g., coalescence) over the height range examined. In contrast, the convective parameter distributions showed a more complicated structure, probably as a consequence of the complex microphysical processes occurring in the convective precipitation category.

Reflectivity–rainfall (Z–R) relations of the form Z = AR B were developed for each precipitation category as a function of height using linear regressions to the profiler retrievals of R and Z in log space. Similar to findings from previous studies, the rainfall decreased for a given reflectivity as the precipitation type changed from convective to stratiform. This result primarily was due to the fact that the coefficient A in the best-fit stratiform Z–R was approximately a factor of 2 greater than the convective A at all heights. The coefficient A generally increased downward with height in each category; the exponent B showed a small decrease (stratiform), almost no change (convective), or a slight increase (mixed convective–stratiform). Consequently, the amount by which convective rain rate exceeded stratiform (for a given reflectivity) varied significantly as a function of height, ranging from about 15% to over 80%.

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Deepak K. Rajopadhyaya, Peter T. May, Robert C. Cifelli, Susan K. Avery, Christopher R. Willams, Warner L. Ecklund, and Kenneth S. Gage

Abstract

Two different frequency radar wind profilers (920 and 50 MHz) were used to retrieve rain rates from a long-lasting rainfall event observed near Darwin, Northern Territory, Australia, during the 1993–94 wet season. In this technique, 50-MHz data are used to derive the vertical air motion parameters (vertical velocity and spectral width); the 920-MHz data are then used to obtain the precipitation characteristics with the vertical air motion corrections. A comparison of the retrieved rain rates with rain gauge measurements shows excellent agreement. A detailed examination of the mean vertical velocity and spectral width corrections in the rain retrieval shows that the error due to an uncorrected mean vertical velocity can be as large as 100%, and the error for an uncorrected spectral width was about 10% for the range of mean vertical velocity and spectral width considered. There was a strong functional dependence between the retrieved mean vertical velocity and percentage difference between observed and retrieved rain rates with and without vertical air motion corrections. The corresponding functional dependence with and without the spectral width corrections was small but significant. An uncorrected upward mean vertical velocity overestimates rain rates, whereas an uncorrected downward mean vertical velocity underestimates rain rates. Uncorrected spectral width estimates have a tendency to overestimate rain rates. There are additional errors in the width correction because of antenna beam mismatching. A method is discussed to quantitatively evaluate this effect, and it is shown to be relatively small compared to the first-order mean vertical velocity correction.

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