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K. R. Hardy

Abstract

Computations of the changes of the raindrop-size distributions with distance fallen are made with a computer. With the assumption of a steady mass flux of raindrops just below the melting level, changes brought about in the distribution through coalescence among raindrops, by accretion of cloud droplets, and by evaporation are considered. The numerical procedures which are used remove all restraints on the form of the initial raindrop-size distribution and on the properties of the cloud and the atmosphere through which the drops are falling.

Raindrop-size distributions may frequently be expressed satisfactorily by a function of the form:where D is the drop diameter, NDdD the number of drops of diameter between D and D + dD in unit volume of space, N 0 the value of ND for D = 0, and Λ is the magnitude of the slope of the distribution. It is found that an initial distribution having a relatively large slope is considerably modified by the processes of coalescence, accretion and evaporation. Whereas the number of smaller drops is markedly depleted by each process, the number of larger drops is increased by coalescence and accretion but is decreased by evaporation. A distribution with a relatively small slope is only slightly modified by the three processes. By considering raindrop-size distributions with various slopes but equal rainfall intensity, it is found that the depletion of cloud liquid water content increases as the slope of the distribution becomes larger. The amount of evaporation also increases as the slope of the distribution increases.

A procedure is presented whereby the raindrop-size distribution at the melting level can be deduced. This is possible by combining the information obtained from the computations of the change in the distribution below the melting level with the observed distribution at the ground. One study of this type for the light rain of 31 July 1961 at Flagstaff, Ariz., shows that the observed distribution at the surface must develop from a distribution aloft which has more large drops and fewer small drops than indicated by the Marshall and Palmer distribution.

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P. W. Rosenkranz, K. D. Hutchison, K. R. Hardy, and M. S. Davis

Abstract

Cross-track and conical scan microwave sounder designs are compared with respect to temperature profile retrieval accuracy in nonprecipitating atmospheres and with respect to the beamfilling effect of precipitation. The conical design shows slightly better accuracy at pressure levels of 3 hPa or less, while the cross-track design performs slightly better at pressure levels of 850 hPa or greater. Under the assumption that precipitation-contaminated fields of view would be rejected, consideration of beamfilling by rain cells indicates that retrieval yield would be higher for the cross-track design.

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Sam Hardy, Juliane Schwendike, Roger K. Smith, Chris J. Short, Michael J. Reeder, and Cathryn E. Birch

Abstract

The key physical processes responsible for inner-core structural changes and associated fluctuations in the intensification rate for a recent, high-impact western North Pacific tropical cyclone that underwent rapid intensification [Nepartak (2016)] are investigated using a set of convection-permitting ensemble simulations. Fluctuations in the inner-core structure between ringlike and monopole states develop in 60% of simulations. A tangential momentum budget analysis of a single fluctuation reveals that during the ringlike phase, the tangential wind generally intensifies, whereas during the monopole phase, the tangential wind remains mostly constant. In both phases, the mean advection terms spin up the tangential wind in the boundary layer, whereas the eddy advection terms deepen the storm’s cyclonic circulation by spinning up the tangential wind between 1.5 and 4 km. Calculations of the azimuthally averaged, radially integrated vertical mass flux suggest that periods of near-constant tangential wind tendency are accompanied by a weaker eyewall updraft, which is unable to evacuate all the mass converging in the boundary layer. Composite analyses calculated from 18 simulations produce qualitatively similar results to those from the single case, a finding that is also in agreement with some previous observational and modeling studies. Above the boundary layer, the integrated contribution of the eddy term to the tangential wind tendency is over 80% of the contribution from the mean term, irrespective of inner-core structure. Our results strongly indicate that to fully understand the storm’s three-dimensional evolution, the contribution of the eddies must be quantified.

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W. Ho, H. H. Wang, W. F. Hall, W. Norris, W. N. Hardy, K. W. Gray, and G. M. Hidy

Abstract

No abstract available.

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R. Ware, M. Exner, D. Feng, M. Gorbunov, K. Hardy, B. Herman, Y. Kuo, T. Meehan, W. Melbourne, C. Rocken, W. Schreiner, S. Sokolovskiy, F. Solheim, X. Zou, R. Anthes, S. Businger, and K. Trenberth

This paper provides an overview of the methodology of and describes preliminary results from an experiment called GPS/MET (Global Positioning System/Meteorology), in which temperature soundings are obtained from a low Earth-orbiting satellite using the radio occultation technique. Launched into a circular orbit of about 750-km altitude and 70° inclination on 3 April 1995, a small research satellite, MicroLab 1, carried a laptop-sized radio receiver. Each time this receiver rises and sets relative to the 24 operational GPS satellites, the GPS radio waves transect successive layers of the atmosphere and are bent (refracted) by the atmosphere before they reach the receiver, causing a delay in the dual-frequency carrier phase observations sensed by the receiver. During this occultation, GPS limb sounding measurements are obtained from which vertical profiles of atmospheric refractivity can be computed. The refractivity is a function of pressure, temperature, and water vapor and thus provides information on these variables that has the potential to be useful in weather prediction and weather and climate research.

Because of the dependence of refractivity on both temperature and water vapor, it is generally impossible to compute both variables from a refractivity sounding. However, if either temperature or water vapor is known from independent measurements or from model predictions, the other variable may be calculated. In portions of the atmosphere where moisture effects are negligible (typically above 5–7 km), temperature may be estimated directly from refractivity.

This paper compares a representative sample of 11 temperature profiles derived from GPS/MET soundings (assuming a dry atmosphere) with nearby radiosonde and high-resolution balloon soundings and the operational gridded analysis of the National Centers for Environmental Prediction (formerly the National Meteorological Center). One GPS/MET profile was obtained at a location where a temperature profile from the Halogen Occultation Experiment was available for comparison. These comparisons show that accurate vertical temperature profiles may be obtained using the GPS limb sounding technique from approximately 40 km to about 5–7 km in altitude where moisture effects are negligible. Temperatures in this region usually agree within 2°C with the independent sources of data. The GPS/MET temperature profiles show vertical resolution of about 1 km and resolve the location and minimum temperature of the tropopause very well. Theoretical temperature accuracy is better than 0.5°C at the tropopause, degrading to about 1°C at 40-km altitude.

Above 40 km and below 5 km, these preliminary temperature retrievals show difficulties. In the upper atmosphere, the errors result from initial temperature and pressure assumptions in this region and initial ionospheric refraction assumptions. In the lower troposphere, the errors appear to be associated with multipath effects caused by large gradients in refractivity primarily due to water vapor distribution.

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