Journal of Applied Meteorology

Seeding-Opportunity Recognition in Winter Orographic Clouds

Mon, 01 Jan 1979 00:00:00 GMT

Seeding-Opportunity Recognition in Winter Orographic Clouds Hill, Geoffrey E. Detailed measurements of supercooled cloud water, precipitation, cloud-top temperature and vertical air motion in winter orographic clouds are used to develop criteria for the seedability of those clouds. Winter orographic clouds over the upwind mountain base with cloud-top temperatures between 0 and ?22°C are found to be primarily composed of supercooled water and are therefore seedable. The supercooled water concentration is empirically found to depend on the updraft velocity. The potential precipitation yield is dependent on the flux of supercooled water over the barrier. Because the updraft velocity is approximately proportional to the cross-barrier wind, the potential precipitation yield is approximately proportional to the square of the cross-barrier wind, provided that the cloud-top temperature is in the seedable range of temperatures. These findings are strongly substantiated by systematic use of aircraft icing reports over a full winter season (November?March 1978?79). It is shown that a cloud-top temperature of about ?22°C separates clouds with a precipitation enhancement potential from those without such a potential. It is found that aircraft icing is approximately proportional to the cross-barrier wind, and that the flux of supercooled water over the barrier for cloud-top temperatures warmer than ?22°C is (as derived from the research data) approximately proportional to the square of the cross-barrier wind. About 20% of cloud episodes over the mountains of northern Utah may be expected to have a high modification potential.

Trajectory Analysis of Summertime Sulfate Concentrations in the Northeastern United States

Mon, 01 Jan 1979 00:00:00 GMT

Trajectory Analysis of Summertime Sulfate Concentrations in the Northeastern United States Samson, Perry J. This paper presents a technique for quantifying the relationships between observed concentrations of atmospheric sulfate aerosol and their corresponding upstream history of sulfur dioxide emissions, wind speed and mixing height. Using reported sulfate concentrations from several sampling sites in the northeastern United States, 72 h upstream trajectories have been computed for winds in the mixed layer of the atmosphere over the duration of their respective sampling periods. Trajectories from one site were computed for four sublayers, each 400 m thick, extending from the surface to 1600 m. The deviations in along-trajectory and cross-trajectory directions of each of the sublayers from the position of the whole mixed layer were computed. From this the functions σy(t) and σx(t) for travel times of 6 to 72 h were derived for each layer individually and collectively for the whole layer. The values of σy(t) and σx(t) for the whole mixed-layer were found to be roughly equivalent over this time period and to grow linearly in time. The growth can be described by the relationship σy(t) = 5.4t , where σy is in kilometers and t is in hours. Using these statistics to describe the potential impact from upstream sources, each trajectory was integrated over finite time steps to estimate the potential emissions loading along that trajectory as a function of time upstream. Correspondingly, estimates were made as a function of time upstream of the wind speed in the layer and the depth of the mixed layer. It was found that sulfate concentrations were insensitive to upstream mixing height, as determined in this study, but were almost always positively correlated with the inverse of wind speed occurring 24 h or more upstream of the sampling point. No consistent relationship was evident between sulfate concentrations and potential upstream SO2 emissions loading. The ratio of observed to potential sulfate generation, as determined from the total upstream sulfur dioxide input, indicates an average net efficiency of roughly 15?30% conversion of sulfate dioxide to sulfate before deposition. These values vary dramatically with increasing efficiency corresponding to increasing resultant sulfate concentration.

Some Turbulence and Diffusion Parameter Estimates within Cooling Tower Plumes Derived from Sodar Data

Mon, 01 Jan 1979 00:00:00 GMT

Some Turbulence and Diffusion Parameter Estimates within Cooling Tower Plumes Derived from Sodar Data Coulter, R. L.; Underwood, K. H. Temperature and velocity fluctuations within a cooling tower plume in stable conditions at the Keystone power plant in Pennsylvania have been measured by use of a calibrated sodar. Monostatic and bistatic systems probed the plume at several positions 40 to 300 m downwind of the cooling tower. Comparison of the sodar estimates of the temperature and velocity structure parameters (CT2 and CV2) with those derived from measurements taken by aircraft at the same location shows acceptable agreement. Alternate methods of averaging profiles of CT2 through the plume are used to investigate single and relative dispersion coefficients. Both methods describe a linear increase of plume width with distance from the cooling tower. Combining values of temperature and velocity structure parameters leads to estimates of turbulence kinetic energy dissipation rate ? near 0.3 m2 s?3 and temperature fluctuation destruction rate N of 0.01?0.21 K2 s?1. The decrease in CT2 and CV2 is found to be exponential with horizontal distance from the tower; CT2 decreases more rapidly than CV2. Time-lapse photography used simultaneously with the sodar measurements indicates that the thermal turbulence plume is larger than the condensation plume by a factor of 2?5.

Wind Speeds in Two Tornadic Storms and a Tornado, Deduced from Doppler Spectra

Mon, 01 Jan 1979 00:00:00 GMT

Wind Speeds in Two Tornadic Storms and a Tornado, Deduced from Doppler Spectra Zrnic, Dusan; Istok, Michael Doppler spectra of a tornado were collected with a radar having a large unambiguous velocity range, ±91 m s?1. Thus for the first time a presentation of nonaliased spectra was possible, showing direct measurement of radial velocities. By fitting the tornado model spectrum to data, the radius of maximum winds and tornado center location are deduced. Tornado spectral signature is defined as a double peak, symmetric with respect to the mean wind spectrum. Histograms of maximum measured wind speeds (from spectrum skirts) for two tornadic storms are obtained, and the histograms of velocity difference (between the left and right spectrum skirt) suggest that smaller scale turbulence (<500 m) is principally responsible for spectrum broadness.