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Michael P. Meyers and William R. Cotton

Abstract

A prolonged orographic precipitation event occurred over the Sierra Nevada in central California on 12–13 February 1986. This well-documented case was investigated via the nonhydrostatic version of the Colorado State University (CSU) Regional Atmospheric Modeling System (RAMS). The two-dimensional, cross-barrier simulations produced flow fields and microphysical structure, which compared well with observations. The feasibility of producing quantitative precipitation forecasts (QPF) with an explicit cloud model was also demonstrated.

The experiments exhibited a profound sensitivity to the input sounding. Initializing with a sounding, which is representative of the upstream environment, was the most critical factor to the success of the simulation. The QPF was also quite sensitive to input graupel density. Decreasing the density of graupel led to increases in the overall precipitation. Sensitivities to other microphysical parameters as well as orography and dynamics were also examined.

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T. P. Meyers and R. F. Dale

Abstract

Solar radiation information is used in crop growth, boundary layer, entomological and plant pathological models, and in determining the potential use of active and passive solar energy systems. Yet solar radiation is among the least measured meteorological variables.

A semi-physical model based on standard meteorological data was developed to estimate solar radiation received at the earth's surface. The radiation model includes the effects of Rayleigh scattering, absorption by water vapor and permanent gases, and absorption and scattering by aerosols and clouds. Cloud attenuation is accounted for by assigning transmission coefficients based on cloud height and amount. The cloud transmission coefficients for various heights and coverages were derived empirically from hourly observations of solar radiation in conjunction with corresponding cloud observations at West Lafayette, Indiana. The model was tested with independent data from West Lafayette and Indianapolis, Madison, WI, Omaha, NE, Columbia, MO, Nashville, TN, Seattle, WA, Los Angeles, CA, Phoenix, AZ, Lake Charles, LA, Miami, FL, and Sterling, VA. For each of these locations a 16% random sample of days was drawn within each of the 12 months in a year for testing the model. Excellent agreement between predicted and observed radiation values was obtained for all stations tested. Mean absolute errors ranged from 1.05 to 1.80 MJ m−2 day−1 and root-mean-square errors ranged from 1.31 to 2.32 MJ m−2 day−1. The model's performance judged by relative error was found to be independent of season and cloud amount for all locations tested.

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Michael P. Meyers, Paul J. DeMott, and William R. Cotton

Abstract

Two new primary ice-nucleation parameterizations are examined in the Regional Atmospheric Modeling System (RAMS) cloud model via sensitivity tests on a wintertime precipitation event in the Sierra Nevada region. A model combining the effects of deposition and condensation-freezing nucleation is formulated based on data obtained from continuous-flow diffusion chambers. The data indicate an exponential variation of ice-nuclei concentrations with ice supersaturation reasonably independent of temperatures between −7° and −20°C. Predicted ice concentrations from these measurements exceed values predicted by the widely used temperatures dependent Fletcher approximation by as much as one order of magnitude at temperatures warmer than −20°C. A contact-freezing nucleation model is also formulated based on laboratory data gathered by various authors using techniques that isolated this nucleation mode. Predicted contact nuclei concentrations based on the newer measurements are as much as three orders of magnitude less than values estimated by Young's model, which has been widely used for predicted schemes.

Simulations of the orographic precipitation event over the Sierra Nevada indicate that the pristine ice fields are very sensitive to the changes in the ice-nucleation formulation, with the pristine ice field resulting from the new formulation comparing much better to the observed magnitudes and structure from the case study. Deposition-condensation-freezing nucleation dominates contact-freezing nucleation in the new scheme, except in the downward branch of the mountain wave, where contact freezing dominates in the evaporating cloud. Secondary ice production is more dominant at warm temperatures in the new scheme, producing more pristine ice crystals over the barrier. The old contact-freezing nucleation scheme overpredicts pristine ice-crystal concentrations, which depletes cloud water available for secondary ice production. The effect of the new parameterizations on the precipitating hydrometeors is substantial with nearly a 10% increase in precipitation across the domain. Graupel precipitation increased dramatically due to more cloud water available with the new scheme.

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Michael P. Meyers, Paul J. Demott, and William R. Cotton

Abstract

Ice initiation by specific cloud seeding aerosols, quantified in laboratory studies, has been formulated for use in mesoscale numerical cloud models. This detailed approach, which explicitly represents artificial ice nuclei activation, is unique for mesoscale simulators of cloud seeding. This new scheme was applied in the simulation of an orographic precipitation event seeded with the specific aerosols on 18 December 1986 from the Sierra Cooperative Pilot Project using the Regional Atmospheric Modeling System (RAMS). Total ice concentrations formed following seeding agreed well with observations. RAMS's three-dimensional results showed that the new seeding parameterization impacted the microphysical fields producing increased pristine ice crystal, aggregate, and graupel mass downstream of the seeded regions. Pristine ice concentration also increased as much as an order of magnitude in some locations due to seeding. Precipitation augmentation due to the seeding was 0.1–0.7 mm, similar to values inferred from the observations. Simulated precipitation enhancement occurred due to increased precipitation efficiency since no large precipitation deficits occurred in the simulation. These maxima were collocated with regions of supercooled liquid water where nucleation by man-made ice nucleus aerosols was optimized.

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Matthew R. Kumjian, Yvette P. Richardson, Traeger Meyer, Karen A. Kosiba, and Joshua Wurman

Abstract

Two of the “Doppler on Wheels” facility radars (DOW6 and DOW7) have been upgraded to dual-polarization capabilities and operate at two closely spaced X-band frequencies. For particles with sizes that are large relative to the wavelength, resonance scattering effects may lead to differences in the backscattered radiation between these two frequencies. This study investigates the utility of dual-frequency, dual-polarization DOW radars for hail detection and sizing. T-matrix scattering calculations at the two X-band DOW7 frequencies reveal that dual-frequency differences in the radar reflectivity factors at horizontal polarization (Δλ Z H) and differential reflectivities (Δλ Z DR) exist for hailstones, whereas negligible differences exist for raindrops. These differences are enhanced for wet or melting hailstones. Further, these dual-frequency differences may be positive or negative, thereby defining four distinct quadrants in the Δλ Z H–Δλ Z DR parameter space that occur for narrow bands of hail sizes. DOW7 data from two hail-bearing storms are analyzed: one produced only small hail, and the other produced severe hail up to ~3.8 cm in diameter. The analysis reveals dual-frequency signals that are consistent with the scattering calculations for those sizes, including consistent changes in the signatures below the melting layer in the first storm as hailstones acquire more liquid meltwater and a shift in the Δλ Z H–Δλ Z DR parameter space over time as the second storm grew upscale and hail sizes decreased. Implications for further applications and suggestions about closely spaced dual-frequency observations at other wavelengths are discussed.

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Jordan R. Bell, Esayas Gebremichael, Andrew L. Molthan, Lori A. Schultz, Franz J. Meyer, Christopher R. Hain, Suravi Shrestha, and K. Cole Payne

Abstract

The normalized difference vegetation index (NDVI) has been frequently used to map hail damage to vegetation, especially in agricultural areas, but observations can be blocked by cloud cover during the growing season. Here, the European Space Agency’s Sentinel-1A/1B C-band synthetic aperture radar (SAR) imagery in co- and cross polarization is used to identify changes in backscatter of corn and soybeans damaged by hail during intense thunderstorm events in the early and late growing season. Following a June event, hail-damaged areas produced a lower mean backscatter when compared with surrounding, unaffected pixels [vertical–vertical (VV): −1.1 dB; vertical–horizontal (VH): −1.5 dB]. Later, another event in August produced an increase in co- and cross-polarized backscatter (VV: 0.7 dB; VH: 1.7 dB) that is hypothesized to result from the combined effects of crop growth, change in structure of damaged crops, and soil moisture conditions. Hail damage regions inferred from changes in backscatter were further assessed through coherence change detections to support changes in the structure of crops damaged within the hail swath. While studies using NDVI have routinely concluded a decrease in NDVI is associated with damage, the cause of change with respect to the damaged areas in SAR backscatter values is more complex. Influences of environmental variables, such as vegetation structure, vegetation maturity, and soil moisture conditions, need to be considered when interpreting SAR backscatter and will vary throughout the growing season.

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