Search Results

You are looking at 1 - 10 of 27 items for

  • Author or Editor: R. Meyer x
  • User-accessible content x
Clear All Modify Search
R. E. Meyer

Abstract

The reflection of water surface waves by long undersea ridges and valleys is studied on the basis of linear longwave theory and of refraction theory. If L is the wavelength and h the local water depth, then for small (L/h)dh/dx the first approximation to the reflection coefficient is established for a large class of smooth depth distributions h(x).

Full access
Andreas Schiller, Gary Meyers, and Neville R. Smith

Abstract

No abstract available.

Full access
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.

Full access
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.

Full access
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.

Full access
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.

Full access
Patrick C. Meyers, Ralph R. Ferraro, and Nai-Yu Wang

Abstract

The Goddard profiling algorithm 2010 (GPROF2010) was revised for the Advanced Microwave Scanning Radiometer for Earth Observing System (EOS; AMSR-E) instrument. The GPROF2010 land algorithm was developed for the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), which observes slightly different central frequencies than AMSR-E. A linear transfer function was developed to convert AMSR-E brightness temperatures to their corresponding TMI frequency for raining and nonraining instantaneous fields of view (IFOVs) using collocated brightness temperature and TRMM precipitation radar (PR) measurements. Previous versions of the algorithm separated rain from surface ice, snow, and desert using a series of empirical procedures. These occasionally failed to separate raining and nonraining scenes, leading to failed detection and false alarms of rain. The new GPROF2010, version 2 (GPROF2010V2), presented here, prefaced the heritage screening procedures by referencing annual desert and monthly snow climatologies to identify IFOVs where rain retrievals were unreliable. Over a decade of satellite- and ground-based observations from the Interactive Multisensor Snow and Ice Mapping System (IMS) and AMSR-E allowed for the creation of a medium-resolution (0.25° × 0.25°) climatology of monthly snow and ice cover. The scattering signature of rain over ice and snow is not well defined because of complex emissivity signals dependent on snow depth, age, and melting, such that using a static climatology was a more stable approach to defining surface types. GPROF2010V2 was subsequently used for the precipitation environmental data record (EDR) for the AMSR2 sensor aboard the Global Change Observation Mission–Water 1 (GCOM-W1).

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

Abstract

An effort to improve descriptions of ice initiation processes of relevance to cirrus clouds for use in regional-scale numerical cloud models with bulk microphysical schemes is described. This is approached by deriving practical parameterizations of the process of ice initiation by homogeneous freezing of cloud and haze (CCN) particles in the atmosphere. The homogeneous freezing formulations may be used with generalized distributions of cloud water and CCN (pure ammonium sulfate assumed). Numerical cloud model sensitivity experiments were made using a microphysical parcel model and a mososcale cloud model to investigate the impact of the homogeneous freezing process and heterogeneous ice nucleation processes on the formation and makeup of cirrus clouds. These studies point out the critical nature of assumptions made regarding the abundance and character of heterogeneous ice nuclei (IN) present in the upper troposphere. Conclusions regarding the sources of ice crystals in cirrus clouds and the potential impact of human activities on these populations must await further measurements of CCN and particularly IN in upper-tropospheric and lower-stratospheric regions.

Full access
T. M. Georges, J. A. Harlan, L. R. Meyer, and R. G. Peer

Abstract

Hurricane Claudette was successfully tracked for three days using the 2-s (7 m) surface wave direction field mapped by the U.S. Air Force OTH-B over-the-horizon radar 2400 km away on the coast of Maine. Inflow and fine structure of the surface circulation are apparent in streamline plots derived from surface wave direction measured with 60-km resolution in the vicinity of the storm for five radar runs. The radar-derived track is within 60 km of that published by the NOAA National Hurricane Center.

Full access
A. Schiller, J. S. Godfrey, P. C. McIntosh, G. Meyers, and R. Fiedler

Abstract

A global ocean circulation model, driven by observed interannual fluxes, is used to gain insight into how sea surface temperature anomalies (SSTAs, i.e., variations from the mean seasonal signal) in the tropical and subtropical Indian and Pacific Ocean are maintained and changed on interannual timescales. This is done by investigation of heat in the upper ocean at five selected sites and by comparison to observations based on expendable bathythermograph data and the TOGA/TAO moored buoys. A 6-yr simulation between 1985 and 1990 reveals that the model’s simulated interannual temperature variability in the upper 450 m of the ocean is in reasonable agreement with observations. However, the model overestimates the meridional extent and amplitude of SST variability in parts of the equatorial Pacific and Indian Oceans. The problem is associated with the choice of heat flux boundary condition: the ratio of air humidity to saturated humidity over freshwater at SST in the latent heat flux term is independent of the spatial scale of SSTA pattern, which implies a weaker negative feedback on SST change.

In the central Pacific at (0°, 140°W), budgets for the surface mixed layer and over the top 300 m both show the primary causes of temperature change to be zonal and vertical advection, with their sum generally less than half of either term individually. At (0°, 110°W), the mixed layer is much thinner so that the temperature changes result from a small disturbance of a basic balance between the vertical convergence of heat flux and vertical and zonal advection. At both sites the zonal flow (and hence the zonal heat advection) is determined by a sum of several terms, none of which are small. It is therefore difficult to find a clear physical basis in the model for the Kessler–McPhaden empirical rule for SSTAs, which correlates highly with observed SSTAs. However, this rule suggests that differences between wind stress products that exceed 0.04 N m−2 over several months (as occurs at 140°W in 1989) could lead to differences in SSTAs of up to 4°C. This may help explain the occurrence of a short but intense La Niña episode that occurred in the model, but not in the observed SST. Comparison with earlier model results tends to confirm that FSU winds were in error in the east Pacific in late 1989 and suggests that the use of a realistic (thin) surface mixed layer exacerbates the problem by strengthening the sensitivity of SSTAs to wind errors.

A simple time integral of the depth-averaged (0–350 m) current at 140°E, near the western boundary of the equatorial Pacific, shows a clear correlation with the zonal movements of the eastern edge of the warm pool, lagged by about six months. This is qualitatively as expected from “delayed oscillator” theory and confirms that the basic current structure of our model is in close agreement with observations.

Model and XBT observations show strong similarities in the depth of the 20°C isotherm and SSTA along the IX1 section from western Australia to Java during 1985–90. SST close to the southern end of this section (23°S, 112°E) is dominated by the annual signal with a superimposed weak interannual signal. The time rate of change of accumulated temperature anomalies in the top 450 m is dominated by anomalous cold vertical advection from late 1986 to early 1988 with the opposite happening from late 1988 to early 1990. Both signals indicate the arrival of the ENSO signal along the northwest Australian coast with a reduced (increased) thermocline thickness during the El Niño (La Niña) event. SSTA at 23°S, 112°E in the model is controlled by a balance between anomalous vertical advection and total diffusion; SSTA is not driven by local heat fluxes.

Full access