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At least 145 records · Page 8

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NASA NTRS1985

A new convective parameterization scheme for global models

A convective adjustment scheme is presented for parameterization of the vertical temperature and moisture structure for large-scale numerical weather models. The temperature and moisture structure is adjusted toward observed quasi-equilibrium structures to maintain a realistic vertical temperature and moisture structure in grid-scale global models during convection. Two reference thermodynamic structures are modeled analytically to account for shallow and deep convection. The thermodynamic structure obtained is permitted to change as a function of large-scale advective terms, radiation and surface fluxes. The reference profiles are applied when an energy integral provides a description of the shallow or deep convective processes occurring. Comparisons with GATE, BOMEX and ATEX data and a 10 day forecast for weather over the tropical ocean illustrate the climate and precipitation forecast accuracy improvements available with the adjustment scheme relative to standard parameterization schemes.

Miller, M. J.↗

NASA NTRS1986

A method for coupling a parameterization of the planetary boundary layer with a hydrologic model

Deardorff's parameterization of the planetary boundary layer is adapted to drive a hydrologic model. The method converts the atmospheric conditions measured at the anemometer height at one site to the mean values in the planetary boundary layer; it then uses the planetary boundary layer parameterization and the hydrologic variables to calculate the fluxes of momentum, heat and moisture at the atmosphere-land interface for a different site. A simplified hydrologic model is used for a simulation study of soil moisture and ground temperature on three different land surface covers. The results indicate that this method can be used to drive a spatially distributed hydrologic model by using observed data available at a meteorological station located on or nearby the site.

Lin, J. D.↗

NASA NTRS1987

The role of convective parameterization in the simulation of a Gulf Coast precipitation system

Three simulations were performed with the Limited Area Mesoscale Prediction System (LAMPS) model in order to examine the role of convective parameterization in the simulation of the Gulf Coast precipitation system of March 6-7, 1982. The first simulation neglected moist processes, the second used the representation for moist physics of Orlanksi and Ross (1984) and Rosenthal (1978, 1979), and the third simulation incorporated the LAMPS sequential plume cumulus cloud model as convective parameterization for the model. The effects of various representations of moist physics on the evolution of the model's precipitation, kinematic and diabatic heating, and mass and wind fields are investigated. It is determined that the representation of moist thermodynamics in the meso-alpha-scale model influences the manner in which the model describes the mesoscale structures.

Kalb, Michael W.↗

NASA NTRS1987

Parameterization of radiative flux profiles within layer clouds

This paper presents simple formulations for the vertical structure of solar and infrared radiative fluxes within a layer cloud overlying a partially reflective surface. The parameterized profile shapes are analytic, with governing parameters derived from bulk radiative properties and more physically based radiative transfer models. The bulk cloud, subcloud layer, and surface radiative properties are assumed to be known. The parameterizations are based on exponential functions of height, with decay scales related to cloud liquid water content. Although the results presented here are based on very simple assumptions about the cloud structure in the vertical, the method used is applicable to more general cases as well as to various other analytic and/or numerical radiative transfer calculations.

Hanson, Howard P.↗

NASA NTRS1987

Development of a two-dimensional zonally averaged statistical-dynamical model. II - The role of eddy momentum fluxes in the general circulation and their parameterization

The role of eddy momentum fluxes in the general circulation was investigated using a two-dimensional zonally averaged statistical-dynamical model described by Yao and Stone (1987), which is almost two orders of magnitude faster than the three-dimensional climate model of Hansen et al. (1983). Results show that the vertical structure of the meridional eddy flux has relatively little impact on the general circulation, presumably because the vertical structure is strongly constrained by the thermal wind relation and surface friction. On the other hand, it was found that, in order to simulate accurately the general circulation and its response to climate changes, parameterization of the vertically integrated meridional eddy flux of angular momentum is necessary. A new parameterization of this eddy momentum transport was carried out, which is intended to represent the transport due to large-scale transient eddies arising from baroclinic instability.

Stone, Peter H.↗

NASA NTRS1988

Shortwave radiative parameterization of large atmospheric aerosols - Dust and water clouds

A parameterization of the single scattering properties of an aerosol for a dust layer in the solar spectral region is presented. The parameterization scheme accounts for the microphysical changes of an aerosol layer and is applied to dust and water clouds. A scaling parameter is presented which describes, simply and effectively, the optical properties of an aerosol layer with a large effective size parameter.

Ackerman, Steven A.↗

NASA NTRS1988

Assessment of recent resolution and parameterization changes in the GLA fourth order GCM

The Goddard Laboratory for Atmospheres' fourth-order GCM is under evaluation for the impact on model integrations of enhanced horizontal and vertical resolution, as well as the effects of such novel parameterization schemes as that of gravity-wave-drag, the Arakawa-Schubert (1974) cumulus parameterization, and an explicitly-resolved planetary boundary layer. While the doubling of the GMC's horizontal resolution to 2 deg in latitude and 2.5 deg in longitude has improved the model's predictive skill for 6-7 day forecasts, systematic errors associated with the model's climate drift lead to a deterioration in predictions for longer forecasts.

Helfand, H. M.↗

NASA NTRS1989

A parameterization for longwave surface radiation from sun-synchronous satellite data

A parameterization is presented for computing downward, upward, and net longwave radiation at the earth's surface using data from NOAA sun-synchronous satellites. The parameterization is applied to satellite soundings for April, 1982 over a large region of the tropical Pacific Ocean. Sensitivity studies were used to estimate the random and systematic errors in computed fluxes due to probable errors in TOVS-derived parameters. It is suggested that large biases in the results due to errors in TOVS-derived parameters may be corrected with data from the International Satellite Cloud Climatology Project.

Gupta, Shashi K.↗

NASA NTRS1989

Parameterized desert/clear atmosphere limb-darkening model derived from earth radiation budget satellite along-track measurements

A parameterized desert/clear atmosphere limb-darkening model was derived using longwave measurements from the Earth Radiation Budget Satellite operating in a unique along-track mode that allows all points along the ground track to be viewed over an entire range of viewing zenith angles at essentially constant solar zenith angle. Application of the model to radiances for scenes defined as clear desert by the Earth Radiation Budget Experiment data analysis algorithms shows that this parameterized model reflects the geographical and diurnal behavior expected for the limb-darkening phenomenon.

Brooks, David R.↗

NASA NTRS1989

A scheme for parameterizing ice cloud water content in general circulation models

A method for specifying ice water content in GCMs is developed, based on theory and in-cloud measurements. A theoretical development of the conceptual precipitation model is given and the aircraft flights used to characterize the ice mass distribution in deep ice clouds is discussed. Ice water content values derived from the theoretical parameterization are compared with the measured values. The results demonstrate that a simple parameterization for atmospheric ice content can account for ice contents observed in several synoptic contexts.

Heymsfield, Andrew J.↗

NASA NTRS1990

Validation of parameterization scheme for eddy diffusion from satellite data

The eddy diffusion coefficient K(yy) has been calculated usign LIMS for the months of December 1978 and January and February 1979. Two methods have been used. The first implements the suggestion made by Tung (1987) to parameterize the eddy transport as a diffusive process along isentropes. The second method integrates the equation relating the parcel displacements to the eddy velocity fields. The latter method uses a filtering on both space and time domains to isolate transients and is referred to as the 'spectral method'. Results from the first method are shown to be reliable only for quiescent periods, breaking down when the meridional gradient of potential vorticity is negligible. Results from the two methods are in agreement only for very disturbed conditions, when transience is readily isolated. It is concluded that the parameterizations suggested for eddy transport and calculated in this paper may be meaningful for quiet periods, but are not reliable for unsteady and very large amplitude disturbances.

Sassi, F.↗

NASA NTRS1990

A parameterization for the shortwave transmissivity of stratiform water clouds based on empirical data and radiative transfer theory

Surface measurements of solar flux and total integrated liquid-water content, radiosonde data, and infrared satellite images are analyzed in conjunction with radiative transfer calculations to derive an empirical parameterization for the shortwave transmissivity of continental stratiform water clouds. The data were collected near Denver, Colorado, over a period of six years. A multiple reflection radiative transfer model is employed to estimate stratus cloud transmissivity in terms of the measurable liquid-water path (LWP). A nonlinear fit of estimated transmissivities to the corresponding observations of LWP yields close agreement with a previous, more complicated parameterization. The derived expression for cloud transmissivity is used to predict mean daily surface fluxes for 61 days during which periods of stratiform clouds were observed over the Denver area. A comparison between predicted and measured fluxes shows agreement to within + or - 4 percent, with best agreement for clouds of moderate optical thickness. Potential sources of error are identified with sensitivity studies.

Derr, V. E.↗

NASA NTRS1991

Evaluation of cloudiness parameterizations using a cumulus ensemble model

Diagnostic cloudiness parameterizations in large-scale models are evaluated by using a two-dimensional numerical cumulus ensemble model. The model covers a large horizontal domain (512 km) but resolves individual clouds. This study explores the dependence of diagnostic relations (between cloud amount and a large-scale variable) on cloud regime, horizontal averaging distance, and cloud type for tropical convective cloud regimes. Large-scale variables, including relative humidity, cumulus mass flux, large-scale vertical velocity and surface precipitation rate, are examined. It is shown that the total cloud amount can be better estimated as the sum of separate estimates of stratiform and convective cloud amounts using different large-scale variables than by an estimate of the total cloud amount using any single large-scale variable. The stratiform cloud amount can best be estimated by using relative humidity. The convective cloud amount can be diagnosed by using cumulus mass flux. Neither set of diagnostic relations depends significantly on the stimulated cloud regime or horizontal averaging distance, but other diagnostic relations do show some such dependence. These results are interpreted and their implications for cloudiness parameterization are discussed.

Xu, Kuan-Man↗

NASA NTRS1992

Cross section parameterizations for cosmic ray nuclei. 1: Single nucleon removal

Parameterizations of single nucleon removal from electromagnetic and strong interactions of cosmic rays with nuclei are presented. These parameterizations are based upon the most accurate theoretical calculations available to date. They should be very suitable for use in cosmic ray propagation through interstellar space, the Earth's atmosphere, lunar samples, meteorites, spacecraft walls and lunar and martian habitats.

Norbury, John W.↗

NASA NTRS1990

Parameterizing mesoscale and large-scale ice clouds in general circulation models

The paper discusses GCM parameterizations for two types of ice clouds: (1) ice clouds formed by large-scale lifting, often of limited vertical extent but usually of large-scale horizontal extent; and (2) ice clouds formed as anvils in convective systems, often of moderate vertical extent but of mesoscale size horizontally. It is shown that the former type of clouds can be parameterized with reference to an equilibrium between ice generation by deposition from vapor, and ice removal by crystal settling. The same mechanisms operate in the mesoscale clouds, but the ice content in these cases is considered to be more closely linked to the moisture supplied to the anvil by cumulus towers. It is shown that a GCM can simulate widespread ice clouds of both types.

Donner, Leo J.↗

NASA NTRS1990

A physically based fractional cloudiness parameterization

A bulk PBL model with a simple internal vertical structure and a simple second-order closure has been developed for use as a PBL parameterization in a large-scale model. A method to determine the value of sigma, the fractional area covered by rising motion, is given. Large-eddy simulations of Moeng (1984, 1986) are used to validate various aspects of the parameterization.

Randall, David A.↗

NASA NTRS1992

An intracloud lightning parameterization scheme for a storm electrification model

The parameterization of an intracloud lightning discharge has been implemented in the present storm electrification model. The initiation, propagation direction, and termination of the discharge are computed using the magnitude and direction of the electric field vector as the determining criteria. The charge redistribution due to the lightning is approximated assuming the channel to be an isolated conductor with zero net charge over its entire length. Various simulations involving differing amounts of charge transferred and distribution of charges have been done. Values of charge transfer, dipole moment change, and electrical energy dissipation computed in the model are consistent with observations. The effects of the lightning-produced ions on the hydrometeor charges and electric field components depend strongly on the amount of charge transferred. A comparison between the measured electric field change of an actual intracloud flash and the field change due to the simulated discharge shows favorable agreement. Limitations of the parameterization scheme are discussed.

Helsdon, John H., Jr.↗

NASA NTRS1992

Relaxed Arakawa-Schubert - A parameterization of moist convection for general circulation models

A simple implementation of the Arakawa and Schubert (1974) cumulus parameterization is presented. The major simplification made is to 'relax' the state toward equilibrium each time the parameterization is invoked, rather than requiring that the final state be balanced, as in the original Arakawa-Schubert implementation. This relaxed Arakawa-Schubert (RAS) scheme is evaluated in off-line tests using the Global Atmospheric Research Program (GARP) Atlantic Tropical Experiment (GATE) Phase III data. The results show that RAS is equivalent to the standard implementation of Arakawa-Schubert but is more economical and simpler to code. RAS also avoids the ill-posed problem that occurs in Arakawa-Schubert as a result of having to solve for a balanced state.

Moorthi, Shrinivas↗