Showing papers in "Monthly Weather Review in 1972"

On the Assessment of Surface Heat Flux and Evaporation Using Large-Scale Parameters

Abstract: In an introductory review it is reemphasized that the large-scale parameterization of the surface fluxes of sensible and latent heat is properly expressed in terms of energetic considerations over land while formulas of the bulk aerodynamic type are most suitahle over the sea. A general framework is suggested. Data from a number of saturated land sites and open water sites in the absence of advection suggest a widely applicable formula for the relationship between sensible and latent heat fluxes. For drying land surfaces, we assume that the evaporation rate is given by the same formula for evaporation multiplied by a factor. This factor is found to remain at unity while an amount of water, varying from one site to another, is evaporated. Following this a linear decrease sets in, reducing the evaporation rate to zero after a further 5 cm of evaporation, the same at several sites examined.

Frequency Filter for Time Integrations

Abstract: A simple filter for controlling high-frequency computational and physical modes arising in time integrations is proposed. A linear analysis of the filter with leapfrog, implicit, and semi-implicit, differences is made. The filter very quickly removes the computational mode and is also very useful in damping high-frequency physical waves. The stability of the leapfrog scheme is adversely affected when a large filter parameter is used, but the analysis shows that the use of centered differences with frequency filter is still more advantageous than the use of the Euler-backward method. An example of the use of the filter in an actual forecast with the meteorological equations is shown.

Parameterization of the Planetary Boundary layer for Use in General Circulation Models1

Abstract: The surface stress and fluxes of heat and moisture are parameterized for use in numerical models of the general circulation of the atmosphere. The parameterization is designed to be consistent with recent advances in knowledge of both the planetary boundary layer and the surface layer. A key quantity throughout is the height, h, of the planetary boundary layer, which appears in the governing stability parameter, a bulk Richardson number. With upward heat flux, a time-dependent prediction equation is proposed for h that incorporates penetrative convection and vertical motion. Under stable conditions, h is assumed to depart from the neutral value and to become nearly proportional to the Monin-Obukhov length. The roughness length, Zo, is incorporated in the combination h/zo, and the parameterization is consistent with h/zo affecting only the wind component in the direction of the surface velocity. The direction of the surface wind and stress is derived in a manner consistent with the known value of ...

Conservative Finite-Difference Approximations of the Primitive Equations on Quasi-Uniform Spherical Grids

Abstract: A class of conservative finite-difference approximations of the primitive equations is given for quasi-uniform spherical grids derived from regular polyhedrons. The earth is split into several contiguous regions. Within each region, a coordinate system derived from central projections is used, instead of the spherical coordinate system, to avoid the use of inconsistent boundary conditions at the poles. The presence of artificial internal boundaries has no effect on the conservation properties of the approximations. Examples of conservative schemes, up to the second order in the case of a cube, are given. A selective damping operator is needed to remove the two-grid interval waves generated by the existence of internal boundaries.

Response of the Tropical Atmosphere to Local, Steady Forcing

Abstract: A theoretical analysis is made of the large-scale, stationary, zonally asymmetric motions that result from heating and the orographic effect in the tropical atmosphere. The release of latent heat dominates the sensible and radiational heating and the latter two effects are ignored. The first linear model is a continuous stratified atmosphere in solid westward rotation with no dissipation. Of all the modes, only the rotationally trapped Kelvin wave exhibits a significant response. Because the Kelvin wave response does not compare well with the observed flow, we concluded that the neighboring westerlies in the real atmosphere are important even if the forcing is in low latitudes. The second linear model is a two-layer numerical model including parameterized dissipation and realistic basic currents. Realistic forcing is considered, following an analysis of the response to especially simple forms of heating and orographic forcing. Dissipative effects close to the Equator are very important in this mo...

Vertically Integrated Liquid Water—A New Analysis Tool

Abstract: Through the use of digital radar data measured at successive elevation angles in a storm system, we developed a technique that presents a new dimension in mesoscale analysis. This technique, mapped vertically integrated liquid-water content (VIL), presents the three-dimensional characteristics of a storm system in a two-dimensional display. This analysis technique appears to hold real promise for both severe storm and hydrologic applications.

An Implicit Time Integration Scheme for Baroclinic Models of the Atmosphere

Abstract: A semi-implicit time integration algorithm developed earlier for a barotropic model resulted in an appreciable economy of computing time. An extension of this method to baroclinic models is formulated, including a description of the various steps in the calculations. In the proposed scheme, the temperature is separated into a basic part dependent only on the vertical coordinate and a corresponding perturbation part. All terms involving the perturbation temperature are calculated from current values of the variables, while a centered finite-difference time average is applied to the horizontal pressure gradient, the divergence, and the vertical motion in the remaining terms. This method gives computationally stable integrations with relatively large time steps. The model used to test the semi-implicit scheme does not include topography, precipitation, diabatic heating, and other important physical processes. Five-day hemispheric integrations from real data with time steps of 60 and 30 min show diff...

Surges of Maritime Tropical Air Northward Over the Gulf of California

Abstract: Several surges of maritime tropical air into the deserts of Arizona and California are investigated. These surges are related to the large cloud masses of tropical origin located over the Gulf of California. A natural channel exists for this surge transport; it is about 200 mi in width, the western boundary being the rugged ridge line extending the length of Baja California and the eastern boundary being the broad Sierra Madre Range. The surge process resembles that of a very large-scale sea breeze with the greatest energy transport occurring near the surface and disappearing in the middle troposphere. Four case studies are analyzed in detail.

An Efficient, One-Level, Primitive-Equation Spectral Model

Abstract: A one-level, global, spectral model using the primitive equations is formulated in terms of a concise form of the prognostic equations for vorticity and divergence. The model integration incorporates a grid transform technique to evaluate nonlinear terms; the computational efficiency of the model is found to be far superior to that of an equivalent model based on the traditional interaction coefficients. The transform model, in integrations of 116 days, satisfies principles of conservation of energy, angular momentum, and square potential vorticity to a high degree.

Cumulative Results of Extended Forecast Experiments I. Model Performance for Winter Cases

Abstract: Two-week experimental forecasts were carried for 12 July Cases with a nine vertical level, 270 km grid-size hemispheric model, and the results were examined statistically. The solutions studied were the stationary (10-day average) and transient components of the general circulation; in particular, the ensemble mean of temperature, zonal wind and eddy kinetic energy; and the hemispheric maps of ensemble mean height fields. The predictive ability of this model was examined by comparing the results with observation and calculating statistical scores such as standard deviation, correlation coefficient and horizontal gradient (S1) score for 1000, 500 and 50 mb geopotential height verified against the NMC (National Meteorological Center) analysis. The results were also analyzed in terms of zonal wavenumbers of geopotential. So far as this model (1967 version) is concerned, the predictability in the lower atmosphere seems to decay more rapidly in July than in January. However, the predictability of the ...

Numerical Simulation of Precipitation Development in Supercooled Cumuli-Part I

Abstract: A numerical model of supercooled cumuli is developed and discussed. Water substance in the model is idealized to be partitioned into the five phase components; namely, water vapor, liquid cloud water, liquid rainwater, frozen rainwater, and ice crystals. Continuity equations are developed that predict the distribution of water substance among the five phase components. The cloud dynamic framework consists of a simple one-dimensional Lagrangian model that includes the effects of entrainment. The model is able to operate either as a steady-state model or as a spherical vortex model. The results of two case study experiments illustrated that the principle action of ice particles nucleated on sublimation nuclei, or by the freezing of cloud droplets in cumulus clouds containing moderate to heavy amounts of supercooled rainwater, is to promote the freezing of supercooled rainwater. On the other hand, clouds containing small amounts of supercooled rainwater are dynamically insensitive to moderate concen...

Development of Asymmetries in a Three-Dimensional Numerical Model of the Tropical Cyclone1

Abstract: Notable asymmetric features of an early experiment with a three-dimensional hurricane model were spiral bands of convection and large-scale asymmetries (eddies) in the outflow layer. Using an improved version of the model, we describe the formation and maintenance of these features in greater detail in this paper. The spiral bands in the model propagate cyclonically outward in agreement with bands in nature. The breakdown of symmetry into a chaotic pattern of eddies in the outflow region is shown to be the result of dynamic (inertial) instability, with the eddy kinetic energy derived from the kinetic energy of the azimuthal flow. This instability does not contribute to the overall intensification of the model storm, however. We observe a curious anticyclonic looping of the vortex center in these experiments. This looping appears to be associated with asymmetries in the divergence pattern associated with the eddies in the outflow layer. This paper also summarizes improvements made in the original ...

Ground-Based Determination of Low Altitude Temperature Profiles by Microwaves

Abstract: Vertical temperature profiles of the lower atmosphere are determined from clear air ground-based measurements of microwave thermal emission by oxygen. Angular emission data from two diverse meteorological locations are mathematically inverted by statistical techniques to recover the vertical profiles. Inversion of 52.5 GHz data, gathered at Upolu Point, Hawaii, Hawaii, resulted in an average root-mean-square (rms) difference of 1.27°K between inverted and radiosonde measured profiles from 0 to 10 km. Pressure and humidity profiles are simultaneously estimated from the data; numerical integration of the inverted humidity profiles results in a determination of total vertical water content with a relative accuracy of about 10 percent. Radiometer emission data at 54.0, 54.5, and 55.0 GHz, taken at Salt Lake City, Utah, are inverted with resulting average rms differences of 1.17°K over the height interval from 0 to 6.4 km. A priori temperature variance, corresponding to known surface conditions, is re...

Use of the gamma distribution in single-cloud rainfall analysis

Abstract: This study is based on the radar-evaluated rainfall data from 52 south Florida cumulus clouds, 26 seeded and 26 control clouds, selected by a randomization procedure. The fourth root of the rainfall for both seeded and control populations was well fitted by a gamma distribution for probability density. The gamma distribution is prescribed by two parameters, one for scale and one for shape. Since the coefficient of variation of seeded and control cloud populations was the same, the shape parameters for the two gamma distributions were the same, while the seeded population's scale parameter was such as to shift the distribution to higher rainfall values than the control distribution. The best-fit gamma functions were found by application of the principle of maximum entropy. Specification of tractable distributions for natural and modified rainfall populations provides an important prerequisite for the evaluation of seeding effects by Bayesian statistics, a continuing objective in the Experimental M...

Distribution of Graupel and Hail With Size

Abstract: Observations have been made on the size and concentration of graupel and hail occurring in untreated convective cloud systems over the High Plains of the United States. From these data, an average graupel and hail distribution has been generated. The comparison of this .,ize spectrum with data from other studies, along with the implications to hail detection and hail suppression, is also discussed.

Planetary Variations of Stratospheric Temperatures

Abstract: Radiances emitted from the atmosphere near 669.3 cm−1 in the center of the 15-µm CO2 band were measured from the Nimbus 3 satellite. Changes in observed radiance correspond to weighted temperature changes of the upper 100 mb of air. The seasonal march of latitudinally averaged radiances is presented from 80°N to 80°S. The latitudinal and seasonal variations of radiance are removed from the data. The residuals emphasize the fact that winter polar stratospheric warmings are accompanied by stratospheric coolings in the Tropics and summer hemisphere. A warming of about 7°K near latitude 50° is accompanied by a cooling of about 1°K at the Equator. However, much larger warmings at latitudes near the poles do not produce correspondingly large coolings at the Equator. The synoptic distributions of radiances on 2 days are discussed: (1) the day when the average latitudinal radiance was a minimum in polar latitudes and (2) the day when the polar warming reached its maximum. The wave number 1 pattern in mid...

Optimum Interpolation From Observations of Mixed Quality1

Abstract: On the basis of a 10-yr record of rawinsonde observations in the Tropics, experiments were run to illustrate the manner in which climatology may be used to minimize the root-mean-square errors of interpolation from data of mixed quality that are irregularly located in time and space. The procedure, based on the theory of optimum interpolation, determines the relative weights of the data used in the interpolation on the basis of their error characteristics, their location, and the scale and variability of the meteorological fields that they sample.

Response of the Joint Ocean-Atmosphere Model to the Seasonal Variation of the Solar Radiation

Abstract: The effect of the seasonal variation of solar radiation is incorporated into the joint ocean-atmosphere model developed at the Geophysical Fluid Dynamics Laboratory of the National Oceanic and Atmospheric Administration, and the resulting system is integrated for the 11/2-yr model time. The purpose of this study is to analyze the response of the joint air-sea model to seasonal changes in the solar zenith angle rather than to obtain a true equilibrium state. Comparisons are also made with results previously presented for the case of annual mean conditions. The most important feature that emerges as a direct result of this seasonal variation is a significant warming of the lower troposphere in high latitudes. This warming is found to be caused by (1) the removal of the snowpack during the summer season, which decreases the earth's albedo there during this time, and (2) a net rise in the temperature of the ocean surface in high latitudes as a result of the seasonal variation of convective activity i...

Numerical Experiments on the Relation Between Microphysics and Dynamics in Cumulus Convection

Abstract: An existing numerical model of cumulus growth, treating condensation but not precipitation, is modified by the incorporation of a parameterized treatment of liquid phase microphysics. This modification improves the realism of the results in several important respects; among them are maximum height of cloud growth, maximum liquid content, amount and distribution of temperature departure, cloud shape, and occurrence and strength of subcloud downdraft. We found that one of the most important controlling features is the rate of evaporation of droplets. In particular, the introduction of a class of large particles with relatively slow evaporation rate produces a smaller temperature deficit at the cloud summit, hence more vigorous cloud growth. In this model, the upper and lower parts of the cloud are, to a large extent, decoupled dynamically, the development of a strong subcloud downdraft by evaporation of precipitation having little effect on the ultimate extent of cloud growth.

Numerical Solutions of the One-Dimensional Primitive Equations Using Galerkin Approximations With localized Basis Functions

Abstract: The Galerkin method is applied to a pair of linear and then nonlinear primitive (wave) equations. This results in a system of ordinary differential equations. Procedures are included for generating the coefficient matrices of the system of ordinary differential equations when piecewise Hermite cubic functions are used as basis functions. It is demonstrated that this system can be efficiently solved by an implicit method. Numerical examples show that integration using the Galerkin method is more efficient than the corresponding finite-difference method with central differences in space.

Snowfall From Lake-Effect Storms

Abstract: Three yr of winter lake-storm data were analyzed to determine snowfall distribution patterns downwind of Lake Erie and Lake Ontario. The total amount of snowfall and the area of ground cover in each of 23 lake-effect storms were determined for both lakes. Total snowfall mass was highly dependent on time of year; November and early December storms were two to five times more productive than January storms. A considerable variation in snow density (snowfall depth to melt water ratio) could be attributed mainly to differences in snow crystal type.

The Fleet Numerical Weather Central Operational Primitive-Equation Model

Abstract: The U.S. Navy Fleet Numerical Weather Central (FNWC), Monterey, Calif., five-layer, primitive-equation, atmospheric prediction model has been under development since late 1968 and became operational in September 1970. Seventy-two-hour prognoses are generated twice daily, requiring 2 hr per run. The conservation forms of the difference equations are based on the Arakawa technique and are integrated, using a 381-km space step (at 60°N) and a 10-min time step, on sigma surfaces. Realistic mountains are used. Pressure-force terms are replaced by a single geopotential gradient on pressure surfaces synthesized locally to reduce inconsistent truncation error. Lateral diffusion is performed on forecast difference fields to prevent systematic distortions of sigma-surface state parameter distributions. Stress is applied at the lowest level. Restoration boundaries and centered time differencing are used. The integrations are restarted each 6 hr with a Euler-backward step to reduce solution separation. The m...

Global Sea-Surface Temperature Distribution Determined From an Environmental Satellite

Abstract: A composite histogram method is used to objectively derive sea-surface temperature distribution from satellite radiation measurements for the Northern and Southern Hemispheres. Comparisons with conventional observations yield root-mean-square differences of 2°–3°K. Some of the differences can be accounted for by factors such as the coherent noise introduced by the onboard tape recorder, insufficient atmospheric attenuation corrections, and basic differences between the two types of temperature measurements.

Summer Thunderstorms Over Southern California

Abstract: Ten-day fire weather records from forestry lookouts in the southern California mountains are used to describe summer thunderstorm activity. The distribution of thunderstorms during a typical season closely resembles rainy-day frequencies for summer precipitation in Arizona, thereby supporting the concept that most southern California summer thunderstorms are caused by a westward extension of the same circulation that brings Arizona its “monsoon” rains. Thunderstorms are also induced by dissipating eastern North Pacific tropical cyclones that move up the west coast of Mexico. Tropical storm tracks, local National Weather Service reports, and fire weather records are used to show which occurrences of thunderstorms can be credited to this tropical cyclone activity and the attendant influx of upper level moisture. When the data from the summers of 1947–68 are divided into two 11-yr periods using 1957/58 as the break point, a significant increase in thunderstorms and tropical cyclones affecting southe...

Visibility Changes in Ohio, Kentucky, and Tennessee From 1962 to 1969

Abstract: Previous studies of changes in visibility over a period of years have indicated either a general trend toward better horizontal visibilities or no change. In this study, visibility, relative humidity, wind direction, and other related data from three National Weather Service Offices (Akron, Ohio; Lexington, Ky.; Memphis, Tenn.) are used to determine changes in daylight visibilities during the summer seasons of 1962–69. Analyses of the data indicate that the percent of restricted visibility was greater during the period 1966–69 than during the period 1962–65 both before and after adjustment for the effects of location, time of day, humidity, and wind.

Study of Heat Sources and Sinks and the Generation of Available Potential Energy in the Indian Region During the Southwest Monsoon Season

Abstract: The total vertical velocity and the heating field were evaluated for a portion of the Indian region for a typical monsoon day in July 1966 using a two-level geostrophic model. The generation of zonal and eddy available potential energy was computed with these values of heating and cooling. Both forms of available potential energy seem to be generated by diabatic processes. The generation of eddy available potential energy may be due to the released latent heat of condensation.

Performance Analysis of the HURRAN Tropical Cyclone Forecast System

Abstract: The HURRAN (hurricane analog) technique, a fully computerized objective forecast aid making use of past tracks in forecasting hurricane motion, was developed prior to the 1969 hurricane season. Encouraging operational results during the 1969 and 1970 hurricane seasons suggested further evaluation of the technique. To this end, HURRAN computations were made for approximately 1,000 forecast situations. Results are stratified according to initial direction and speed of movement of the sample storms and the number of analogs selected. The utility of the technique is discussed, and the importance of position accuracy at forecast time is demonstrated. Initial indications of the value of the technique are substantiated.

Vertical Temperature Structure During the 1966 Thanksgiving Week Air Pollution Episode in New York City

Abstract: This paper presents time cross-sections of vertical temperature structure during the 1966 Thanksgiving week air pollution episode in New York City, based on 6-hourly soundings at Kennedy Airport. The analyses depict numerous inversions in the lower 10,000 ft of the atmosphere, including an interesting sequence of surface-based inversions and an outstanding inversion aloft. Diurnal and daily variations in the height of the mixing layer can also be seen. Some possible influences of the vertical temperature structure on SO2 concentrations in Manhattan are discussed, and it is suggested that exceptional peak SO2 concentrations were largely due to the fumigation process.

Numerical Study of the Effects of the Great Lakes on a Winter Cyclone

Abstract: A vigorous winter storm over North America is studied by means of an eight-level primitive-equation model. Included are orography, surface and internal friction, long-wave radiation from clouds and water vapor, large-scale release of latent heat, and fluxes of sensible heat and water vapor from water surfaces. Sigma coordinates are employed. The grid size is 190 km at 60°N. Two 36-hr numerical integrations are performed, one with and one without the effects of the Great Lakes and other water surfaces. When these influences are included, lower tropospheric temperatures are raised by more than 7°C and 1000-mb heights are reduced as much as 70 m. Ekman layer wind speeds are modified by up to 6 m/s. The maximum increase in large-scale precipitation over the Great Lakes is 0.5 cm with decreases to the southeast. Below 800 mb, isobaric surfaces are lowered; they are raised at higher levels. Effects on vorticity and divergence also change sign between lower and upper troposphere. Near the earth's surfac...

Mesoscale Wind Fields and Transport Estimates Determined From a Network of Wind Towers1

Abstract: Techniques are developed to utilize wind data from a network of stations to study mesoscale flow patterns and transport in the lower planetary boundary layer. The data sample for this study is hourly averaged wind data for the year 1969 from 21 stations over the Upper Snake River Plain of southeastern Idaho. Examination of 6-hourly wind field plots for the entire year reveals a strong correlation of the flow patterns with a topographic variation of relatively minor amplitude. Plots of patterns of trajectories of hypothetical “particles” released once an hour from a single location for the entire year are typed according to season and release period during the day. These patterns provide an estimate of a transport climatology; a climatology not available from a wind rose at the source. Evidence is presented that demonstrates that estimates of transport from the source wind can be seriously in error due to marked spatial variations in the flow.