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Charles Cohen
and
William M. Frank

Abstract

Observations in the tropics have shown that the lapse rate of virtual Potential temperature θ v normally resembles that of a reversible moist adiabat. In the present study, a mesoscale numerical model with parameterized convection is used to examine the adjustment to equilibrium of the tropical atmosphere in the presence of large-scale forcing. The physical processes that enable the model to produce an equilibrium lapse rate similar to what is observed are examined. The results indicate that a spectrum of cloud sizes, which is produced by a variable entrainment rate, is essential to enable the atmosphere to approach an equilibrium state of small conditional instability in the presence of large-scale destabilization. Lateral detrainment is not essential for modeling the large-scale stability of the atmosphere, despite its significance for modeling individual mesoscale convective systems.

When downdrafts are removed from the parameterized convection, the model only temporarily produces a realistic large-scale lapse rate. Other results suggest that the departure of observed θ ν soundings from adiabats around the melting level is due to ice processes. The feedbacks between convective-scale process and the environmental stability that allow the atmosphere to maintain a state of small conditional instability are discussed.

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Nouhou T. Diallo
and
William M. Frank

Abstract

Numerical simulations of a 24-h period during GATE are performed to evaluate the impact of enhanced initial moisture fields on short range forecasts of tropical rainfall. The domain covers West Africa and most of the eastern half of the Atlantic Ocean. A version of the Pennsylvania State University Mesoscale Model is used, and results are verified against satellite rainfall estimates. The initial moisture fields are enhanced subjectively using visible satellite imagery, available rawinsonde soundings and monthly mean soundings.

The rainfall forecasts with initial moistures enhanced throughout the analysis region clearly outperform forecasts which use either the unaltered initial moisture analysis or an analysis performed with selectively enhanced moisture soundings. The former produce better zonal and meridional distributions of rain, narrower and more intense intertropical convergence zone (ITCZ) regions and stronger local maxima in agreement with verification. The success of the relatively simple moisture enhancement used in this study suggests that satellite-enhanced moisture analyses will have a major impact on short-range numerical forecasts of tropical rainfall.

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Raymond W. Arritt
and
William M. Frank

Abstract

Modifications to current model output statistics procedures for quantitative precipitation forecasting were explored. Probability of precipitation amount equations were developed for warm and cool seasons in a region in the eastern United States. Twelve-term equations, which were simultaneously regressed for four precipitation categories, were compared to equations that were regressed independently for each of the categories. The effect of varying the number of terms in the independently regressed equations was also considered. The utilities of linear predictors not presently considered and of multiplicative predictors selected with the aid of a one parameter multiplicative model were investigated.

All forecast equations were evaluated using threat scores and biases achieved upon verification for one year of independent data. The independently regressed equations generally achieved threat scores similar to the twelve-term simultaneously regressed equations, and usually required fewer terms to do so. These more compact equations could be more readily interpreted by individual forecasters than could the twelve-term equations, making it easier to develop techniques for local adjustments to the objective forecasts. The prediction of the higher precipitation amount categories may benefit from the inclusion of predictor variables not presently considered.

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Jenn-Luen Song
and
William M. Frank

Abstract

Rawinsonde data from CIATE were used to investigate further the relationships between deep convection and large-scale circulations. The data were analyzed at all of the individual observation times and with composites ~ on rainfall rate. The net latent heating rate was found to he strongly related to horizontal divergence between the surface and 700 mb and from 150 to 350 mb, but not to middle level divergence. It was also wrongly correlated with horizontal moisture convergence below 500 mb and with vertical velocity, particularly in the middle levels. The latter result is in close agreement with the findings of earlier studies which estimated convection from diagnostic models.

Spectral analyses of radar-es6mated rainfall showed that the convection varied significantly only at frequencies corresponding to those of easterly waves and the diurnal cycle supporting the satellite data analyses of Murakami.

Conditional instability averaged over the A/B array varied substantially with time and was poorly related to instantaneous rainfall rates, although an apparent threshold value required for development of widespread deep convection was noted. Vertical profiles of convective heating also varied substantially with time and were generally unrelated to profiles of the buoyancy of rising, undilute surface parcels.

Sensitivity tests indicated that studies which analyze both A/B- and B-army data with quadratic fitting schemes should be regarded as A/B-array averages due to domination of the fields by the outer data points. Analyses of kinematic vertical velocities at 100 mb suggest that use of zero velocity as an upper boundary condition may cause difficulties in some circumstances.

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William M. Gray
,
William M. Frank
,
Myron L. Corrin
, and
Charles A. Stokes

Abstract

Growing global population pressures and predicted future food and energy shortages dictate that man fully explore his potential use of solar energy. This paper investigates the possibility of beneficial weather modification through artificial solar energy absorption. A variety of physical ideas related to artificial heat sources on different scales of motion are considered. Interest is concentrated on the feasibility of mesoscale (∼100–300 km) weather modification through solar energy absorption by carbon aerosol particles of size ∼0.1 μm or less. Particles of this size maximize solar energy absorption per unit mass.

It is hypothesized that significant beneficial influences can be derived through judicious exploitation of the solar absorption potential of carbon black dust. There is an especially high potential for this in the boundary layer over tropical oceans and in the formation of cirrus clouds and the consequent alteration of the tropospheric IR energy budget. If dispersed in sizes ≤0.1 μm, solar energy absorption amounts as high as ∼2 × 1010 cal lb−1 per 10 h or about 4 × 1011 cal per dollar per 10 h can be obtained. This is a tremendously powerful heat source, especially if it stimulates additional radiation energy gains from extra cloud formation and/or enhanced surface evaporation. Preliminary observational and modeling information indicates that this artificial heat source can be employed on the mesoscale (∼100–300 km) to achieve significant economic gains by means of precipitation enhancement and tropical storm destruction alleviation. It may also be possible to use carbon dust to enhance precipitation over interior land areas, alter extratropical cyclones, inhibit high daytime summer temperatures and severe weather, prevent frosts, and speed up springtime snowmelt in agriculturally marginal regions.

A discussion of this physical hypothesis from the meteorological, radiational, engineering and ecological points of view is made.

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Jeffrey S. Gall
,
William M. Frank
, and
Young Kwon

Abstract

Under high-wind conditions, breaking waves and whitecaps eject large numbers of sea spray droplets into the atmosphere. The spray droplets originate with the same temperature and salinity as the ocean surface and thus increase the effective surface area of the ocean in contact with the atmosphere. As a result, the spray alters the total sensible and latent heat fluxes in the near-surface layer. The spray drops in the near-surface layer also result in horizontal and vertical spray-drag effects. The mass of the spray introduces an additional drag in the vertical momentum equation and tends to stabilize the lower boundary layer (BL).

An initially axisymmetric control hurricane was created from the output of a real-data simulation of Hurricane Floyd (1999) using the nonhydrostatic fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5, version 3.4). The subsequent simulations, however, are not axisymmetric because the mass, wind, and spray fields are allowed to develop asymmetries. While such a design does not result in an axisymmetric simulation, the mass, wind, and spray fields develop more realistic structures than in an axisymmetric simulation. Simulations of the hurricane were conducted using a version of the Fairall et al. (1994) sea spray parameterization, which includes horizontal and vertical spray-drag effects. The simulations were run using varying spray-source function intensities and with and without horizontal and vertical spray-drag effects. At present, the relationship of spray production to surface wind speed is poorly known for hurricane-force wind regimes.

Results indicate that spray modifies the hurricane structure in important but complex ways. Spray moistens the near-surface layer through increased evaporation. The effect of spray on the near-surface temperature profile depends on the amount of spray and its location in the hurricane. For moderate spray amounts, the near-surface layer warms within the high-wind region of the hurricane and cools at larger radii. For larger spray amounts, the near-surface layer warms relative to the moderate spray case.

The moderate spray simulations (both with and without drag effects) have little net effect on the hurricane intensity. However, in the heavier spray runs, the total sensible heat flux is enhanced by 200 W m−2, while the total latent heat flux is enhanced by over 150 W m−2 in the high-wind region of the storm. Horizontal spray drag decreases wind speeds between 1 and 2 m s−1, and vertical spray drag increases the stability of the lower BL. In these heavy spray runs, the effect of the enhanced spray sensible and latent heat fluxes dominates the negative spray-drag effects, and as a result, the modeled storm intensity is upward of 10 mb stronger than the control run by the end of the simulation time. This study shows that spray has the capability of significantly affecting hurricane structure, but to do so, the amount of spray ejected into the BL of the hurricane would need to lie near the upper end of the currently hypothesized spray-source functions.

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Jeffrey S. Gall
,
William M. Frank
, and
Matthew C. Wheeler

Abstract

This two-part series of papers examines the role of equatorial Rossby (ER) waves in tropical cyclone (TC) genesis. To do this, a unique initialization procedure is utilized to insert n = 1 ER waves into a numerical model that is able to faithfully produce TCs. In this first paper, experiments are carried out under the idealized condition of an initially quiescent background environment. Experiments are performed with varying initial wave amplitudes and with and without diabatic effects. This is done to both investigate how the properties of the simulated ER waves compare to the properties of observed ER waves and explore the role of the initial perturbation strength of the ER wave on genesis. In the dry, frictionless ER wave simulation the phase speed is slightly slower than the phase speed predicted from linear theory. Large-scale ascent develops in the region of low-level poleward flow, which is in good agreement with the theoretical structure of an n = 1 ER wave. The structures and phase speeds of the simulated full-physics ER waves are in good agreement with recent observational studies of ER waves that utilize wavenumber–frequency filtering techniques. Convection occurs primarily in the eastern half of the cyclonic gyre, as do the most favorable conditions for TC genesis. This region features sufficient midlevel moisture, anomalously strong low-level cyclonic vorticity, enhanced convection, and minimal vertical shear. Tropical cyclogenesis occurs only in the largest initial-amplitude ER wave simulation. The formation of the initial tropical disturbance that ultimately develops into a tropical cyclone is shown to be sensitive to the nonlinear horizontal momentum advection terms. When the largest initial-amplitude simulation is rerun with the nonlinear horizontal momentum advection terms turned off, tropical cyclogenesis does not occur, but the convectively coupled ER wave retains the properties of the ER wave observed in the smaller initial-amplitude simulations. It is shown that this isolated wave-only genesis process only occurs for strong ER waves in which the nonlinear advection is large. will look at the more realistic case of ER wave–related genesis in which a sufficiently intense ER wave interacts with favorable large-scale flow features.

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Liying Qian
,
George S. Young
, and
William M. Frank

Abstract

In the atmosphere, the cold, dry, precipitation-driven downdrafts from deep convection spread laterally after striking the surface, inhibiting new convection in the disturbed wake (cold pool) area. In contrast, the leading edge of this cold air lifts unstable environmental air helping to trigger new convection. A GCM cannot resolve the disturbed and undisturbed regions explicitly, so some parameterizations of these critical mesoscale phenomena are needed. The simplest approaches are to either instantly mix downdraft air with the environment, or instantly recover the downdraft air. The instant-mixing approach tends to lead to unrealistic pulsing of convection in environments that would otherwise be able to support long-lived mesoscale convective systems while the instant recovery approach usually overestimates surface energy fluxes. By replacing these simplistic approaches with a physically based convective wake–gust front model, these problems are substantially remedied.

The model produced realistic parameterized wakes that closely resemble those observed in the Global Atmospheric Research Program’s Atlantic Tropical Experiment and Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment when given reasonable inputs based on observations taken during these experiments. For realistic downdraft characteristics, wake recovery time is on the order of hours, which is significantly different from the instant recovery or instant mixing assumed in previous parameterizations. A preliminary test in midlatitude continental conditions also produced reasonable wake characteristics. Sensitivity tests show the model sensitivities to variations in downdraft mass flux, downdraft thermodynamic characteristics, and surface wind/downdraft traveling velocity. Prognostic studies using a simple coupled cloud model successfully simulated the convective termination due to stabilization of the boundary layer by precipitation-driven downdrafts, the initiation of convection after the boundary layer recovery by surface fluxes, and the phenomenon of surface flux enhancement during the convective phase.

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Klaus Fraedrich
,
John L. McBride
,
William M. Frank
, and
Risheng Wang

Abstract

Empirical orthogonal function (EOF) analyses are performed of time–height series of zonal and meridional winds and of cumulonimbus heating and drying in the Tropics. The data are from a rawinsonde array in the western Pacific located between the equator and 10°S during the intensive observation period of the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The EOF analyses are performed by applying a window of 20 days to the data and thus calculating the EOFs of the time development of the vertical structure.

The wind time series is found to be well represented by two pairs of EOFs, each representing an oscillation. The first oscillation has a period of approximately 40 days, is predominantly in the zonal wind component, and has a first internal mode vertical structure with westerly anomalies in the lower troposphere corresponding to easterly perturbations in the upper troposphere. This pair describes 48.3% of the variance. A second EOF pair in the wind is a zonal variation that occurs predominantly in the upper troposphere. It has a period of approximately 24 days and describes 13.9% of the variance.

The heating–drying series is described by a dominant oscillation of period 40 days representing 41% of the variance. The structure is maximum in the middle troposphere and is associated with the same physical phenomeon as the dominant (u, υ) oscillation. The second EOF pair for heating–drying has a period of 13 days, so there is a large time separation in periodiocities for heating–drying compared to that for winds. The second (13 day) oscillation in heating–drying has the same vertical structure as the dominant (40 day) oscillation.

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William M. Frank
,
Houjun Wang
, and
John L. McBride

Abstract

During the 120 days of the TOGA COARE intensive observation period, there was an enhanced network of rawinsonde stations covering a large portion of the equatorial West Pacific. These soundings were of sufficient quality and frequency to permit computation of line integral beat and moisture budgets over a variety of large-scale arrays. In this study an enhanced operational dataset is used to compute rainfall, surface beat, and moisture fluxes, and vertical profiles of diabatic and/or subgrid-scale heating and moistening over these arrays.

Time series of daily rainfall computed from beat and moisture budgets are presented over seven arrays, including the intensive flux array, outer sounding array, and large-scale array. Vertical profiles of apparent beat source and apparent moisture sink are analyzed and presented for different arrays and for different rainfall rates.

The mean budget-derived rainfall ranged from 4 to 12 mm day−1 over the various arrays, with the most rain occurring within the intensive flux array and the least over Papua New Guinea. Correlations between convective indicators, low-level winds, and surface fluxes indicate that convection tends to precede or be coincident with increased surface fluxes in the more active regions south of the equator but not in the less convectively active regions.

Convective heating in this region tends to be vertically distributed in a dominant single mode, apparently a characteristic blend of convective and stratiform rain heating, with a broad peak in the midtroposphere around 400–500 mb. This distribution varies surprisingly little from day to day or with rainfall intensity. In contrast, convection over Papua New Guinea differs from the maritime convection. The convection over this large island produces more beating at upper-tropospheric levels than does the surrounding maritime convection. This indicates a fundamental difference between maritime and island rainfall that may well have significant effects on global-scale circulations.

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