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Xubin Zeng
,
Qiang Zhang
,
D. Johnson
, and
W-K. Tao

Abstract

Analysis of the Goddard cloud-ensemble (GCE) model output forced by observational data over the tropical western Pacific and eastern tropical North Atlantic has shown that ocean surface latent and sensible heat fluxes averaged in a typical global-model grid box are reproduced well using bulk algorithms with grid-box-average scalar wind speed but could be significantly underestimated under weak wind conditions using average vector wind speed. This is consistent with previous observational and modeling studies. The difference between scalar and vector wind speeds represents the subgrid wind variability (or wind gustiness) that is contributed by boundary layer large eddies, convective precipitation, and cloudiness. Based on the GCE data analysis for a case over the tropical western Pacific, a simple parameterization for wind gustiness has been developed that considers the above three factors. This scheme is found to fit well the GCE data for two other cases over the tropical western Pacific and eastern tropical North Atlantic. Its fit is also much better than that of the traditional approach that considers the contribution to wind gustiness by boundary layer large eddies alone. A simple formulation has also been developed to account for the dependence of the authors' parameterization on spatial scales (or model grid size). Together, the preliminary parameterization and formulation can be easily implemented into weather and climate models with various horizontal resolutions.

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G. M. Martin
,
D. W. Johnson
, and
A. Spice

Abstract

Observations from the Meteorological Research Flight's Hercules C-130 aircraft of the microphysical characteristics of warm stratocumulus clouds have been analyzed to investigate the variation of the effective radius of cloud droplets in layer clouds. Results from experiments in the eastern Pacific, South Atlantic, subtropical regions of the North Atlantic, and the sea areas around the British Isles are presented. In situations where entrainment effects are small the (effective radius)3 is found to be a linear function of the (volume-averaged radius)3 in a given cloud and can thus be parameterized with respect to the liquid water content and the droplet number concentration in the cloud. However, the shape of the droplet size spectrum is very dependent on the cloud condensation nuclei (CCN) characteristics below cloud base, and the relationship between effective radius and volume-averaged radius varies between maritime air masses and continental air masses. This study also details comparisons that have been made in stratocumulus between the droplet number concentrations and (a) aerosol concentrations below cloud base in the size range 0.1 to 3.0 μm and (b) CCN supersaturation spectra in the boundary layer. A parameterization relating droplet concentration and aerosol concentration is suggested. The effects of nonadiabatic processes on the parameterization of effective radius are discussed. Drizzle is found to have little effect near cloud top, but in precipitating stratocumulus clouds the parameterization breaks down near cloud base. Comparisons are made between this parameterization of effective radius and others used currently or in the past.

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D. E. Johnson
,
W-K. Tao
, and
J. Simpson

Abstract

The Goddard Cumulus Ensemble (GCE) model is used to examine the sensitivities of multiday 2D simulations of deep tropical convection to surface fluxes, interactive radiation, and ice microphysical processes. The simulations incorporate large-scale temperature, moisture, and momentum forcings, from the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) for the period 19–27 December 1992.

This study shows that, when surface fluxes are eliminated, the mean simulated atmosphere is much cooler and drier, convection and CAPE are much weaker, precipitation is less, and low-level to midlevel cloudiness is much greater. Surface fluxes using the TOGA COARE flux algorithm are weaker than with the aerodynamic formulation, but closer to the observed fluxes. In addition, trends similar to those noted above for the case without surface fluxes are produced for the TOGA COARE flux case, albeit to a much lesser extent. The elimination of shortwave and longwave radiation is found to have only minimal effects on the mean thermodynamics, convection, and precipitation. However, exclusion of radiation in the model does have a significant impact on cloud temperatures and structure above 200 mb.

The removal of ice microphysical processes produces major changes in the structure of the clouds. Much of the liquid water is transported to the upper levels of the troposphere and evaporates, resulting in less mean total surface precipitation. The precipitation primarily occurs in regions of narrow, but intense, convective rainfall bands. The elimination of melting processes (diabatic cooling and conversions to rain) leads to greater (ice) hydrometeor mass below the 0°C level and reduced latent cooling. This, along with weaker vertical cloud mass fluxes, produces a much warmer and moister boundary layer, and a greater mean CAPE. Finally, the elimination of the graupel species has only a small impact on mean total precipitation, thermodynamics, and dynamics of the simulation, but does produce much greater snow mass just above the melting layer.

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W-K. Tao
,
D. Johnson
,
C-L. Shie
, and
J. Simpson

Abstract

A two-dimensional version of the Goddard Cumulus Ensemble (GCE) model is used to simulate convective systems that developed in various geographic locations (east Atlantic, west Pacific, South China Sea, and Great Plains in the United States). Observed large-scale advective tendencies for potential temperature, water vapor mixing ratio, and horizontal momentum derived from field campaigns are used as the main forcing. The atmospheric temperature and water vapor budgets from the model results show that the two largest terms are net condensation (heating/drying) and imposed large-scale forcing (cooling/moistening) for tropical oceanic cases though not for midlatitude continental cases. These two terms are opposite in sign, however, and are not the dominant terms in the moist static energy budget.

The balance between net radiation, surface latent heat flux, and net condensational heating vary in these tropical cases, however. For cloud systems that developed over the South China Sea and eastern Atlantic, net radiation (cooling) is not negligible in the temperature budget; it is as large as 20% of the net condensation. However, shortwave heating and longwave cooling are in balance with each other for cloud systems over the west Pacific region such that the net radiation is very small. This is due to the thick anvil clouds simulated in the cloud systems over the Pacific region. The large-scale advection of moist static energy is negative, as a result of a larger absolute value of large-scale advection of sensible heat (cooling) compared to large-scale latent heat (moistening) advection in the Pacific and Atlantic cases. For three cloud systems that developed over a midlatitude continent, the net radiation and sensible and latent heat fluxes play a much more important role. This means that the accurate measurement of surface fluxes and radiation is crucial for simulating these midlatitude cases.

The results showed that large-scale mean (multiday) precipitation efficiency (PE) varies from 24% to 31% (or 32% to 45% using a different definition of PE) between cloud systems from different geographic locations. The model results showed that there is no clear relationship between the PE and rainfall, the positive cloud condensation (condensation plus deposition), or the large-scale forcing. But, the model results suggest that cases with large, positive net condensation terms in the moist static energy budget tend to have a large PE.

The PE and its relationship with relative humidity and the vertical shear of the horizontal wind are also examined using 6-hourly model data. The model results suggest that there is no clear relationship between the individual PE and total mass-weighted relative humidity or the middle- and upper-tropospheric moisture for each case. The model results suggest that for the west Pacific and east Atlantic cases, PE slightly decreases with increasing middle-tropospheric wind shear in low to moderate shear regimes. The correlation (based on the best polynomial fit) is quite weak however. No strong relationship between PE and wind shear was found for the South China Sea and cases over the United States.

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K. A. Browning
,
J. Hallett
,
T. W. Harrold
, and
D. Johnson

Abstract

Attempts have been made to obtain samples of freshly fallen hailstones from severe storms in Oklahoma with the purpose of studying the nature and extent of spongy ice within natural hail. Interception by automobile of radar echoes with Ze > 105 mm6 m−3 has been found to provide a workable technique for collecting large hailstones as they fall to the ground. Observations suggest that the regions of highest reflectivity were associated more closely with the falls of large hail than with the accompanying heavy rain.

Immediate sectioning of the freshly fallen hailstones revealed the presence of thin shells of spongy ice in many of the larger stones. Calorimetric analyses gave liquid water contents of up to 12 ± 4% of the mass of the stones. Some of the hailstones were aspherical owing to preferential melting of the regions of spongy ice during fall. In the case of hailstones that were stored at sub-freezing temperatures, spongy ice shells could often still be identified from the presence of millimeter size cavities embedded within ice composed of large crystals.

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W. J. MAUNDER
,
STANLEY R. JOHNSON
, and
J. D. McQUlGG

Abstract

Daily engineering records from two road construction projects near Jefferson City, Mo., for the years 1965–68 were combined with soil moisture and precipitation measurements from nearby meteorological stations to develop a model capable of producing an experimental series describing conditions suitable for road building activities. This model was then applied to a long-term series of daily precipitation records for Jefferson City (1918–65) to calculate road construction conditions over this period. Monthly and seasonal statistics describing the feasibility of various levels of road building activity are presented for the 48-yr period. These statistics include second-order Markov chain probability estimates of working and nonworking days. Aside from the inferences which can be directly drawn from the seasonal and monthly descriptive data, the statistics may have value in developing further simulation models for estimating the effects of various management strategies.

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W. J. MAUNDER
,
STANLEY R. JOHNSON
, and
J. D. McQUIGG

Abstract

A simulation model is applied to a long series (1918–65) of daily rainfall observations to produce an experimental series of operational records on the weather-sensitive portion of a road building project. These records are analyzed statistically for various periods of time within the normal construction season and the resulting statistics are examined for their potential usefulness in the management of road construction.

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EXECUTIVE COMMITTEE
,
D. S. Johnson
,
D. Atlas
,
W. W. Kellogg
,
F. G. Shuman
,
W. H. Best
,
P. D. McTaggart-Cowan
,
K. C. Spengler
, and
D. F. Landrigan
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R. D. Harmel
,
C. W. Richardson
,
C. L. Hanson
, and
G. L. Johnson

Abstract

Weather simulation models are commonly used to generate synthetic daily weather for use in studies of crop growth, water quality, water availability, soil erosion, climate change, and so on. Synthetic weather sequences are needed if long-term measured data are not available, measured data contain missing records, collection of actual data is cost or time prohibitive, or when necessary to simulate impacts of future climate scenarios. Most weather generators are capable of producing one or more components of weather such as precipitation, temperature, solar radiation, humidity, and wind speed. This study focused on one generation component, the procedure commonly used by weather simulation models to generate daily maximum and minimum temperature. The normal distribution is used by most weather generators (including USCLIMATE, WXGEN, LARS-WG, CLIMGEN, and CLIGEN) to generate daily maximum and minimum temperature values. The objective of this study was to analyze the adequacy of generating temperature data from the normal distribution. To accomplish this objective, the assumption of normality in measured daily temperatures was evaluated by testing the hypothesis that daily minimum and maximum temperature are normally distributed for each month. In addition, synthetic temperature records generated with the normal distribution were compared with measured temperature records. Based on these analyses, it was determined that measured daily maximum and minimum temperature are generally not normally distributed in each month but often are slightly skewed, which contradicts the assumption of normality used by most weather generators. In addition, generating temperature from the normal distribution resulted in several physically improbable values.

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D. E. Johnson
,
W-K. Tao
,
J. Simpson
, and
C-H. Sui

Abstract

Interactions between deep tropical clouds over the western Pacific warm pool and the larger-scale environment are key to understanding climate change. Cloud models are an extremely useful tool in simulating and providing statistical information on heat and moisture transfer processes between cloud systems and the environment, and can therefore be utilized to substantially improve cloud parameterizations in climate models. In this paper, the Goddard Cumulus Ensemble (GCE) cloud-resolving model is used in multiday simulations of deep tropical convective activity over the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). Large-scale temperature and moisture advective tendencies, and horizontal momentum from the TOGA COARE Intensive Flux Array region, are applied to the GCE version that incorporates cyclical boundary conditions. Sensitivity experiments show that the horizontal extent (size) of the domain produces the largest response to domain-mean temperature and moisture deviations, as well as cloudiness, in comparison with grid horizontal or vertical resolution, and advection scheme. It is found that a domain size of at least 512 km is needed to adequately contain the convective cloud features and to replicate both the eastward and westward movements of the observed precipitating systems. The control experiment shows that the atmospheric heating and moistening is primarily a response to cloud latent processes of condensation/evaporation, and deposition/sublimation. Air–sea exchange of heat and moisture is found to be of secondary importance, while the net radiational heating–cooling is small except above cloud tops. A convective–stratiform breakdown of the precipitating systems shows that while 55% of the total rainfall occurs in convective regions, 90% of the total rainfall coverage occurs in stratiform regions. The simulated rainfall and atmospheric heating and moistening rates agree very well with observations, and the results compare favorably to other models simulating this case.

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