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S-T. Soong
and
W-K. Tao

Abstract

The two-dimensional cloud ensemble model developed by Soong and Ogura (1980) is used to simulate the response of deep clouds to mesoscale lifting using data obtained in the Global Atmospheric Research Program (GARP) Atlantic Tropical Experiment (GATE). The input to the model includes the mesoscale vertical velocity, horizontal advections of temperature and mixing ratio of water vapor, radiative cooling and sea surface temperature. The cloud ensemble feedback effects due to the condensation and evaporation of cloud liquid drops and vertical fluxes of heat and moisture are determined by the model.

The simulated upward mass flux inside the model clouds is about three times the mass flux due to mesoscale lifting. The downward mass flux inside clouds is also large, leaving a small downward mass flux in the cloud-free area. The major portion of the heat flux is produced by the updraft inside clouds. On the other hand, the moisture fluxes due to both updraft and downdraft are important. In the cloud-free area, the heat and moisture fluxes are both small due to the small mass flux in that area.

Experiments with different magnitudes of mesoscale lifting generate different sizes of clouds and different cloud heating and moistening profiles. However, in each simulation, the changes of temperature and mixing ratio due to mesoscale processes are almost balanced by the cloud heating and drying effects, leaving only small temporal changes in the horizontal mean temperature and mixing ratio.

In a simulation with only low-level lifting, a warming is generated in the middle levels. This warming can be important in producing higher level vertical lifting, which in turn could produce even higher clouds.

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W.-K. Tao
,
T. Iguchi
, and
S. Lang

Abstract

The Goddard convective–stratiform heating (CSH) algorithm has been used to retrieve latent heating (LH) associated with clouds and cloud systems in support of the Tropical Rainfall Measuring Mission and Global Precipitation Measurement (GPM) mission. The CSH algorithm requires the use of a cloud-resolving model to simulate LH profiles to build lookup tables (LUTs). However, the current LUTs in the CSH algorithm are not suitable for retrieving LH profiles at high latitudes or winter conditions that are needed for GPM. The NASA Unified-Weather Research and Forecasting (NU-WRF) Model is used to simulate three eastern continental U.S. (CONUS) synoptic winter and three western coastal/offshore events. The relationship between LH structures (or profiles) and other precipitation properties (radar reflectivity, freezing-level height, echo-top height, maximum dBZ height, vertical dBZ gradient, and surface precipitation rate) is examined, and a new classification system is adopted with varying ranges for each of these precipitation properties to create LUTs representing high latitude/winter conditions. The performance of the new LUTs is examined using a self-consistency check for one CONUS and one West Coast offshore event by comparing LH profiles retrieved from the LUTs using model-simulated precipitation properties with those originally simulated by the model. The results of the self-consistency check validate the new classification and LUTs. The new LUTs provide the foundation for high-latitude retrievals that can then be merged with those from the tropical CSH algorithm to retrieve LH profiles over the entire GPM domain using precipitation properties retrieved from the GPM combined algorithm.

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K. M. Lau
,
C. H. Sui
, and
W. K. Tao

This paper presents the preliminary findings of an investigation of the water budget of tropical cumulus convection using the Goddard Cumulus Ensemble Model (GCEM). Results of an experiment designed to obtain a “fingerprint” in the tropical hydrologic cycle in response to surface warming are also presented. The ensemble mean water budget shows that the distribution of water vapor and cloud water in the tropical atmosphere is maintained as a result of a balance between moisture convergence (including cloud scale and large scale) and condensation and reevaporation by various microphysical species within the cumulus clusters. Under radiative convective equilibrium conditions, 66% of the precipitation reaching the ground comes from the convective region and 34% from the stratiform region. In a climate with above-normal sea surface temperature but fixed large-scale vertical velocity, tropical convection is enhanced with more abundant moisture sources. Water vapor is increased throughout the troposphere with the surplus largest near the surface and decreases monotonically up to 10 km. However, the percentage increase in water vapor is largest near 8 to 16 km. As a result of the warming, the freezing level in clouds is elevated resulting in a large increase (decrease) in cloud water just above (below) 5 km. As with water vapor, the fractional increase in cloud water and cloudiness amount is largest at the upper troposphere.

In spite of the detailed microphysics and cloud-scale dynamical processes included in the GCEM, the results on changes in temperature and water vapor induced by surface warming are in agreement with those from general circulation models that use crude cumulus parameterization. This is consistent with previous findings that equilibrium water vapor distribution is a strong function of temperature. In an open domain such as the tropical convective environment, with a specified climatological vertical velocity, the ratio of increased precipitation to increased surface evaporation due to a 2°C surface warming is approximately 5. The increase is mostly found for convective rain and is negligible for stratiform rain. The climate implication of these changes is also discussed.

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Tao Wang
,
K. S. Lam
,
Agatha S. Y. Lee
,
S. W. Pang
, and
W. S. Tsui

Abstract

As in many metropolitan areas around the world, air pollution in Hong Kong is an increasing concern. In this paper the authors present the observations of ozone (O3) pollution episodes made at a nonurban coastal location in Hong Kong. Four O3 episodes were observed in 1994, during which hourly averaged O3 concentrations exceeded 100 ppbv and in one case reached 162 ppbv. Recirculation of urban air caused by the reversal of surface winds was found to be an important mechanism for transporting the “aged” urban plumes to the monitoring site. Concurrent measurements of CO, SO2, NO, and O3 provided an insight to the chemical characteristics of the air masses, and the chemical data appeared to suggest that the high levels of O3 during the episodes were produced in the urban plumes that were mainly characteristic of vehicle emissions. The relationship between O3 and CO in two of the episodes may be represented by a linear approximation, and a nonlinear relationship between O3 and CO was found in another. Ozone levels observed at the nonurban site were higher than those at two urban locations.

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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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J. Halverson
,
M. Garstang
,
J. Scala
, and
W-K. Tao

Abstract

Mesoscale water and energy budgets are diagnosed for a squall line during the Convection and Precipitation Electrification Experiment and combined with the results of the two-dimensional Goddard Cumulus Ensemble Model. The fine temporal and spatial resolution of cloud-scale processes contained in the model is used to reduce uncertainty in the diagnosed water budget residual and, thus, to arrive at a good estimate of storm-total rainfall. Profiles of cumulus heating (Q 1) and drying (Q 2) inferred from the sounding observations are in turn compared with the cloud-scale energy budget terms calculated from the model. This comparison reveals near-agreement in the magnitude and vertical distribution of the peak Q 1 and Q 2, and also the relative size of the heating and drying at different levels in the column.

When the size of the mesoscale convective disturbance is approximately the same as the sounding observation network, it may be wrong to assume that the diagnosed vertical eddy heat transport accounts for most of the total eddy transport of moist static energy, F. The cloud model is used to resolve the relative contribution of the horizontal and vertical eddy flux convergence of heat and moisture, and thus it serves as a guide to interpreting the sounding-diagnosed total flux. The model results suggest that although the mean column vertical flux convergence is significantly larger than the column-mean horizontal flux convergence, the horizontal flux convergence does play a significant role in midlevels of the convective region. This flux convergence may be associated with a strong front-to-rear inflow that develops during the mature stage of the squall line.

This study suggests that when combined with the independent results of a mesoscale cloud model, the sounding diagnostics can provide a sensitivity test for the Tropical Rainfall Measuring Mission measurements of rainfall and diabatic heating over the life cycle of an entire mesoscale convective system.

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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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S. Lang
,
W-K. Tao
,
J. Simpson
, and
B. Ferrier

Abstract

Six different convective–stratiform separation techniques are compared and evaluated using 2D numerical simulations of a tropical and a midlatitude continental squall line. The techniques used include a texture algorithm applied to surface rainfall, a similar algorithm but with additional criteria applied to vertical velocity and cloud, a texture algorithm applied to vertical velocities below the melting layer, a simple approach that assumes a constant characteristic width for the convective region, a more sophisticated texture algorithm applied to radar reflectivities below the melting layer, and a new technique based on the premise that the fall speed of precipitation particles is large relative to air velocity in regions of stratiform precipitation. Comparisons are made in terms of rainfall, mass fluxes, apparent heating and moistening, hydrometeor contents, reflectivity and vertical-velocity contoured-frequency-with-altitude diagrams (CFAD), microphysics, and latent heating retrieval. Overall, it was found that the different separation techniques produced results that qualitatively agreed. However, the quantitative differences were significant. The texture algorithm applied to surface rain consistently produced the most stratiform rain while the texture algorithm applied to radar reflectivities below the melting layer and the new method comparing air velocities to terminal velocities consistently produced the most convective rain. The simple constant-area method performed comparably to the others in this squall line setting. Observational comparisons within the context of the model were unable to identify a superior technique. However, all of the methods were able to generate CFADs that were consistent with observations. Latent heating retrieval was shown to be sensitive to the use of separation technique mainly as a result of differences in the stratiform region. Methods that found very little stratiform rain resulted in exaggerated rain-normalized stratiform heating profiles.

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