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Hung-Neng S. Chin

Abstract

A two-dimensional cloud model is used to study the interrelationships among cloud microphysics, radiation, and dynamics in a midlatitude broken-line squall system. The impact of the ice phase, longwave and shortwave radiation on the dynamic and microphysical structures of this multicellular storm, the thermodynamic properties of the cloud ensemble, and their cloud-radiative feedback to the modeled squall line system is investigated in detail. In addition, partitioned heat, moisture, and water budgets are used to assess quantitatively the role of anvil clouds on the modeled squall line system. The major conclusions are as follows.

1) Both ice phase and radiation have little influence on the multicellular characters of the modeled squall line system. However, the ice phase and longwave radiation significantly impact the mesoscale structure and lead to a more realistic feature having an evident transition zone between the bright melting band and the convective region in the model-derived radar reflectivity.

2) The development of rear inflow in the modeled squall line system is attributed to the upshear tilt of the convective system. The intensity of rear inflow is also modulated by the ice phase and radiation. This rear inflow is found to play an important role in the cloud-radiative feedback to the modeled squall line system.

3) For this type of squall line system, the ice phase and radiation do not considerably change the heating and drying profiles of the cloud ensemble (10% ∼ 20% difference in the maximum heating and drying). Due to the dominance of convective clouds, the contributions of stratiform clouds to the total heat and moisture budgets of the cloud ensemble account for only a relatively small portion (10% and 20% ∼ 30% for the maximum heat and moisture budgets, respectively).

4) Horizontal transport of hydrometeors from deep convection is the primary source (∼2/3) of the water budget for anvil clouds in ice simulations; the rest (∼1/3) is contributed by the mesoscale lifting associated with the tilting convective system.

5) Longwave optical properties of anvils are insensitive to the ice phase. However, the ice phase can significantly impact shortwave optical properties of anvils. In contrast to the destabilization of longwave radiation, shortwave radiation acts to stabilize the stratiform and convective clouds.

6) Model simulations imply that the feedback of anvil clouds to the large-scale system is most likely dominated by radiative processes. Owing to the large coverage of convectively generated anvil clouds, the present study suggests that the missing physics of cumulus-anvil interactions in general circulation models may result in an underestimated cloud albedo and an overestimated surface insolation.

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Hung-Neng S. Chin

Abstract

No abstract available.

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Hung-Neng S. Chin and Robert B. Wilhelmson

Abstract

A three-dimensional cloud model is used to simulate the early development and propagation of an arc-shaped line element similar to that found in the GARP (Global Atmospheric Research Program) Tropical Atlantic Experiment (GATE) 4–5 September 1974 squall line system. The simulated squall line element forms in a relatively unexplored environment with moderate convective available potential energy and a strong low-level jet (bulk Richardson number = 37) associated with an easterly wave. The simulated line element develops in a large-scale convergence region from an initial cell that splits and elongates in a manner reminiscent of some midlatitude lines. Simulation features compare favorably with observed characteristics of some of the line elements including line orientation (approximately perpendicular to the average wind shear below the low-level jet), propagation speed (11 m s−1 to the southwest), length (75 km), and maximum precipitation rate (187 mm h−1). In addition, the simulated line merges with cells that form ahead as observed.

The simulated arc-shaped line element consists of four regions several hours after its initiation. The northern region or right flank of the line contains a long-lived cell exhibiting three-dimensional characteristics similar to midlatitude supercells including long-lasting and steady updrafts with midlevel cyclonic rotation and movement to the right of the mean winds. Although the simulated supercell characteristics cannot be confirmed because of insufficient data from the limited GATE 4–5 September observations, updrafts with strong vertical vorticity have been observed to occur in other GATE rainbands where waterspouts have been seen. The region just to the south of this cell is the site where a cell that formed out ahead of the line segment merges with the line and also develops supercell-like characteristics. Farther to the south, convection develops with quasi two-dimensional characteristics below 3–4 km but breaking into several multicells above. A rear-inflow jet does not accompany these features and the winds in the downdraft are oblique too and nonuniform along the associated gust front. Finally, on the southernmost or left flank of the line element, cells with both long and short lifetimes develop.

Sensitivity tests indicate that once the early line structure has developed, its evolution and structure are not seriously altered by the removal of large-scale forcing. Further, the formation of the cold pool and gust front between the initial separating (splitting) cells is crucial to the filling in of the line. Changes in the thermodynamic profile consistent with nonsquall observations one-half day earlier result in only modest differences. Changes in the wind profile (based on the same observations) led to significant differences, such as the lack of convection between the initial separating cells and the merger taking place to the north of the right flank creating a convective complex in this region.

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Hung-Neng S. Chin, Peter M. Caldwell, and David C. Bader

Abstract

The Weather Research and Forecasting (WRF) model version 3.0.1 is used in both short-range (days) and long-range (years) simulations to explore the California wintertime model wet bias. California is divided into four regions (the coast, central valley, mountains, and Southern California) for validation. Three sets of gridded surface observations are used to evaluate the impact of measurement uncertainty on the model wet bias. Short-range simulations are driven by the North American Regional Reanalysis (NARR) data and designed to test the sensitivity of model physics and grid resolution to the wet bias using eight winter storms chosen from four major types of large-scale conditions: the Pineapple Express, El Niño, La Niña, and synoptic cyclones. Control simulations are conducted with 12-km grid spacing (low resolution) but additional experiments are performed at 2-km (high) resolution to assess the robustness of microphysics and cumulus parameterizations to resolution changes. Additionally, long-range simulations driven by both NARR and general circulation model (GCM) data are performed at low resolution to gauge the impact of the GCM forcing on the model wet bias.

These short- and long-range simulations show that low-resolution runs tend to underpredict precipitation in the coast region and overpredict it elsewhere in California. The sensitivity test of WRF physics in short-range simulations indicates that model precipitation depends most strongly on the microphysics scheme, though convective parameterization is also important, particularly near the coast. In contrast, high-resolution (2 km) simulation increases model precipitation in all regions. As a result, it improves the forecast bias in the coast region while it downgrades the model performance in the other regions. It is also found that the choice of validation dataset has a significant impact on the model wet bias of both short- and long-range simulations. However, this impact in long-range simulations appears to be a secondary contribution as compared to its counterpart from the GCM forcing.

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Hung-Neng S. Chin, Michael M. Bradley, Qiang Fu, and Chares R. Molenkamp

Abstract

A two-dimensional cloud model is used to study a tropical oceanic squall-line system. The dynamical and microphysical structures of the simulated squall-line system and the impact of environmental wind profiles on these structures are presented. The influence of the microphysics treatment on cloud radiative properties and the sensitivity of this simulated system to radiation is also investigated. In addition, partitioned heat, moisture and water budgets, and two radiative transfer schemes are used to assess the role of anvil clouds on the simulated system and on the assumption used in a bulk parameterization for cloud radiative properties. The comparison with a midlatitude study is also made to show its climatic implication. The major conclusions are as follows.

  1. The simulated tropical squall-line system replicates many observed features. A transition zone in the simulated multicellular storm is primarily caused by the jetlike wind profile, while it is due to longwave radiation in the midlatitude system.
  2. The effect of a jetlike wind profile is to weaken/strengthen the convective/anvil portion of the simulated system, which leads to an overall decrease of total surface precipitation by 17%.
  3. The moisture budgets indicate that tropical deep convection serves as a more efficient engine, pumping low-level moisture upward to form the upper-level anvil cloud, than its midlatitude counterpart although the convective instability is lower in the tropical environment.
  4. Microphysical production is the primary source of the water budget (∼3/5) in the simulated tropical anvil, and rest (∼2/5) is contributed by horizontal transport of hydrometeors from deep convection. This is just the reverse of the midlatitude case.
  5. The simulated tropical oceanic anvil has a stronger shortwave radiative forcing than the midlatitude continental anvil, although they have comparable longwave forcings.
  6. The small difference in total precipitation of the simulated system caused by different radiation transfer schemes appears to justify the assumption of using a bulk parameterization for cloud radiative properties.
  7. Comparisons of water budges and cloud radiative properties between simulated tropical and midlatitude anvils suggest the need to parameterize the tilting structure of mesoscale convective systems for improving the representation of cloud processes in general circulation models.
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Steven K. Krueger, Qiang Fu, K. N. Liou, and Hung-Neng S. Chin

Abstract

It is important to properly simulate the extent and ice water content of tropical anvil clouds in numerical models that explicitly include cloud formation because of the significant effects that these clouds have on the radiation budget. For this reason, a commonly used bulk ice-phase microphysics parameterization was modified to more realistically simulate some of the microphysical processes that occur in tropical anvil clouds. Cloud ice growth by the Bergeron process and the associated formation of snow were revised. The characteristics of graupel were also modified in accord with a previous study. Numerical simulations of a tropical squall line demonstrate that the amount of cloud ice and the extent of anvil clouds are increased to more realistic values by the first two changes.

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Hung-Neng S. Chin, Daniel J. Rodriguez, Richard T. Cederwall, Catherine C. Chuang, Allen S. Grossman, John J. Yio, Qiang Fu, and Mark A. Miller

Abstract

Using measurements from the Department of Energy’s Atmospheric Radiation Measurement Program, a modified ground-based remote sensing technique is developed and evaluated to study the impacts of the subadiabatic character of continental low-level stratiform clouds on microphysical properties and radiation budgets. Airborne measurements and millimeter-wavelength cloud radar data are used to validate retrieved microphysical properties of three stratus cloud systems occurring in the April 1994 and 1997 intensive observation periods at the Southern Great Plains site.

The addition of the observed cloud-top height into the Han and Westwater retrieval scheme eliminates the need to invoke the adiabatic assumption. Thus, the retrieved liquid water content (LWC) profile is represented as the product of an adiabatic LWC profile and a weighting function. Based on in situ measurements, two types of weighting functions are considered in this study: one is associated with a subadiabatic condition involving cloud-top entrainment mixing alone (type I) and the other accounts for both cloud-top entrainment mixing and drizzle effects (type II). The adiabatic cloud depth ratio (ACDR), defined as the ratio of the actual cloud depth to the one derived from the adiabatic assumption, is found to be a useful parameter for classifying the subadiabatic character of low-level stratiform clouds. The type I weighting function only exists in the lower ACDR regime, while the type II profile can appear for any adiabatic cloud depth ratio.

Results indicate that the subadiabatic character of low-level stratiform clouds has substantial impacts on radiative energy budgets, especially those in the shortwave, via the retrieved LWC distribution and its related effective radius profile of liquid water. Results also show that this subadiabatic character can act to stabilize the cloud deck by reducing the in-cloud radiative heating/cooling contrast. As a whole, these impacts strengthen as the subadiabatic character of low-level stratiform clouds increases.

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Hung-Neng S. Chin, Martin J. Leach, Gayle A. Sugiyama, John M. Leone Jr., Hoyt Walker, J. S. Nasstrom, and Michael J. Brown

Abstract

A modified urban canopy parameterization (UCP) is developed and evaluated in a three-dimensional mesoscale model to assess the urban impact on surface and lower-atmospheric properties. This parameterization accounts for the effects of building drag, turbulent production, radiation balance, anthropogenic heating, and building rooftop heating/cooling. U.S. Geological Survey (USGS) land-use data are also utilized to derive urban infrastructure and urban surface properties needed for driving the UCP. An intensive observational period with clear sky, strong ambient wind, and drainage flow, and the absence of a land–lake breeze over the Salt Lake Valley, occurring on 25–26 October 2000, is selected for this study.

A series of sensitivity experiments are performed to gain understanding of the urban impact in the mesoscale model. Results indicate that within the selected urban environment, urban surface characteristics and anthropogenic heating play little role in the formation of the modeled nocturnal urban boundary layer. The rooftop effect appears to be the main contributor to this urban boundary layer. Sensitivity experiments also show that for this weak urban heat island case, the model horizontal grid resolution is important in simulating the elevated inversion layer.

The root-mean-square errors of the predicted wind and temperature with respect to surface station measurements exhibit substantially larger discrepancies at the urban locations than their rural counterparts. However, the close agreement of modeled tracer concentration with observations fairly justifies the modeled urban impact on the wind-direction shift and wind-drag effects.

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