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S. H. Melfi, J. D. Spinhirne, S-H. Chou, and S. P. Palm

Abstract

Observations of a convective planetary boundary layer (PBL) were made with an airborne, downward-looking lidar system over the Atlantic Ocean during a cold air outbreak. The lidar data revealed well-organized, regularly spaced cellular convection with dominant spacial scales between two and four times the height of the boundary layer. It is demonstrated that the lidar can accurately measure the structure of the PBL with high vertical and horizontal resolution. Parameters important for PBL modeling such as entrainment zone thickness, entrainment rate, PBL height and relative heat flux can be inferred from the lidar data. It is suggested that wind shear at the PBL top may influence both entrainment and convective cell size.

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W-K. Tao, S. Lang, J. Simpson, C-H. Sui, B. Ferrier, and M-D. Chou

Abstract

Radiative forcing and latent heat associated with precipitation are the two most important diabatic processes that drive the circulation of the atmosphere. Clouds can affect radiation and vice versa. It is known that longwave radiative processes can enhance precipitation in cloud systems. This paper concentrates on determining the relative importance of three specific longwave radiative mechanisms by comparing cloud-resolving models with and without one or more of these processes. Three of the ways that longwave radiation is thought to interact with clouds are as follows: 1) cloud-top cooling and cloud-base warming may alter the thermal stratification of cloud layers, 2) differential cooling between clear and cloudy regions might enhance convergence into the cloud system, and 3) large-scale cooling could change the environment. A two-dimensional version of the Goddard Cumulus Ensemble model has been used to perform a series of sensitivity tests to identify which is the dominant cloud-radiative forcing mechanism with respect to the organization, structure, and precipitation processes for both a tropical (EMEX) and a midlatitude (PRE-STORM) mesoscale convective system.

The model results indicate that the dominant process for enhancing the surface precipitation in both the PRE-STORM and EMEX squall cases is the large-scale radiative cooling. However, the overall effect is really to increase the relative humidity and not tie convective available potential energy (CAPE). Because of the high moisture in the Tropics, the increase in relative humidity by radiative cooling can have more of an impact on precipitation in the tropical case than in the midlatitude case. The large-scale cooling led to a 36% increase in rainfall for the tropical cast. The midlatitude model squall with a higher CAPE and lower humidity environment was only slightly affected (8%) by any of the longwave mechanisms. Our results also indicated that the squall systems' overall (convective and stratiform) precipitation is increased by turning off the cloud-top cooling and cloud-base warming. Therefore, the cloud-top cooling-cloud-base warming mechanism was not the responsible cloud-radiative mechanism for enhancing the surface precipitation. However, the circulation as well as the microphysical processes were indeed (slightly) enhanced in the stratiform region by the cloud-top cooling and cloud-base warming mechanism for the midlatitude squall case.

For both cases, the model results show that the mechanism associated with differential cooling between the clear and cloudy regions may or may not enhance precipitation processes. However, this mechanism is definitely less important than the large-scale longwave radiative cooling. Solar heating was run from 0900 to 1300 LST in both environments and was found to decrease the precipitation by 7% in each case compared to the runs with longwave radiation only. This result suggests that solar heating may play a significant role in the daytime minimum/nighttime maximum precipitation cycle found over most oceans.

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W. -K. Tao, J. Simpson, C. H. Sui, B. Ferrier, S. Lang, J. Scala, M. D. Chou, and K. Pickering

Abstract

A two-dimensional, time-dependent, and nonhydrostatic numerical cloud model is used to estimate the heating (Q 1, moisture (Q 2), and water budgets in the convective and stratiform regions for a tropical and a midlatitude squall line (EMEX and PRE-STORM). The model is anelastic and includes a parameterized three-class ice-phase microphysical scheme and longwave radiative transfer processes. A quantitative estimate of the impact of the longwave radiative cooling on the total surface precipitation as well as on the development and structure of these two squall lines is presented.

It was found that the vertical eddy moisture fluxes are a major contribution to the model-derived Q 2 budgets in both squall cases. A distinct midlevel minimum in the Q 2 profile for the EMEX case is due to vertical eddy transport in the convective region. On the other hand, the contribution to the Q 1 budget by the cloud-scale fluxes is minor for the EMEX case. In contrast, the vertical eddy heat flux is relatively important for the PRE-STORM case due to the stronger vertical velocities present in the PRE-STORM convective cells. It was found that the convective region plays an important role in the generation of stratiform rainfall for both cases. Although the EMEX case has more stratiform rainfall than its PRE-STORM counterpart, the relative contribution to the stratiform water budget made by the horizontal transfer of hydrometeors from the convective region is less. But the transfer of condensate from the convective region became relatively less important with time in the stratiform water budget of the PRE-STORM system as it developed from its initial stage, such that the relative contribution to the stratiform water budget made by the horizontal transfer of hydrometeors from the convective region is similar at the mature stages of both systems.

Longwave radiative cooling enhanced the total surface precipitation about 14% and 31% over a 16-h simulation time for the PRE-STORM and EMEX cases, respectively. The relative contribution to the stratiform water budget from the convective region is, however, more sensitive to the longwave radiative cooling for the PRE-STORM case than for the EMEX case. These results are due to the relatively moist environment and comparatively earlier development of the stratiform cloud in the EMEX squall system. Nevertheless, the effect of radiative cooling is shown to increase as systems age in both cases. It was also determined that the Q 1 and Q 2 budgets in the convective and stratiform regions are only quantitatively, not qualitatively, altered by the inclusion or exclusion of longwave radiative transfer processes.

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J. A. Curry, A. Bentamy, M. A. Bourassa, D. Bourras, E. F. Bradley, M. Brunke, S. Castro, S. H. Chou, C. A. Clayson, W. J. Emery, L. Eymard, C. W. Fairall, M. Kubota, B. Lin, W. Perrie, R. A. Reeder, I. A. Renfrew, W. B. Rossow, J. Schulz, S. R. Smith, P. J. Webster, G. A. Wick, and X. Zeng

High-resolution surface fluxes over the global ocean are needed to evaluate coupled atmosphere–ocean models and weather forecasting models, provide surface forcing for ocean models, understand the regional and temporal variations of the exchange of heat between the atmosphere and ocean, and provide a large-scale context for field experiments. Under the auspices of the World Climate Research Programme (WCRP) Global Energy and Water Cycle Experiment (GEWEX) Radiation Panel, the SEAFLUX Project has been initiated to investigate producing a high-resolution satellite-based dataset of surface turbulent fluxes over the global oceans to complement the existing products for surface radiation fluxes and precipitation. The SEAFLUX Project includes the following elements: a library of in situ data, with collocated satellite data to be used in the evaluation and improvement of global flux products; organized intercomparison projects, to evaluate and improve bulk flux models and determination from the satellite of the input parameters; and coordinated evaluation of the flux products in the context of applications, such as forcing ocean models and evaluation of coupled atmosphere–ocean models. The objective of this paper is to present an overview of the status of global ocean surface flux products, the methodology being used by SEAFLUX, and the prospects for improvement of satellite-derived flux products.

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