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Observations and Parameterization of Boundary Layer Structures and Clouds at the ARM TWP Nauru Site

Observations and Parameterization of Boundary Layer Structures and Clouds at the ARM TWP Nauru Site. Bruce Albrecht, Virendra Ghate, and Dan Voss 1 Pavlos Kollias 2 1 University of Miami 2 Brookhaven National Laboratory. Model Description:

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Observations and Parameterization of Boundary Layer Structures and Clouds at the ARM TWP Nauru Site

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  1. Observations and Parameterization of Boundary Layer Structures and Clouds at the ARM TWP Nauru Site Bruce Albrecht, Virendra Ghate, and Dan Voss1 Pavlos Kollias21University of Miami 2Brookhaven National Laboratory Model Description: An atmospheric mixed layer representation of the subcloud layer for application in marine fair-weather cumulus regimes (Albrecht et al., 1977) is used to develop a diagnostic model of air-sea temperature and moisture differences. This model assumes local steady state conditions and cloud base height fixed at the mixed layer LCL. The subcloud layer radiative heating rate QRM from this model (where advection is negligible) can be written as zB=cloud base height; k=0.2; CT is a bulk aerodynamic transfer coefficient. Introduction: The observations from the ARM TWP-Nauru site provide a unique opportunity to fair-weather cumulus clouds and the associated boundary layer structure. In this study an atmospheric mixed layer representation of the subcloud layer for application in fair-weather cumulus regimes is used to evaluate a diagnostic model of the near surface temperature and moisture associated with a given sea surface surface temperature and wind speed. The validity of treating the Nauru (night-time) site as a marine site is demonstrated. Monthly averaged values of terms used to evaluate the residual longwave cooling. Cloud base height is from ceilometer (lowest 25%) estimates. Conclusions: Observations from Nauru demonstrate the utility of using a simple boundary layer model to represent air-sea differences. Night-time longwave radiative cooling in the subcloud layer is found to be remarkably constant for the period analyzed. Despite variations in wind speeds, sea-air temperature differences, and boundary layer depth, longwave cooling calculated as a residual from these quantities was on average ~2.5 oC day-1 with only small month-to-month variability. These values compared well with monthly averaged LW cooling rates from calculations by Mather and McFarlane. Temperature, moisture, and LCL/cloud-base variations in the boundary layer showed a dependence consistent with the model. References: Bruce A. Albrecht, Alan K. Betts, Wayne H. Schubert and Stephen K. Cox. 1979: Model of the Thermodynamic Structure of the Trade-Wind Boundary Layer: Part I. Theoretical Formulation and Sensitivity Tests. J. Atmos. Sci., 36,73–89. Mather, J., S. McFarlane, M. Miller, and K. Miller, 2007: Cloud Properties and Associated Radiative Heating Rates in the Tropical Western Pacific. JGR (in Press) Residual longwave cooling rates compared with those calculated (Mather and MacFarlane; Mather et al., 2007). LWup is σTo4 and LWdwn from observations. Residual longwave cooling rates calculated using cloud base height and those using LCL. Monthly averaged sea-air mixing ratio difference as a function of sea-air temperature differences showing consistency with model. SST Air Temp Observations showing correlation between cloud base height (mixed layer depth) and sea-air temperature difference that is consistent with model LW cooling rates (residuals and calculated) as a function of difference between LWup and LWdw. Line is fit to calculated values. Sea surface temperature (NCEP/Reynolds) and Nauru air temperature. SSTs BL Forcing Winds QRM residual cooling and cooling calculated from LW fluxes using regression from above. Monthly averaged Nauru and QuikScat winds showing good correlation with 3 ms-1 offset.

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