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A. K. Betts
and
W. Ridgway

Abstract

An idealized energy-balance model for a closed circulation is first presented to illustrate the coupling between the net tropospheric radiative cooling, the surface fluxes and the mean subsidence away from the precipitation zones. Then a one-dimensional diagnostic model and a radiation model with boundary layer clouds are combined to explore this coupling for a specific region using mean sounding data over the tropical Pacific. The radiatively driven subsidence rate at the top of the convective boundary layer is approximately 35 mb day−1 (0.04 Pa s−1 and is largely independent of boundary layer cloud fraction. The sensitivity of the corresponding convective heat flux profiles to the mass divergence profile and cloud fraction within the boundary layer is explored. Reasonable assumptions give realistic surface sensible and latent heat fluxes for this region of approximately 10 and 130 W m−1. The paper illustrates the important background climatic control of the radiation field on the tropical surface fluxes.

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Alan K. Betts
and
W. Ridgway

Abstract

A one-dimensional thermodynamic model for a partially mixed, partly cloudy, convective boundary layer (CBL) is coupled to a radiation model to compute equilibrium solutions for a tropical CBL and troposphere in energy balance over the ocean. For a sea surface temperature (SST) of 300 K, the model gives an equilibrium cloud base ≈ 950 mb, a CBL top ≈ 800 mb and a low level θ e ≈ 347 K, close to climatic values. The CBL deepens and low level θ e rises with increasing wind speed and SST. We explore the change in CBL structure and surface fluxes with external parameters on three timescales; namely, the CBL (∼1 day); the tropospheric radiative equilibrium (∼10 days); and the oceanic thermal equilibrium (>100 days). The variation in cloud top decreases with greater coupling to atmosphere and ocean. The slope of the latent heat flux with increasing SST decreases with more tropospheric coupling, and reverse sign with a coupled ocean. This simplified model gives an increase of tropical SST with a doubling of CO2 on climatic timscales of 2–3°K, increasing with upper tropospheric moisture.

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Alan K. Betts
,
Patrick Minnis
,
W. Ridgway
, and
David F. Young

Abstract

A mixing-line boundary-layer model is used to retrieve cloud-top height from satellite-derived cloud-top temperature using 700-hPa National Meteorological Center (NMC) analyses and the Comprehensive Ocean and Atmosphere Data Set (COADS) surface data as supporting datasets. Results are compared with the fixed-lapse-rate method of retrieving boundary-layer depth from sea surface temperatures (SST) and cloud-top temperatures. A radiative-convective equilibrium boundary-layer model is used to retrieve boundary-layer structure given SST and surface wind, satellite cloud-top temperatures and cloud fraction, and the 700-hPa NMC thermodynamic analyses. Good agreement is found between the COADS data and the model solutions for low-level temperature and moisture. This suggests that equilibrium boundary-layer models may be of use over remote oceans in the retrieval of boundary-layer structure.

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