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Numerical Simulations of the Marine Stratocumulus-Capped Boundary Layer and Its Diurnal Variation

W. S. SmithEarth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico

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C-Y. J. KaoEarth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico

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Abstract

A high-resolution one-dimensional version of a second-order turbulence radiative–convective model, developed at Los Alamos National Laboratory, is used to simulate the diurnal cycle of the marine stratocumulus cloud-capped boundary layer. The fidelity of the model to the underlying physics is assessed by comparing the model simulation to data taken at San Nicolas Island during the intensive field observation (IFO) of the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE), conducted during June and July 1987. The model is able to reproduce the observed diurnal cycle of the liquid water content, cloud-base height, radiative heating or cooling rates, and the mean and turbulence variables fairly well. The mechanisms that cause the diurnal variation and the decoupling of the boundary layer are examined.

The possible role of an imposed diurnal cycle for the subsidence in inducing the cloud-top diurnal cycle observed during the FIRE IFO is also addressed. Three regimes of subsidence influence are identified for the stratocumulus-capped boundary layer. Regimes I and III are characterized by vertical propagation of the inversion height and erratic fluctuation of turbulence in the region of the inversion. Regime II is characterized by a continuum of quasi-equilibrium states that can exist for a range of subsidence values. In this regime, the boundary layer height is fairly insensitive to changes in the subsidence. The boundary layer behavior implied for these regimes is used to explore the effect of a diurnally varying subsidence rate on the diurnal cycle for the cloud-top height.

Abstract

A high-resolution one-dimensional version of a second-order turbulence radiative–convective model, developed at Los Alamos National Laboratory, is used to simulate the diurnal cycle of the marine stratocumulus cloud-capped boundary layer. The fidelity of the model to the underlying physics is assessed by comparing the model simulation to data taken at San Nicolas Island during the intensive field observation (IFO) of the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE), conducted during June and July 1987. The model is able to reproduce the observed diurnal cycle of the liquid water content, cloud-base height, radiative heating or cooling rates, and the mean and turbulence variables fairly well. The mechanisms that cause the diurnal variation and the decoupling of the boundary layer are examined.

The possible role of an imposed diurnal cycle for the subsidence in inducing the cloud-top diurnal cycle observed during the FIRE IFO is also addressed. Three regimes of subsidence influence are identified for the stratocumulus-capped boundary layer. Regimes I and III are characterized by vertical propagation of the inversion height and erratic fluctuation of turbulence in the region of the inversion. Regime II is characterized by a continuum of quasi-equilibrium states that can exist for a range of subsidence values. In this regime, the boundary layer height is fairly insensitive to changes in the subsidence. The boundary layer behavior implied for these regimes is used to explore the effect of a diurnally varying subsidence rate on the diurnal cycle for the cloud-top height.

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