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  • Author or Editor: Rafael L. Bras x
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Jingfeng Wang
and
Rafael L. Bras

Abstract

An extremum hypothesis of turbulent transport in the atmospheric surface layer is postulated. The hypothesis has led to a unique solution of Monin–Obukhov similarity equations in terms of simple expressions linking shear stress (momentum flux) and heat flux to mean wind shear and temperature gradient. The extremum solution is consistent with the well-known asymptotic properties of the surface layer. Validation of the extremum solution has been made by comparison to field measurements of momentum and heat fluxes. Furthermore, a modeling test of predicting surface heat fluxes using the results of this work is presented. A critical reexamination of the interpretation of the Obukhov length is given.

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Jingfeng Wang
,
Elfatih A. B. Eltahir
, and
Rafael L. Bras

Abstract

Mesoscale circulations forced by a random distribution of surface sensible heat flux have been investigated using a three-dimensional numerical model. The complex land surface is modeled as a homogeneous random field characterized by a spectral distribution. Standard deviation and length scale of the sensible heat flux at the surface have been identified as the important parameters that describe the thermal variability of land surface. The form of the covariance of the random surface forcing is not critical in driving the mesoscale circulation. The thermally induced mesoscale circulation is significant and extends up to about 5 km when the atmosphere is neutral. It becomes weak and is suppressed when the atmosphere is stable. The mesoscale momentum flux is much stronger than the corresponding turbulent momentum flux in the neutral atmosphere, while the two are comparable in the stable atmosphere. The mesoscale heat flux has a different vertical profile than turbulent heat flux and may provide a major heat transport mechanism beyond the planetary boundary layer. The impact of synoptic wind on the mesoscale circulations is relatively weak. Nonlinear advection terms are responsible for momentum flux in the absence of synoptic wind.

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Fabio Castelli
,
Rafael L. Bras
, and
Kerry A. Emanuel

Abstract

The existence of analytical solutions for two-dimensional nonlinear semigeostrophic models of moist frontogenesis is investigated. Two different schemes for the modeling of stratiform cloud thermodynamics are taken into account, one based on the assumption of an everywhere cloudy environment, while in the other the atmosphere is considered to be exactly saturated and only condensation effects are relevant. In the first case, an exact analytical solution is derived for arbitrary boundary conditions, which satisfies the requirements for the validation of the semigeostrophic approximation even when the atmosphere is conditionally unstable with respect to slantwise convection. The growth of symmetric instabilities with no short-wave cutoff is predicted for this conditionally unstable atmosphere, even under the approximations of the semigeostrophic theory. An analogous, but approximate, analytical solution is then proposed for the second case. The errors introduced by the approximation are, however, not bigger than the terms neglected in the semigeostrophic approximation itself.

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