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G. A. Dalu and R. A. Pielke

Abstract

An analytical evaluation of the vertical heat fluxes associated with the mesoscale flow generated by thermal inhomogeneities in the PBL in the absence of a synoptic wind is presented. Results show that the mesoscale fluxes are of the same order as the diabatic heat fluxes.

In the sea-breeze case results show that in the lower layer of the atmosphere the heat flux is positive over the land and negative over the sea with an overall positive horizontal average. In the free atmosphere above the PBL the mesoscale vertical heat flux is negative over the land and over the sea; that is, the lower atmosphere becomes warmer while the free atmosphere above becomes cooler. As a result the mesoscale flow contributes to the weakening of the atmospheric stability within a region that extends a Rossby radius distance from the coastline, and up to an altitude larger than twice the depth of the convective PBL. The average momentum flux equals zero because the momentum removed over the sea is fed back into the atmosphere over the land.

Sinusoidally periodic thermal inhomogeneities induce periodic atmospheric cells of the same horizontal scale. The intensity of mesoscale cells increases for increasing values of the wavenumber, reaches its maximum value when the wavelength of the forcing is of the order of the local Rossby radius, and then decreases as the wavelength of the forcing decreases, because of the destructive interference between mesoscale cells. The intensity of the vertical velocity and vertical fluxes is, however, only a weak function of the wavenumber, at large wavenumber. Therefore, the intensity of the mesoscale heat flux does not decrease substantially at high wavenumbers; however, the transport of cool air over small heated patches of land may cut off the temperature gradient in the atmosphere between the land and water early in the day, thereby reducing the duration of the mesoscale activity. Also horizontal diffusion of heat in the convective boundary layer can significantly weaken horizontal temperature gradients for large wavenumbers. Periodic square-wave thermal inhomogeneities are more effective than sinusoidal waves in generating mesoscale cells; that is, the intensity of the flow is generally stronger. When dealing with low resolution models, which do not resolve explicitly the mesoscale activity, the mesoscale heat fluxes have to be introduced in a parametric form, using this or a similar theory.

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C. Prabhakara, G. Dalu, R. C. Lo, and N. R. Nath

Abstract

From the depth of the water vapor spectral lines in the 8–9 μm window region, measured by the Nimbus 4 Infrared Interferometer Spectrometer (IRIS) with a resolution of about 3 cm−1, the precipitable water vapor w over the oceans is remotely sensed. In addition the IRIS spectral data in the 11–13 μm window region have been used to derive the sea surface temperature (SST). Seasonal maps of w on the oceans deduced from the spectral data reveal the dynamical influence of the large-scale atmospheric circulation. With the help of a model for the vertical distribution of water vapor, the configuration of the atmospheric boundary layer over the oceans can be inferred from these remotely sensed w and SST. The gross seasonal mean structure of the boundary layer inferred in this fashion reveals the broad areas of trade wind inversion and the convectively active areas such as the ITCZ. The derived information is in reasonable agreement with some observed climatological patterns over the oceans.

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G. A. Dalu, R. A. Pielke, M. Baldi, and X. Zeng

Abstract

The authors Present an analytical evaluation of the vertical heat and momentum fluxes associated with mesoscale flow generated by periodic and isolated thermal inhomogeneities within the convective boundary layer (CBL). The influence of larger-scale wind flow is also included.

The results show that, with little or no synoptic wind, the vertical velocity is in phase with the diabatic temperature perturbations and that the mesoscale heat flux is positive and of the same order as the diabatic heat flux within the CBL. Above the CBL, the heat flux is negative and penetrates into the free atmosphere through a depth comparable to the depth of the CBL. In the presence of synoptic flow, the mesoscale perturbation is in the form of propagating waves that penetrate deeply into the free atmosphere. As a result, there is a net downward flux of momentum, which is dissipated within the CBL by turbulence. Furthermore, mixing with the environment of the air particles displaced by the waves results in a net negative mesoscale heat flux, which contributes to the weakening of the stability of the free atmosphere.

Strong synoptic advection can significantly weaken the horizontal temperature gradients in the CBL, thereby weakening the intensity of the mesoscale flow. Turbulent diffusion also weakens the temperature gradients and the intensity of the mesoscale flow at large wavenumbers when the wavelength is comparable to the CBL depth. Finally, when the, synoptic wind is very strong, the mesoscale perturbation is very weak and vertically trapped.

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M. Baldi, G. A. Dalu, and R. A. Pielke Sr.

Abstract

It is shown that landscape variability decreases the temperature in the surface layer when, through mesoscale flow, cool air intrudes over warm patches, lifting warm air and weakening the static stability of the upper part of the planetary boundary layer. This mechanism generates regions of upward vertical motion and a sizable amount of available potential energy and can make the environment of the lower troposphere more favorable to cloud formation. This process is enhanced by light ambient wind through the generation of trapped propagating waves, which penetrate into the midtropospheric levels, transporting upward the thermal perturbations and weakening the static stability around the top of the boundary layer. At moderate ambient wind speeds, the presence of surface roughness changes strengthens the wave activity, further favoring the vertical transport of the thermal perturbations. When the intensity of the ambient wind is larger than 5 m s−1, the vertical velocities induced by the surface roughness changes prevail over those induced by the diabatic flux changes. The analysis is performed using a linear theory in which the mesoscale dynamics are forced by the diurnal diabatic sensible heat flux and by the surface stress. Results are shown as a function of ambient flow intensity and of the wavelength of a sinusoidal landscape variability.

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G. A. Dalu, M. Baldi, R. A. Pielke Sr., and G. Leoncini

Abstract

A theory is presented for the evaluation of the different terms of the pressure gradient force, when mesoscale flow is driven by a sensible heat source in the planetary boundary layer (PBL), or by an elevated confined heat source, such as the release of the latent heat of condensation in a cloud. The nonlinear and linear, and the nonhydrostatic and the hydrostatic pressure gradient contributions are evaluated. The validity of the different approximations is discussed as a function of time and space scales. In addition, the validity of this approach is explored as a function of atmospheric environmental parameters, such as static stability, large-scale flow, and dissipation.

By accessing the relative importance of each contribution, specific solution techniques for mesoscale atmospheric flows can be adopted. For example, when the linear contributions dominate, an exact analytic model could be used, rather than relying on numerical approximation solution techniques. When the hydrostatic contribution dominates, the spatial variation of the vertical temperature profile can be used to uniquely define the horizontal pressure gradient force.

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C. Prabhakara, G. Dalu, G. L. Liberti, J. J. Nucciarone, and R. Suhasini

Abstract

Passive microwave measurements made by the Scanning Multichannel Microwave Radiometer (SMMR) and the Special Sensor Microwave/Imager (SSM/I) reveal information about rain and precipitation-sized ice in the field of view (FOV) of the instruments. The brightness temperature T b measured at 37 GHZ, having an FOV of about 30 km, shows relatively strong emission from rain and only marginal effects caused by scattering by ice above the rain clouds. At frequencies below 37 GHz, where the FOV is larger and the volume extinction coefficient is weaker, it is found that the observations made by these radiometers do not yield appreciable additional information about rain. At 85 GHz (FOV ≈ 15 km), where the volume extinction coefficient is considerably larger, direct information about rain below the clouds is generally masked.

Based on the above considerations, 37-GHz observations with a 30-kin FOV from SMMR and SSM/I are selected for the purpose of rain-rate retrieval over oceans. An empirical method is developed to estimate the rain rate in which it is assumed that over an oceanic area the statistics of the observed T b's at 37 GHz in a rain storm are related to the rain-rate statistics in that storm. Also, in this method, the underestimation of rain rate, arising from the inability of the radiometer to respond sensitively to rain rate above a given threshold, is rectified with the aid of two parameters that depend on the total water vapor content in the atmosphere. The rain rates retrieved by this method compare favorably with radar observation. Monthly mean global maps of rain derived from this technique over the oceans are consistent with climatology.

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R. A. Pielke, G. A. Dalu, J. S. Snook, T. J. Lee, and T. G. F. Kittel

Abstract

This paper demonstrates that the influence of mesoscale landscape spatial variability on the atmosphere must be parameterized (or explicitly modeled) in larger-scale atmospheric model simulations including general circulation models. The mesoscale fluxes of heat that result from this variability are shown to be of the same order of magnitude but with a different vertical structure than found for the turbulent fluxes. These conclusions are based on experiments in which no phase changes of water were permitted. When, for example, cumulus clouds organized in response to the landscape pattern develop, the mesoscale influence on larger-scale climate is likely to be even more important.

To parameterize surface thermal inhomogeneities, the influence of landscape must be evaluated using spectral analysis or an equivalent procedure. For horizontal scales much less than the local Rossby radius, based on the results of Dalu and Pielke, the surface heat fluxes over the different land surfaces can be proportionately summed and an average grid-area value used as proposed by Avissar and Pielke. Moisture fluxes can probably be represented in the same fashion as for heat fluxes. For larger-scale spatial variability, however, the mesoscale fluxes must also be included as shown in this paper. While the linear effect could be parameterized using a procedure such as presented in Dalu and Pielke, where the spectral analysis is used to fractionally weight the contributions of the different spatial scales, the complete vertical mesoscale heat flux requires the incorporation of nonlinear advective effects. To include the nonlinear contribution of each scale, numerical model simulations for the range of observed surface and overlying atmospheric conditions must be performed.

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C. Prabhakara, R. S. Fraser, G. Dalu, Man-Li C. Wu, R. J. Curran, and T. Styles

Abstract

Spectral differences in the extinction between the 10.8 and 12.6 μm bands of the infrared window region, due to optically thin clouds, are observed in the measurements made by a broad-band infrared aircraft radiometer. Similar spectral properties are also revealed by the measurements made by the high-resolution infrared inter-ferometer spectrometer (IRIS) aboard the Nimbus-4 satellite, which had a field of view of ∼ 95 km. These observations show that the extinction due to cloud particles at 12.6 μm is appreciably larger than that at 10.8 μm. Both water or ice particles in the clouds can account for such spectral difference in extinction provided that the particles are smaller than the wavelength of radiation. This spectral effect is demonstrated with the help of multiple scattering radiative transfer calculations. As the IRIS data reveal this spectral feature, about 100 to 200 km away from the center of high altitude cold clouds (∼ 230 K), it is inferred that this feature is related to the spreading of cirrus clouds. Based on this hypothesis, we have deduced mean seasonal maps of the distribution of thin cirrus clouds over the oceans from 50°N to 50°S from the IRIS data. These maps reveal that, over the oceans, such clouds are often present over the convectively active areas such as ITCZ, SPCZ, and the Bay of Bengal. These results have application to studies of earth radiation balance and climate.

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