Boundary Layer Thermodynamics Research Articles

On the Characteristic Height Scales of the Hurricane Boundary Layer

AbstractIn this study, data from 794 GPS dropsondes deployed by research aircraft in 13 hurricanes are analyzed to study the characteristic height scales of the hurricane boundary layer. The height scales are defined in a variety of ways: the height of the maximum total wind speed, the inflow layer depth, and the mixed layer depth. The height of the maximum wind speed and the inflow layer depth are referred to as the dynamical boundary layer heights, while the mixed layer depth is referred to as the thermodynamical boundary layer height. The data analyses show that there is a clear separation of the thermodynamical and dynamical boundary layer heights. Consistent with previous studies on the boundary layer structure in individual storms, the dynamical boundary layer height is found to decrease with decreasing radius to the storm center. The thermodynamic boundary layer height, which is much shallower than the dynamical boundary layer height, is also found to decrease with decreasing radius to the storm center. The results also suggest that using the traditional critical Richardson number method to determine the boundary layer height may not accurately reproduce the height scale of the hurricane boundary layer. These different height scales reveal the complexity of the hurricane boundary layer structure that should be captured in hurricane model simulations.

On the characteristic height scales of the hurricane boundary layer

In this study, data from 794 GPS dropsondes deployed by research aircraft in 13 hurricanes are analyzed to study the characteristic height scales of the hurricane boundary layer. The height scales are defined in a variety of ways: the height of the maximum total wind speed, the inflow layer depth, and the mixed layer depth. The height of the maximum wind speed and the inflow layer depth are referred to as the dynamical boundary layer heights, while the mixed layer depth is referred to as the thermodynamical boundary layer height. The data analyses show that there is a clear separation of the thermodynamical and dynamical boundary layer heights. Consistent with previous studies on the boundary layer structure in individual storms, the dynamical boundary layer height is found to decrease with decreasing radius to the storm center. The thermodynamic boundary layer height, which is much shallower than the dynamical boundary layer height, is also found to decrease with decreasing radius to the storm center. The results also suggest that using the traditional critical Richardson number method to determine the boundary layer height may not accurately reproduce the height scale of the hurricane boundary layer. These different height scales reveal the complexity of the hurricane boundary layer structure that should be captured in hurricane model simulations.

Boundary Layer Dynamics in a Simple Model for Convectively Coupled Gravity Waves

Abstract A simplified model of intermediate complexity for convectively coupled gravity waves that incorporates the bulk dynamics of the atmospheric boundary layer is developed and analyzed. The model comprises equations for velocity, potential temperature, and moist entropy in the boundary layer as well as equations for the free tropospheric barotropic (vertically uniform) velocity and first two baroclinic modes of vertical structure. It is based on the multicloud model of Khouider and Majda coupled to the bulk boundary layer–shallow cumulus model of Stevens. The original multicloud model has a purely thermodynamic boundary layer and no barotropic velocity mode. Here, boundary layer horizontal velocity divergence is matched with barotropic convergence in the free troposphere and yields environmental downdrafts. Both environmental and convective downdrafts act to transport dry midtropospheric air into the boundary layer. Basic states in radiative–convective equilibrium are found and are shown to be consistent with observations of boundary layer and free troposphere climatology. The linear stability of these basic states, in the case without rotation, is then analyzed for a variety of tropospheric regimes. The inclusion of boundary layer dynamics—specifically, environmental downdrafts and entrainment of free tropospheric air—enhances the instability of both the synoptic-scale moist gravity waves and nonpropagating congestus modes in the multicloud model. The congestus mode has a preferred synoptic-scale wavelength, which is absent when a purely thermodynamic boundary layer is employed. The weak destabilization of a fast mesoscale wave, with a phase speed of 26 m s−1 and coupling to deep convection, is also discussed.

Turbulence Structure of the Hurricane Boundary Layer between the Outer Rainbands

Abstract As part of the Coupled Boundary Layers Air–Sea Transfer (CBLAST)-Hurricane program, flights were conducted to directly measure turbulent fluxes and turbulence properties in the high-wind boundary layer of hurricanes between the outer rainbands. For the first time, vertical profiles of normalized momentum fluxes, sensible heat and humidity fluxes, and variances of three-dimensional wind velocities and specific humidity are presented for the hurricane boundary layer with surface wind speeds ranging from 20 to 30 m s−1. The turbulent kinetic energy budget is estimated, indicating that the shear production and dissipation are the major source and sink terms, respectively. The imbalance in the turbulent kinetic energy budget indicates that the unmeasured terms, such as horizontal advection, may be important in hurricane boundary layer structure and dynamics. Finally, the thermodynamic boundary layer height, estimated based on the virtual potential temperature profiles, is roughly half of the boundary layer height estimated from the momentum flux profiles. The latter height where momentum and humidity fluxes tend to vanish is close to that of the inflow layer and also of the maximum in the tangential velocity profiles.

Remote sensing of mineral dust aerosol using AERI during the UAE2: A modeling and sensitivity study

Numerical simulations and sensitivity studies have been performed to assess the potential for using brightness temperature spectra from a ground‐based Atmospheric Emitted Radiance Interferometer (AERI) during the United Arab Emirates Unified Aerosol Experiment (UAE2) for detecting/retrieving mineral dust aerosol. A methodology for separating dust from clouds and retrieving the dust IR optical depths was developed by exploiting differences between their spectral absorptive powers in prescribed thermal IR window subbands. Dust microphysical models were constructed using in situ data from the UAE2 and prior field studies while composition was modeled using refractive index data sets for minerals commonly observed around the UAE region including quartz, kaolinite, and calcium carbonate. The T‐matrix, finite difference time domain (FDTD), and Lorenz‐Mie light scattering programs were employed to calculate the single scattering properties for three dust shapes: oblate spheroids, hexagonal plates, and spheres. We used the Code for High‐resolution Accelerated Radiative Transfer with Scattering (CHARTS) radiative transfer program to investigate sensitivity of the modeled AERI spectra to key dust and atmospheric parameters. Sensitivity studies show that characterization of the thermodynamic boundary layer is crucial for accurate AERI dust detection/retrieval. Furthermore, AERI sensitivity to dust optical depth is manifested in the strong subband slope dependence of the window region. Two daytime UAE2 cases were examined to demonstrate the present detection/retrieval technique, and we show that the results compare reasonably well to collocated AERONET Sun photometer/MPLNET micropulse lidar measurements. Finally, sensitivity of the developed methodology to the AERI's estimated MgCdTe detector nonlinearity was evaluated.

Investigation of the first and second aerosol indirect effects using data from the May 2003 Intensive Operational Period at the Southern Great Plains

The Active Tracer High‐Resolution Atmospheric Model is used to examine the aerosol indirect effect (AIE) for a spring continental stratus cloud on the basis of data collected during the 17 May 2003 Aerosol Intensive Operation Period (AIOP) at the Atmospheric Radiation Measurement (ARM) Program Southern Great Plains site. Model results for our base case, which uses observed aerosol concentrations, agree reasonably well with the available observations, giving confidence that the basic model is reasonable. Sensitivity tests are performed to explore the response of the clouds to changes in the aerosol number concentration and surface fluxes. During the major part of the simulation, from 0630 through 1400 local standard time (LST), an increase in the aerosol number concentration (Na) results in a decrease of the mean cloud droplet size and an increase of the cloud liquid water path (LWP) until aerosol number concentration levels reach 1200 cm−3. Further increases in aerosol concentration do not increase the liquid water path because the depletion of cloud water by precipitation is negligible above this number concentration. After 1400 LST, the liquid water path decreases when aerosols increase as long as Na < 600 cm−3 and remains unchanged for higher aerosol concentrations. The decrease of LWP is associated with the evaporative cooling below cloud base which leads to more condensation of water vapor, a result that is consistent with afternoon satellite observations of the response of continental clouds to increases in droplet concentrations. A sensitivity test with a stronger surface latent flux increases both the cloud geometrical thickness and cloud water content. On the other hand, a sensitivity test with a stronger surface sensible heat flux leads to a higher cloud base and a shallower and drier cloud. The response of the cloud geometrical thickness to changes in surface sensible heat flux dominates that of the cloud water content. The cloud fraction is also reduced at the end of the simulation time period. Because the surface heat fluxes will likely change when aerosol and droplet number concentrations change, these sensitivity tests show that a fully coupled simulation with a land surface model will be needed to fully assess the response of the cloud to changing aerosol concentrations. Nevertheless, since the thermodynamic boundary layer profiles do not change significantly when aerosol concentrations are changed, our results for changing aerosol concentrations are qualitatively correct.

Airborne observations of the impact of a convective system on the planetary boundary layer thermodynamics and aerosol distribution in the inter‐tropical discontinuity region of the West African Monsoon

AbstractThe impact of a convective system downdraught and associated gravity current on the monsoon and harmattan flow structure as well as on aerosol vertical distribution over the Sahel and the Sahara is investigated using dropsondes and an airborne lidar. Complementary ground‐based and satellite observations, as well as European Centre for Medium‐range Weather Forecasts analyses are also used. The mission was conducted on 5 June 2006, in the framework of the African Monsoon Multidisciplinary Analysis Special Observing Period. The targeted area was the Saharan heat low region, and Mali in particular, over which the inter‐tropical discontinuity was strongly perturbed by the convective system. To the north, away from the influence of the gravity current, the atmosphere exhibited a two‐layer structure, with a well mixed 3 km deep internal boundary layer capped by a stable layer 2–2.5 km deep layer referred to as the Saharan aerosol layer (SAL). The aerosol loading in the internal boundary layer (as determined by lidar) was observed to be much less than in the SAL above. Plumes of dust generated by strong low‐level winds west of the Hoggar as observed in Spinning Enhanced Visible and Infra‐Red Imager (SEVIRI) images, and possibly injected in the upper SAL through orographic lifting, contributed to the dust loading of the elevated SAL over much of the domain observed by the lidar. In the northernmost part of the domain investigated, the depth of the SAL top was decreased as the result of subsidence enhanced by the presence of an elevated jet associated with the monsoon trough and/or related to the return circulation of plain‐mountain wind system involving the Atlas range. Associated with the gravity current passing over dust sources, a large aerosol plume was observed by means of lidar measurements and SEVIRI imagery. The plume was seen to reach heights of approximately 3 km above ground level a few tens of kilometres behind the leading edge. Lidar measurements suggest that a fraction of the dust lifted by the gravity current is mixed into the SAL, and in some instances injected above the SAL. Copyright © 2007 Royal Meteorological Society

Alpine lee cyclogenesis influence on air‐sea heat exchanges and marine atmospheric boundary layer thermodynamics over the western Mediterranean during a Tramontane/Mistral event

Data from a recent field campaign are used to analyze the nonstationary aspects of air‐sea heat exchanges and marine atmospheric boundary layer (MABL) thermodynamics over the Gulf of Lion (GoL) in connection with synoptic forcing. The data set includes measurements made from a wide range of platforms (sea‐borne, airborne, and space‐borne) as well as three‐dimensional atmospheric modeling. The analysis focuses on the 24 March 1998 Tramontane/Mistral event. It is shown that the nonstationary nature of the wind regime over the GoL was controlled by the multistage evolution of an Alpine lee cyclone over the Tyrrhenian Sea (between Sardinia and continental Italy). In the early stage (low at 1014 hPa) the Tramontane flow prevailed over the GoL. As the low deepened (1010 hPa), the prevailing wind regime shifted to a well‐established Mistral that peaked around 1200 UTC. In the afternoon the Mistral was progressively disrupted by a strengthening outflow coming from the Ligurian Sea in response to the deepening low over the Tyrrhenian Sea (1008 hPa) and the channelling induced by the presence of the Apennine range (Italy) and the Alps. In the evening the Mistral was again well established over the GoL as the depression continued to deepen (1002 hPa) but moved to the southeast, reducing the influence of outflow from the Ligurian Sea on the flow over the GoL. The air‐sea heat exchanges and the structure of the MABL over the GoL were observed to differ significantly between the established Mistral period and the disrupted Mistral period. In the latter period, surface latent and sensible heat fluxes were reduced by a factor of 2, on average. During that latter period, air‐sea moisture exchanges were mainly driven by dynamics, whereas during the former period, both winds and vertical moisture gradients controlled moisture exchanges. The MABL was shallower during the latter period (0.7 km instead of 1.2 km) because of reduced surface turbulent heat fluxes and increased wind shear at the top of the MABL in connection with the outflow from the Ligurian Sea. In the period of established Mistral, gravity waves above the MABL were observed to have an influence on the MABL structure. In the perturbed Mistral case this influence was not observed. Over the GoL the ubiquitous presence of sheltered regions (i.e., regions of reduced wind speed in the MABL) in the lee of the three major mountain ranges surrounding the GoL (namely, the Pyrénées, the Massif Central, and the Alps) was shown to have an impact on surface turbulent heat fluxes. The position of these sheltered regions, which evolved with the synoptic conditions, was the key to a correct interpretation of multiplatform surface turbulent flux measurements made over the GoL on 24 March 1998.

The Role of Longwave Radiation and Boundary Layer Thermodynamics in Forcing Tropical Surface Winds*

This paper reveals major deficiencies of the existing intermediate climate models for tropical surface winds and elaborates the important roles of cloud-longwave radiational forcing and boundary layer thermodynamics in driving the tropical surface winds. The heat sink associated with the cloud-longwave radiation is demonstrated as an important driving force for boreal summer northeast trades and Indian Ocean southwest monsoons. Over the western North Pacific and Atlantic Oceans, low cloudiness and high sea surface temperature enhance longwave radiation cooling, strengthening subtropical high and associated trades. On the other hand, in the regions of heavy rainfall over South Asia, reduced cloud-longwave radiation cooling enhances monsoon trough and associated southwest monsoons. The boundary layer thermodynamic processes, primarily both the surface heat fluxes and the vertical temperature advection, are shown to be critical for a realistic simulation of the intertropical convergence zone, the equatorial surface winds, and associated divergence field. To successfully simulate the tropical surface winds, it is essential for intermediate models to adequately describe the feedback of the boundary layer frictional convergence to convective heat source, cloud-longwave radiation forcing, boundary layer temperature gradient forcing, and their interactions. The capability and limitations of the intermediate tropical climate model in reproducing both climatology and interannual variations are discussed.

The vertical structure of aerosol and its relationship to boundary-layer thermodynamics over the rural UK

The vertical structure of aerosol and its relationship to boundary-layer thermodynamics over the rural UK

Statistical Relation between Monthly Mean Precipitable Water and Surface-Level Humidity over Global Oceans

Monthly summaries of atmospheric soundings taken over 17 years from 49 midocean stations at small islands and weather ships distributed over major oceans are examined. Over tropical oceans, precipitable water is found to be a better predictor of surface-level humidity than surface-level air temperature. A statistical relation in the form of a polynomial is derived; from this relation, the monthly-mean, surface-level mixing ratio can be computed from monthly-mean precipitable water. The root-mean-square differences between the measured and derived values were found to be less than 8 × 10−4 over most ocean areas. Such a relation is useful in deriving large-scale evaporation and latent heal flux data from the ocean, using spaceborne observations. The temporal and spatial variabilities of data deviations from this relation are examined. This relation is found to be applicable to all major ocean basins and can be used to monitor interannual variability. Boundary-layer thermodynamics of different air masses are suggested as an explanation of some characteristics of this relation.

Boundary Layer Thermodynamics of a High Plains Severe Storm

Abstract A new analysis method based on air parcel saturation point is used to intercompare data sampled by different systems (aircraft, rawinsondes, and a surface mesonet) during the passage of one severe storm in the 1981 Cooperative Convective Precipitation Experiment. The low-level thermodynamic structure simplifies into two distinct branches on a thermodynamic diagram. The lower tropospheric environment ahead of the severe storm has a characteristic mixing line structure while the storm low-level outflow has a distinct evaporation line structure.

Radial distribution function for hard spheres from scaled particle theory, and an improved equation of state

Previous applications of scaled particle theory have been limited to the calculation of thermodynamic properties of fluids rather than structure. In the present paper, the theory is expanded so that it is capable of yielding the radial distribution function. The method is first illustrated by applying it to one- dimensional fluids of hard rods where, as in other theories, the radial distribution function is obtained exactly. It is then applied to a fluid of hard spheres where a closure condition is necessary. This condition is supported by recent work in scaled particle theory dealing with the thermodynamics of boundary layers. It is used to calculate the radial distribution function around a λ-cule of varying size, including one of the size of a typical hard sphere solvent molecule. The results are very good, yielding (at the highest density at which numerical solutions have been possible, ρ=0.372ρ0, where ρ0 is the density of close packing) results for the contact radial distribution function and the equation of state which were somewhat better than the original scaled particle and Percus-Yevick theories. These theories yield results which are very close to the predictions of Monte Carlo calculations. As in the original scaled particle theory directed at the contact distribution function alone, exact results (exact closure) are possible for λ-cules of radii corresponding to a point or smaller. Our numerical method becomes unstable for densities higher than ρ=0.372ρ0, and we are attempting to improve the procedure.