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Larry J. Mahrt

Abstract

Steady, longitudinally invariant, barotropic, boundary layer flow is numerically studied at low latitudes where advective accelerations may he large and the Coriolis parameter is small. Flow is generated by specifying the pressure gradient field independent of the flow.

It is found that as the flow approaches the equator, advective terms associated with the large latitudinal variation of the Coriolis parameter become important. As the flow crosses the equator, adjective accelerations may become important to the extent that the boundary layer downstream from the equator is radically different from the Ekman boundary layer. Compared to Ekman flow, the wind vector may rotate with height in the opposite direction, and the boundary layer depth may be considerably thinner and less dependent on latitude. The cross-isobar flow of this advective boundary layer is deeper and may be stronger, so that spatial transitions between this boundary layer and a quasi-Ekman boundary layer can produce significant vertical motion.

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Larry J. Mahrt

Abstract

The steady Reynolds stress and turbulent energy equations for steady, horizontally homogeneous mean flow are used to relate the Reynolds stress uw to the mean wind shear and heat fluxes in the planetary boundary layer.

The resulting Reynolds stress demonstrates a 3/2 power dependence on the stress Richardson number and a ½ power dependence on the flux Richardson number. Numerical results of Deardorff are used to estimate vertical profiles of a heat flux function which results from the derivation. Such calculations and certain observations suggest that the stress depends mainly on the flux Richardson number in the upper part of the strongly heated boundary layer but more on the stress Richardson number in the lower part of the weakly heated or stable boundary layer. The simple model developed appears to be inadequate in the case of large—z/L where the shear generation of stress becomes negligible and turbulent transports of stress may be significant.

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Larry J. Mahrt

Abstract

The influence of advective accelerations in the low-latitude boundary layer and coupling between this boundary layer and the free atmosphere are numerically examined for steady longitudinally invariant flow. Pressure adjustments in a one-layer representation of the free atmosphere are induced by vertical fluxes of mass, momentum and latent heat out of a multi-level boundary layer model. It is found that vertical motions, resulting from advective accelerations in the boundary layer, can strongly influence pressure adjustments and flow development. As a result, the steady pressure field, generated by specified heating or parameterized latent heating, is quite different than would he predicted by linear boundary layer theory.

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David A. Barber and Larry J. Mahrt

Abstract

Rawinsonde observations taken during the National Hail Research Experiment are analyzed by multiple-linear regression techniques to study the influence of environmental factors on hailstorm severity. The latter is inferred from integrated radar returns. The roles of mixed-layer flow and thermodynamic properties as well as upper tropospheric kinematic properties are emphasized. The low-level properties are found to be more important discriminators of storm severity over the High Plains.

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Larry Mahrt, Dean Vickers, Jielun Sun, and J. Harry McCaughey

Abstract

This study estimates area-averaged fluxes over the Boreal Ecosystem–Atmosphere Study (BOREAS) region using tower and aircraft data. The dependence of the area-averaged flux on various assumptions and external flow characteristics is examined.

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Joshua D. Hoover, David R. Stauffer, Scott J. Richardson, Larry Mahrt, Brian J. Gaudet, and Astrid Suarez

Abstract

To better understand the physical processes of the stable boundary layer and to quantify “submeso motions” in moderately complex terrain, exploratory case-study analyses were performed using observational field data supplemented by gridded North American Regional Reanalysis data and Pennsylvania State University real-time Weather Research and Forecasting Model output. Submeso motions are nominally defined as all motions between the largest turbulent scales and the smallest mesoscales. Seven nighttime cases from August and September of 2011 are chosen from a central Pennsylvania [“Rock Springs” (RS)] network of eight ground-based towers and two sound detection and ranging (sodar) systems . The observation network is located near Tussey Ridge, ~15 km southeast of the Allegheny Mountains. The seven cases are classified by the dominant synoptic-flow direction and proximity to terrain to assess the influence of synoptic conditions on the local submeso and mesogamma motions. It is found that synoptic winds with a large crossing angle over nearby Tussey Ridge can generate mesogamma wave motions and larger-magnitude submeso temperature and wind fluctuations in the RS network than do winds from the direction of the more distant Allegheny Mountains. Cases with synoptic winds that are nearly parallel to the topographic contours or are generally weak exhibit the smallest fluctuations. Changes in the magnitude of near-surface submeso temperature and wind fluctuations in response to local indicator variables are also analyzed. The observed submeso wind and temperature fluctuations are generally larger when the low-level wind speed and thermal stratification, respectively, are greater, but the synoptic flow and its relation to the terrain also play an important role.

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Nelson L. Seaman, Brian J. Gaudet, David R. Stauffer, Larry Mahrt, Scott J. Richardson, Jeffrey R. Zielonka, and John C. Wyngaard

Abstract

Numerical weather prediction models often perform poorly for weakly forced, highly variable winds in nocturnal stable boundary layers (SBLs). When used as input to air-quality and dispersion models, these wind errors can lead to large errors in subsequent plume forecasts. Finer grid resolution and improved model numerics and physics can help reduce these errors. The Advanced Research Weather Research and Forecasting model (ARW-WRF) has higher-order numerics that may improve predictions of finescale winds (scales <~20 km) in nocturnal SBLs. However, better understanding of the physics controlling SBL flow is needed to take optimal advantage of advanced modeling capabilities.

To facilitate ARW-WRF evaluations, a small network of instrumented towers was deployed in the ridge-and-valley topography of central Pennsylvania (PA). Time series of local observations and model forecasts on 1.333- and 0.444-km grids were filtered to isolate deterministic lower-frequency wind components. The time-filtered SBL winds have substantially reduced root-mean-square errors and biases, compared to raw data. Subkilometer horizontal and very fine vertical resolutions are found to be important for reducing model speed and direction errors. Nonturbulent fluctuations in unfiltered, very finescale winds, parts of which may be resolvable by ARW-WRF, are shown to generate horizontal meandering in stable weakly forced conditions. These submesoscale motions include gravity waves, primarily horizontal 2D motions, and other complex signatures. Vertical structure and low-level biases of SBL variables are shown to be sensitive to parameter settings defining minimum “background” mixing in very stable conditions in two representative turbulence schemes.

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Robert M. Banta, Larry Mahrt, Dean Vickers, Jielun Sun, Ben B. Balsley, Yelena L. Pichugina, and Eric J. Williams

Abstract

The light-wind, clear-sky, very stable boundary layer (vSBL) is characterized by large values of bulk Richardson number. The light winds produce weak shear, turbulence, and mixing, and resulting strong temperature gradients near the surface. Here five nights with weak-wind, very stable boundary layers during the Cooperative Atmosphere–Surface Exchange Study (CASES-99) are investigated. Although the winds were light and variable near the surface, Doppler lidar profiles of wind speed often indicated persistent profile shapes and magnitudes for periods of an hour or more, sometimes exhibiting jetlike maxima. The near-surface structure of the boundary layer (BL) on the five nights all showed characteristics typical of the vSBL. These characteristics included a shallow traditional BL only 10–30 m deep with weak intermittent turbulence within the strong surface-based radiation inversion. Above this shallow BL sat a layer of very weak turbulence and negligible turbulent mixing. The focus of this paper is on the effects of this quiescent layer just above the shallow BL, and the impacts of this quiescent layer on turbulent transport and numerical modeling. High-frequency time series of temperature T on a 60-m tower showed that 1) the amplitudes of the T fluctuations were dramatically suppressed at levels above 30 m in contrast to the relatively larger intermittent T fluctuations in the shallow BL below, and 2) the temperature at 40- to 60-m height was nearly constant for several hours, indicating that the very cold air near the surface was not being mixed upward to those levels. The presence of this quiescent layer indicates that the atmosphere above the shallow BL was isolated and detached both from the surface and from the shallow BL.

Although some of the nights studied had modestly stronger winds and traveling disturbances (density currents, gravity waves, shear instabilities), these disturbances seemed to pass through the region without having much effect on either the SBL structure or on the atmosphere–surface decoupling. The decoupling suggests that under very stable conditions, the surface-layer lower boundary condition for numerical weather prediction models should act to decouple and isolate the surface from the atmosphere, for example, as a free-slip, thermally insulated layer.

A multiday time series of ozone from an air quality campaign in Tennessee, which exhibited nocturnal behavior typical of polluted air, showed the disappearance of ozone on weak low-level jets (LLJ) nights. This behavior is consistent with the two-stratum structure of the vSBL, and with the nearly complete isolation of the surface and the shallow BL from the rest of the atmosphere above, in contrast to cases with stronger LLJs, where such coupling was stronger.

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James B. Edson, Venkata Jampana, Robert A. Weller, Sebastien P. Bigorre, Albert J. Plueddemann, Christopher W. Fairall, Scott D. Miller, Larry Mahrt, Dean Vickers, and Hans Hersbach
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Alexander A. Baklanov, Branko Grisogono, Robert Bornstein, Larry Mahrt, Sergej S. Zilitinkevich, Peter Taylor, Søren E. Larsen, Mathias W. Rotach, and H. J. S. Fernando

Abstract

No Abstract available.

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