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Larry Mahrt

Abstract

Common large shifts of wind direction in the weak-wind nocturnal boundary layer are poorly understood and are not adequately captured by numerical models and statistical parameterizations. The current study examines 15 datasets representing a variety of surface conditions to study the behavior of wind direction variability. In contrast to previous studies, the current investigation directly examines wind direction changes with emphasis on weak winds and wind direction changes over smaller time periods of minutes to tens of minutes, including large wind direction shifts. A formulation of the wind direction changes is offered that provides more realistic behavior for very weak winds and for complex terrain.

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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 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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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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Edgar L Andreas and Larry Mahrt

Abstract

Nature is wild, unconstrained, and often dangerous. In particular, studying air–sea interaction in winds typical of tropical cyclones can place researchers, their instruments, and even their research platforms in jeopardy. As an alternative, laboratory wind–water tunnels can probe 10-m equivalent winds of hurricane strength under conditions that are well constrained and place no personnel or equipment at risk. Wind–water tunnels, however, cannot simulate all aspects of air–sea interaction in high winds. The authors use here the comprehensive data from the Air–Sea Interaction Salt Water Tank (ASIST) wind–water tunnel at the University of Miami that Jeong, Haus, and Donelan published in this journal to demonstrate how spray-mediated processes are different over the open ocean and in wind tunnels. A key result is that, at all high-wind speeds, the ASIST tunnel was able to quantify the so-called interfacial air–sea enthalpy flux—the flux controlled by molecular processes right at the air–water interface. This flux cannot be measured in high winds over the open ocean because the ubiquitous spray-mediated enthalpy transfer confounds the measurements. The resulting parameterization for this interfacial flux has implications for modeling air–sea heat fluxes from moderate winds to winds of hurricane strength.

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Dean Vickers, Larry Mahrt, and Edgar L Andreas

Abstract

Over 5000 aircraft eddy-covariance measurements from four different aircraft in nine different experiments are used to develop a simple model for the friction velocity over the sea. Unlike the widely used Coupled Ocean–Atmosphere Response Experiment (COARE) bulk flux scheme, the simple model (i) does not use Monin–Obukhov similarity theory (MOST) and therefore does not require an estimate of the Obukhov length, (ii) does not require a correction to the wind speed for height or stability, (iii) does not require an estimate of the aerodynamic roughness length, and (iv) does not require iteration. In comparing the model estimates developed in this work and those of the COARE algorithm, comparable fitting metrics for the two modeling schemes are found. That is, using Monin–Obukhov similarity theory and the Charnock relationship did not significantly improve the predictions. It is not clear how general the simple model proposed here is, but the same model with the same coefficients based on the combined dataset does a reasonable job of describing the datasets both individually and collectively. In addition, the simple model was generally able to predict the observed friction velocities for three independent datasets that were not used in tuning the model coefficients. Motivation for the simple model comes from the fact that physical interpretation of MOST can be ambiguous because of circular dependencies and self-correlation. Additional motivation comes from the large uncertainty associated with estimating the Obukhov length and, especially, the aerodynamic roughness length.

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Edgar L Andreas, Larry Mahrt, and Dean Vickers

Abstract

From almost 7000 near-surface eddy-covariance flux measurements over the sea, the authors deduce a new air–sea drag relation for aerodynamically rough flow:
eq1
Here u* is the measured friction velocity, and UN 10 is the neutral-stability wind speed at a reference height of 10 m. This relation is fitted to UN 10 values between 9 and 24 m s−1. A drag relation formulated as u* versus UN 10 has several advantages over one formulated in terms of . First, the multiplicative coefficient on UN 10 has smaller experimental uncertainty than do determinations of CDN 10. Second, scatterplots of u* versus UN 10 are not ill posed when UN 10 is small, as plots of CDN 10 are; u*UN 10 plots presented here suggest aerodynamically smooth scaling for small UN 10. Third, this relation depends only weakly on Monin–Obukhov similarity theory and, consequently, reduces the confounding effects of artificial correlation. Finally, with its negative intercept, the linear relation produces a CDN 10 function that naturally rolls off at high wind speed and asymptotically approaches a constant value of 3.40 × 10−3. Hurricane modelers and the air–sea interaction community have been trying to rationalize such behavior in the drag coefficient for at least 15 years. This paper suggests that this rolloff in CDN 10 results simply from known processes that influence wind–wave coupling.
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Dean Vickers, Larry Mahrt, and Edgar L Andreas

Abstract

The 10-m neutral drag coefficient (C DN10) over the sea is calculated using a large observational dataset consisting of 5800 estimates of the mean flow and the fluxes from aircraft eddy-covariance measurements. The dataset includes observations from 11 different experiments with four different research aircraft. One of the goals is to investigate how sensitive C DN10 is to the analysis method. As such, C DN10 derived from six unique processing schemes that involve different methods for averaging the surface stress and the wind speed are compared. Especially in weak winds, the resulting C DN10 values depend on the choice of processing.

Four distinct regimes of C DN10 are identified: weak winds where calculating C DN10 is not well posed, moderate winds (4 to 10 m s−1) where C DN10 is a constant, strong winds (10 to 20 m s−1) where C DN10 increases linearly with increasing wind speed, and very strong winds (20 to 24 m s−1) where C DN10 steadily decreases with increasing wind speed. However, as this last regime is based on data from a single experiment, additional data are needed to confirm this apparent decrease in C DN10 for winds exceeding 20 m s−1.

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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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Kyung-Ja Ha, Sun-Hee Shin, and Larry Mahrt

Abstract

This study examines the spatial variation of the wind field observed in the coastal zone of southeast Korea with its complex terrain, using measurements from a regional network 75 km across and centered about Busan. Results are compared with observations from an inland regional network centered around Daegu, Korea, with less dramatic, but still significant, surface heterogeneity. The coherency between stations is examined in terms of the between-station correlations of the wind components for all pairs of stations as a function of separation distance between stations. A mesovelocity scale is defined as a measure of the spatial variability of the wind field within the network. This variability is related to wind speed and cloud cover for the two contrasting regional networks. Additional comparisons are made with a homogenous rural network in Iowa. The results underscore the complexity of flow with topography, urban areas, and land–sea contrasts and demonstrate the inadequacy of existing network strategies.

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