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G. D. Hess

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G. D. Hess

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R. H. Clarke and G. D. Hess

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Data from a recent experiment indicate that the height scale for wind in the planetary boundary layer is u */F, where u */F is the friction velocity and f the Coriolis parameter, rather than zi, the height of the convection-limiting inversion, or |L| the Obukhov length. None of the scales u */f, zi, |L| appears to be appropriate for temperature except in slight-to-moderate stability where again u */f appears to be the most suitable.

The data do not support either Businger's expression which gives the depth of the idealized thermal boundary layer, depending on surface heat flux and rate of temperature change, or Tennekes' expression for bulk temperature difference across the boundary layer in free convection, based on the ratio of −L to the roughness length z 0.

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G. D. Hess and K. T. Spillane

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Following a suggestion by Deardorff, Hess et al. have proposed that a necessary condition for the formation of dust devils is −h/L⩾50, where h is the convective boundary layer height and L the Obukhov length. A survey of pilots, air traffic controllers and meteorologists was conducted in Australia from 1 November 1987 to 31 January 1988 to test this hypothesis.

The results of the survey showed that this criterion was satisfied for each dust devil event. Furthermore, the results were consistent with the idea that convective downdrafts play an important role in initiating dual devils. In strongly convective conditions there were two preferred heights attained by the dust—one at ≈0.09h(the height of the convergence wall between cells) and the other ≈0.51h(the height at which the vertical divergence of the updraft changes sign). The number density in these conditions is ≈4 per area of h 2.

The survey showed that dust devils on density currents have a different geometry; they have diameters five times larger than convective dust devils. Density current dust devils may not be visible and could pose a greater hazard to aircraft than previously realized.

Last, four cases are examined where the dust evils traveled over Bureau of Meteorology recording anemometers. These showed wind speeds greater than usually reported in the literature; wind gusts up to 19.5 m s−1 above the mean wind speed were recorded. Predictions of convective wind gusts and their time intervals were in good agreement with the observations.

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K. T. Spillane and G. D. Hess

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From a review of experimental evidence it is concluded that in confined bodies of water the ratio of surface drift to surface friction velocity, us, u * is a function of water depth that precludes the application of laboratory-derived relationships between these velocities under conditions prevailing in the open sea. A simple empirical model is proposed for contained bodies of water that requires
usw*,skHZ0,s
Uwa½usu*KH
where us, is the surface current, U() the wind speed at height above water surface, w *s the surface friction velocity in the water, u * the surface friction velocity in the air, Z0,s, the roughness length in the water at the surface, H the depth of water, ρw the density of water, ρa the density of air and k the von Kármán constant. These relationships have some support in the experimental evidence presently available.
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B. B. Hicks and G. D. Hess

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Gradients of temperature and humidity above water surfaces are analyzed in order to determine the dependence of the long-term average Bowen ratios β (the ratio of sensible to latent heat flux) on surface temperature. The least-squares fit that results from investigation of six such bodies of data, and which is supported by recent direct measurements of the fluxes by eddy correlation techniques, can be expressed as β = γ/s−0.15, where γ is the ratio of the specific heat of air at constant pressure to the latent heat of vaporization of water and s is the slope of the saturated specific humidity curve at the surface temperature. This expression forms the basis of a model which can be used to determine the average surface temperature from routine observations of air temperature and humidity at sea.

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G. D. Hess and R. H. Clarke

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L. M. Leslie, G. D. Hess, and E. E. Habjan

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National weather services now receive global model forecasts from a number of centers around the world. The existence of these forecasts raises the general question of how the operational forecaster can best use the information that the ensemble of predictions provides. The Australian Bureau of Meteorology receives four global model forecasts in real-time, but at present their performance is evaluated almost entirely in a subjective manner.

In this study, in addition to the standard objective measures (for example, bias and rms error), several alternative objective measures of model performance are calculated (such as the temporal forecast consistency of a given model and divergence between different models), in an attempt to provide the forecasters with more effective tools for model assessment. Both kinds of measures are applied to a two-year dataset (October 1989 to September 1991) of daily sea level pressure predictions from the four models.

There are two main outcomes of this study. First, the current subjective system of ranking the various models has been augmented with more objectively based performance measures. Second, these performance statistics provide guidance to the operational forecasters in a number of ways: geographical regions with large systematic errors can be identified for each model; case studies are presented that illustrate the utility of the regional maps of bias, consistency, and divergence computed in this study; and, finally, there are regions of uncertainty where no model is consistently superior, so forecasts over these regions should be treated with caution.

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G. D. Hess, K. T. Spillane, and R. S. Lourensz

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Observations of funnel clouds over Port Phillip Bay, Victoria, Australia, indicate that they occur during outbreaks of cool air from the Southern Ocean advecting over the relatively warm bay waters. These clouds act as tracers for shallow convection vortices with dynamics similar to large dust devils. The related phenomena of waterspouts and tornadoes differ from these vortices by requiring deep convection and downdraft and updraft interactions associated with rain processes.

Deardorff (1978a) suggests that a necessary condition for the formation of dust devils is −/L of the order of 100 or more, where h is the convective boundary layer height and L the Obukhov length. Calculations of −/L over the bay and over land for the days of observation are consistent with this suggestion. They indicate that significant rotation may occur at −/L as low as 50. This information, if confirmed, may make it possible to use boundary layer numerical models to forecast likely conditions of dust devil occurrence over mesoscale regions, which would be of benefit to pilots of light aircraft and helicopters.

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D. Lombardozzi, Samuel Levis, G. Bonan, P. G. Hess, and J. P. Sparks

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Ozone (O3) is a phytotoxic greenhouse gas that has increased more than threefold at Earth’s surface from preindustrial values. In addition to directly increasing radiative forcing as a greenhouse gas, O3 indirectly impacts climate through altering the plant processes of photosynthesis and transpiration. While global estimates of gross primary productivity (GPP) have incorporated the effects of O3, few studies have explicitly determined the independent effects of O3 on transpiration. In this study, the authors include effects of O3 on photosynthesis and stomatal conductance from a recent literature review to determine the impact on GPP and transpiration and highlight uncertainty in modeling plant responses to O3. Using the Community Land Model, the authors estimate that present-day O3 exposure reduces GPP and transpiration globally by 8%–12% and 2%–2.4%, respectively. The largest reductions were in midlatitudes, with GPP decreasing up to 20% in the eastern United States, Europe, and Southeast Asia and transpiration reductions of up to 15% in the same regions. Larger reductions in GPP compared to transpiration decreased water-use efficiency 5%–10% in the eastern United States, Southeast Asia, Europe, and central Africa; increased surface runoff more than 15% in eastern North America; and altered patterns of energy fluxes in the tropics, high latitudes, and eastern North America. Future climate predictions will be improved if plant responses to O3 are incorporated into models such that stomatal conductance is modified independently of photosynthesis and the effects on transpiration are explicitly considered in surface energy budgets. Improvements will help inform regional decisions for managing changes in hydrology and surface temperatures in response to O3 pollution.

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