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LAWRENCE KYNETT and JOHN LOHNER

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Alan M. Davies and John Lawrence

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A high-resolution (0.5′ north–south by 1.0′ west–east) three-dimensional hydrodynamic model of the eastern Irish Sea is used to examine the influence of enhanced bed friction due to wind–wave effects upon the three-dimensional, wind-induced circulation of the eastern Irish Sea.

The model uses a mixed finite difference-modal approach, in which a standard finite-difference grid is employed in the horizontal, with the Galerkin method, with an expansion of functions (modes) in the vertical, giving a continuous current profile from sea surface to seabed. Vertical eddy viscosity within the model is a function of the flow field.

The model is used to examine the wind-induced response of the area to spatially uniform and constant in time northerly and westerly wind stresses of 1 N m−2. The effect upon bed stresses, and hence the three-dimensional circulation of the region of enhanced bed turbulence due to wind wave effects, is also considered using idealized wave fields. Changes in bed stress, particularly in shallow water regions, have a significant influence upon the wind-induced circulation, especially the wind-induced, near-bed currents.

In addition to calculations using a uniform drag coefficient and bed type, the wind-induced circulation using a range of bed types corresponding to the bed composition, from mud to gravel, of the eastern Irish Sea is also considered. The intensity of near-bed turbulence, and hence drag coefficient, due to wave–current interaction is found to vary significantly with bed type, and this also influences the near-bed currents.

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Wendy B. Lawrence and John L. Spiesberger

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An acoustic source and receiver are placed at 800 m depth and are separated by 4000 km in a two-layer, steady-wind driven, flat bottom eddy-resolving quasi-geostrophic circulation model. Time series of sea surface elevation and upper and lower layer meridional currents are generated for comparison against a series of acoustic travel time. The spectra of the time series exhibit a broad mesoscale peak neat a period of 40 days. The spectrum of the acoustic travel time contains a significant peak which is not present in the spectra of the point measurements, due to a resonant barotropic osci11ation with a period of 29.0 days. In this numerical model, basin-scale tomographic measurements are a useful method of sensing the large-scale resonant barotropic oscillations because the tomographic system attenuates the “noise” from the mesoscale.

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Lawrence B. Dunn and John D. Horel

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Output from simulations of the Eta model are compared to special observations collected during the 1990 Southwest Area Monsoon Project (SWAMP). The emphasis is on the model's prediction of the preconvection air mass over Phoenix, Arizona, and on the model's representation of the low-level jet and moisture surge observed over southwest Arizona.

At times the model fails to capture the rapid increase in low- and mid-level moisture that is observed in the hours prior to the onset of convection. Subsequent convection is not predicted by the Eta model. In one event the model very accurately predicts the evolution of the air mass over Phoenix during the period just prior to the outbreak of severe convection. However, no convection is predicted by the model. The model seems unable to generate convection over the high terrain or lower deserts of central Arizona regardless of whether the air mass is simulated correctly.

A low-level jet feature observed over southwest Arizona during SWAMP is not correctly simulated by the Eta model. The model produces a very strong sea-breeze circulation from the Gulf of California into western Arizona in each simulation. The moisture and stability profiles associated with the sea-breeze are inconsistent with observations over southwest Arizona, which leads to a misrepresentation of the low- and midlevel moisture field over the region. Poor initial conditions in the sea surface temperature field over the Gulf of California are, at least in part, responsible for the model error.

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Lawrence B. Dunn and John D. Horel

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The utility of numerical model guidance produced by the National Meteorological Center has been evaluated for the forecast of convection over central Arizona during the summer monsoon season. Model output from the Nested Grid Model (NGM) and Eta model has been compared to observations taken during the 1990 field experiment referred to as the Southwest Area Monsoon Project (SWAMP).

The NGM precipitation forecasts showed little skill for events in which heavy precipitation was observed over Phoenix, Arizona. Selected events during the SWAMP period were simulated using the Eta model. Qualitative comparisons of the Eta model's precipitation forecasts with lightning data and satellite imagery suggest that the model has little skill over Arizona during the warm season. Nocturnal heavy precipitation over the lower deserts of central Arizona is nearly always preceded by afternoon convection over the mountains to the north and east. The convection over the mountains was absent in the model.

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John A. Laurmann and W. Lawrence Gates

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Numerical simulations made with atmospheric general circulation models contain short-term fluctuations that need to be taken into account when considering the significance of long-term climatic means. When such models are used in the study of the climatic changes resulting from either altered forcing or changed boundary conditions, a quantitative evaluation of the effects of the fluctuations, as measured by the variance of model-generated climatic estimates, is required. Such fluctuations are generally of large magnitude, and necessitate extended simulations or repeated Monte-Carlo calculations in order to determine the climatic means with confidence.

Based on a review of statistical questions that are important for climate modeling, this paper develops a rationale for planning the climate simulation trials needed to establish the significance of proposed climatic change experiments. In particular, a quantitative prescription is given for determining the extent of model calculations required to establish the presence of a climatic change at any chosen confidence level between two numerical experiments of unequal variance.

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John E. Geisler and Lawrence A. Mysak

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This paper describes the zonally propagating wave modes of a homogeneous fluid on an equatorial beta-plane with linear bottom topography. The fluid is bounded above by a free surface and bounded below by a depth profile that increases linearly with distance away from a beach that is parallel to the equator and located some distance either north or south of it. The governing equation for the wave amplitude is solved numerically to obtain dispersion relations and mode structures of waves trapped against the beach. These are interpreted in the light of existing knowledge of trapped modes on a sloping beach on an f-plane and of trapped modes on an equatorial beta plane with a flat bottom. Both gravity modes and low-frequency topographic modes are included in the analysis.

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Alexander Ruzmaikin, John Lawrence, and Cristina Cadavid

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A simple dynamic model, truncated from the stratospheric wave–zonal flow interaction Holton and Mass model, is introduced and studied. This model consists of three ordinary differential equations controlled by two parameters: the initial amplitude of planetary waves and the vertical gradient of the zonal wind. The changes associated with seasonal variations and with the solar variability are introduced as periodic modulations of the zonal wind gradient. The major climatic response to these changes is seen through modulation of the number of cold and warm winters.

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John C. Gille and Lawrence V. Lyjak

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One of the limitations to the accurate calculation of radiative heating and cooling rates in the stratosphere and mesosphere has been the lack of accurate data on the atmospheric temperature and composition. Data from the LIMS experiment on Nimbus-7 have been extended to the South Pole with the aid of other observations. The data have been used as input to codes developed by Ramanathan and Dickinson to calculate the individual components and the net radiative heating rates from 100–0.1 mb. Solar heating due to ozone, nitrogen dioxide, carbon dioxide, water vapor and oxygen is shown to be nearly balanced by cooling in the thermal infrared spectral region due to carbon dioxide, ozone and water vapor. In the lower stratosphere, infrared transfer by ozone leads to heating that is sensitive to the distribution of tropospheric ozone, clouds and water vapor.

The heating and cooling rates are adjusted slightly in order to satisfy the global mass balance. The results are in qualitative agreement with earlier calculations, but show additional detail. There is as strong temporal and vertical variation of cooling in the tropics. Radiative relaxation times are as short as 7 days or less at the stratopause.

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William P. Kustas, John H. Prueger, and Lawrence E. Hipps

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A riparian corridor along the Rio Grande dominated by the Eurasian tamarisk or salt cedar (Tamarix spp.) is being studied to determine water and energy exchange rates using eddy covariance instrumentation mounted on a 12-m tower. The potential of using remotely sensed data to extrapolate these local estimates of the heat fluxes to large sections of the Rio Grande basin is under investigation. In particular, remotely sensed (radiometric) surface temperature can be used to estimate partitioning of net radiation energy into sensible and latent heat fluxes from vegetated landscapes. An important issue that has not been addressed adequately in the application of radiometric surface temperature data is the effect of using different time-averaged quantities in heat transfer formulations. This study evaluates the impact on sensible heat flux estimation of using relatively short time-averaged (1 min) canopy temperatures measured from a fixed-head infrared radiometer with 1-, 10-, and 30-min time-averaged micrometeorological input data used in estimating the resistance to heat transfer. The results indicate that, with short time-averaged radiometric surface temperatures (essentially “instantaneous” from a satellite), variations in sensible heat flux strongly correlate to fluctuations in net radiation conditions. Under near-constant net radiation input, natural perturbations in surface temperature also contribute to variations in sensible heat flux but are typically an order of magnitude smaller. The resulting implications for computed heat flux estimates using data from remotely sensing platforms and validation with flux tower measurements along riparian corridors are discussed.

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