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Ping Zhai, Larry J. Pratt, and Amy Bower

Abstract

The west-to-east crossover of boundary currents has been seen in mean circulation schemes from several past models of the Red Sea. This study investigates the mechanisms that produce and control the crossover in an idealized, eddy-resolving numerical model of the Red Sea. The authors also review the observational evidence and derive an analytical estimate for the crossover latitude. The surface buoyancy loss increases northward in the idealized model, and the resultant mean circulation consists of an anticyclonic gyre in the south and a cyclonic gyre in the north. In the midbasin, the northward surface flow crosses from the western boundary to the eastern boundary. Numerical experiments with different parameters indicate that the crossover latitude of the boundary currents changes with f 0, β, and the meridional gradient of surface buoyancy forcing. In the analytical estimate, which is based on quasigeostrophic, β-plane dynamics, the crossover is predicted to lie at the latitude where the net potential vorticity advection (including an eddy component) is zero. Various terms in the potential vorticity budget can be estimated using a buoyancy budget, a thermal wind balance, and a parameterization of baroclinic instability.

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J. Brent Bower and Dale R. Durran

Abstract

Wind profiler data from Lay Creek, Colorado, along with stability data from the Lander and Grand Junction rawinsonde observations, were examined in an attempt to link various parameters in the upstream flow to the onset of strong downslope winds in Boulder. Some correlation was found between the occurrence of high surface winds at Boulder and the upstream wind direction, upper tropospheric wind shear and the vertical phase shift across the troposphere. However, these parameters alone were not able to distinguish between windstorm and nonwindstorm events. It is likely that the remaining ambiguity could be eliminated with information on the location and strength of inversions in the upstream flow.

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M. Susan Lozier, Timothy J. Bold, and Amy S. Bower

Abstract

A kinematic model is developed to examine the relationship between meander propagation and Lagrangian pressure change within a meandering jet. Basically, the model equates changes in pressure along the path of a water parcel with the cross-stream motion of a parcel in a reference frame moving with the meander. The model is tested by combining isopycnal float data from the Gulf Stream with contemporaneous meander phase speed observations from satellite infrared images. Time series of pressure changes along individual float trajectories show a qualitative trend for the amplitude of pressure changes to generally increase in response to large phase speeds. However, the model suggests that the pressure change following a fluid parcel is related to the vector difference between the velocity and phase speed vectors, not just the magnitude of the phase speed. This is confirmed by the data analysis, which shows that Lagrangian pressure changes are more highly correlated with cross-stream flow when both the zonal and meridional components of the meander propagation are included in the kinematic model. Approximately 90% of the variability associated with the floats’ pressure changes can he accounted for by cross-stream flow using this kinematic formulation.

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A. S. Bower, H. T. Rossby, and J. L. Lillibridge

Abstract

The Gulf Stream ’60 hydrographic survey has been used to examine the distribution of water properties across the Gulf Stream as a function of potential density. This survey covered a half million square miles of Slope, Gulf Stream and Sargasso Sea Waters in the western North Atlantic. Quantities plotted as a function of density are acceleration potential, potential temperature, desso1ved oxygen and potential vorticity.

The transition from Sargasso Sea Water to Slope Water in the upper thermocline (σ0<27.1) is sharp and coincides closely with the dynamical boundary of the Gulf Stream, defined by the gradient of acceleration potential. This indicates that water mass exchanges across the Gulf Stream-Slope Water front are limited at these levels. Below the 27.1 Σ0 surface, the gradient of acceleration potential still reveals the position of the Stream, but there is no coincident water man boundary. This and the uniformity of potential vorticity across the Stream suggest that the deep property fields are being efficiently homogenized by mesoscale exchanges across the Gulf Stream. A cross-frontal eddy diffusivity of KH=2.5×106 cm2 s−1 estimated from oxygen flux calculations agrees well with previously published values for frontal regimes.

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K. N. Bower, T. W. Choularton, J. Latham, J. Nelson, M. B. Baker, and J. Jensen

Abstract

Simple parameterizations of droplet effective radius in stratiform and convective clouds are presented for use in global climate models. Datasets from subtropical marine stratocumulus, continental and maritime convective clouds, and hill cap clouds in middle latitudes and a small amount of data from stratocumulus clouds in middle latitudes have been examined. The results suggest strongly that a simple relationship exists between droplet effective radius and liquid water content in layer clouds with the droplet effective radius proportional to the cube root of the liquid water content. The constant of proportionality is different over oceans and continents. In current global climate models liquid water content is not a predicted variable in convective clouds, and the data strongly suggest that a fixed value of droplet effective radius between 9 and 10 μm should be used for continental clouds more than 500 m deep and 16 μm for maritime cumulus more than 1.5 km deep.

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J. M. White, J. F. Bowers, S. R. Hanna, and J. K. Lundquist

Abstract

The mixing depth of the boundary layer is an input to most atmospheric transport and dispersion (ATD) models, which obtain mixing depths in one of four ways: 1) observations by radiosondes, sodars, or other devices; 2) simulations by regional or mesoscale meteorological models; 3) parameterizations based on boundary layer similarity theory; or 4) climatological averages. This paper describes a situation during a field experiment when exceptionally low mixing depths persisted in the morning and led to relatively high observed tracer concentrations. The low mixing depths were caused by synoptic effects associated with a nearby stationary front and the outflow from a mesoscale thunderstorm complex located 20–50 km away. For the same time period, the ATD model-parameterized mixing depth was a factor of 5–10 higher, leading to predicted concentrations that were less than the observations by a factor of 5–10. The synoptic situation is described and local radiosonde and radar observations of mixing depth are presented, including comparisons with other more typical days. Time series of local observations of near-surface sensible heat fluxes are also plotted to demonstrate the suppression of turbulence by negative sensible heat fluxes during the period in question.

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F. I. M. Thomas, S. A. McCarthy, J. Bower, S. Krothapalli, M. J. Atkinson, and P. Flament

Abstract

Response characteristics of a microhole potentiostatic oxygen sensor and a Beckman membrane oxygen sensor were measured in a laboratory over temperatures ranging from 1° to 21°C. The response term τ of the microhole sensor changed 1.7-fold over this temperature range, and τ of the membrane sensor changed 1.6-fold. For the microhole sensor, the effect of temperature on τ can be modeled as lnτ+−6.5 + 1618T −1. For the membrane sensor the temperature effect on τ can be modeled as lnτ = −5.8 + 2116T −1, where T is temperature in kelvins.

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Eli Jacks, J. Brent Bower, Valery J. Dagostaro, J. Paul Dallavalle, Mary C. Erickson, and James C. Su

Abstract

In this paper, we describe the development and use of new nested grid model (NGM)-based model output statistics (MOS) guidance that has been available since 26 July 1989 for 204 stations in the contiguous United States. The new guidance, which replaced the NGM-based perfect prog package that had been operational since May 1987, consists of forecasts of max/min temperature, probability of precipitation, cloud amount, and surface wind. Guidance for all four elements is available for projections of 1 and 2 days from 0000 and 1200 UTC. The limited-area fine-mesh model (LFM)-based MOS guidance package is still available and was not affected by this change. Verification on independent data shows that NGM-based MOS and LFM-based MOS temperature forecasts are about equally accurate and that both sets of MOS guidance are clearly superior to the NGM-based perfect prog guidance. For the probability of precipitation, the NGM-based MOS guidance is consistently more skillful than the perfect prog guidance, and usually more skillful than the LFM-based MOS guidance. For cloud amount, the NGM-based MOS forecasts are more skillful than either the LFM-based MOS or the NGM-based perfect prog. Finally, the NGM-based MOS and perfect prog wind forecasts are about equally skillful, and both sets are superior to the LFM-based MOS guidance.

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Eugene Yee, R. Chan, P. R. Kosteniuk, G. M. Chandler, C. A. Biltoft, and J. F. Bowers

Abstract

High-frequency fluctuations of concentration in a plume dispersing in the atmospheric surface layer have been measured with high-resolution concentration detectors (approximately 270 Hz at the −6-dB point) to extract various concentration statistics of the fluctuating concentration field. Crosswind and alongwind variations of amplitude statistics (e.g., the total and conditional fluctuation intensity, skewness, and kurtosis), the intermittency factor, and the shapes of the concentration probability density function (PDF) are presented. The behavior of temporal concentration statistics such as the autocorrelation function; power spectrum; PDF of upcrossing intervals PDF of excursion durations; various concentration timescales, length scales, and microscales (e.g., Taylor microscale, correlation scale, length scale based on the spectral peak, etc.); as well as the velocity-to-concentration timescale ratio are studied. It is shown that all the concentration length scales and microscales (with the exception of the correlation scale) grow with downwind distance in proportion to the mean plume width.

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G. Lloyd, C. Dearden, T. W. Choularton, J. Crosier, and K. N. Bower

Abstract

Three case studies in frontal clouds from the Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) project are described to understand the microphysical development of the mixed phase regions of these clouds. The cases are a kata-type cold front, a wintertime warm front, and a summertime occluded frontal system. The clouds were observed by radar, satellite, and in situ microphysics measurements from the U.K. Facility for Airborne Atmospheric Measurements (FAAM) research aircraft. The kata cold front cloud was shallow with a cloud-top temperature of approximately −13°C. Cloud-top heterogeneous ice nucleation was found to be consistent with predictions by a primary ice nucleation scheme. The other case studies had high cloud tops (< −40°C) and despite no direct cloud-top measurements in these regions, homogeneous ice nucleation would be expected. The maximum ice crystal concentrations and ice water contents in all clouds were observed at temperatures around −5°C. Graupel was not observed, hence, secondary ice was produced by riming on snow falling through regions of supercooled liquid water. Within these regions substantial concentrations (10–150 L−1) of supercooled drizzle were observed. The freezing of these drops increases the riming rate due to the increase in rimer surface area. Increasing rime accretion has been shown to lead to higher ice splinter production rates. Despite differences in the cloud structure, the maximum ice crystal number concentration in all three clouds was ~100 L−1. Ice water contents were similar in the warm and occluded frontal cases, where median values in both cases reached ~0.2–0.3 g m−3, but lower in the cold front case.

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