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David P. Jorgensen, Margaret A. LeMone, and Stanley B. Trier

Abstract

This study documents the precipitation and kinematic structure of a mature, eastward propagating, oceanic squall line system observed by instrumented aircraft during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). Doppler radar and low-level in situ observations are used to show the evolution of the convection from an initially linear NNW–SSE-oriented convective line to a highly bow-shaped structure with an embedded low- to midlevel counterclockwise rotating vortex on its northern flank. In addition to previously documented features of squall lines such as highly upshear-tilted convection on its leading edge, a channel of strong front-to-rear flow that ascended with height over a “rear-inflow” that descended toward the convective line, and a pronounced low-level cold pool apparently fed from convective and mesoscale downdrafts from the convective line; rearward, the observations of this system showed distinct multiple maxima in updraft strength with height and reflectivity bands extending rearward transverse to the principal convective line. Vertical motions within the active convective region of the squall line system were determined using a new approach that utilized near-simultaneous observations by the Doppler radars on two aircraft with up to four Doppler radial velocity estimates at echo top. Echo-top vertical motion can then be derived directly, which obviates the traditional dual-Doppler assumption of no vertical velocity at the top boundary and results in a more accurate estimate of tropospheric vertical velocity through downward integration of horizontal divergence.

Low-level flight-level observations of temperature, wind speed, and dew point collected rearward of the squall line are used to estimate bulk fluxes of dry and moist static energy. The strong near-surface fluxes, due to the warm sea and high winds, combined with estimates of mesoscale advection, are used to estimate boundary layer recovery time; they indicate that the boundary layer could recover from the effects of the cold dome within about 3 h of first cold air injection if the observed near-surface winds were maintained. However, the injection and spreading of air from above leads to cooling at a fixed spot ∼20 km rearward of the convective line (surface θ e minimum point), suggesting that the cold pool could be still intensifying at the time of observation. Recovery time at a point is probably similar to that measured in previous studies.

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Gary Barnes, George D. Emmitt, Burghard Brummer, Margaret A. LeMone, and Stephen Nicholls

Abstract

A fair weather boundary layer (BL) with light winds and scattered cumulus to 1100 m is examined in the GATE C-scale triangle using data from tethered balloons, surface measurements from the booms of the ships, structure sondes and gust probe aircraft. The original goal was a comparison of the instrumentation in an expected uniform field of wind, temperature and humidity. It became rapidly obvious that nonuniformities existed not only at the turbulence scales (a few meters to 1 km) but also on scales 10 km and larger. Thus the goal evolved into 1) combining the observations to present a coherent picture of the day, 2) putting the results of various observational techniques in perspective and 3) examining the nonuniformity.

Different aspects of the day are revealed by the different observational techniques. The Dallas tethered balloon reveals a noticeable modification of the BL nearly coincident with a change in convective activity. In spite of nonuniformity, and the interception of convective events similar to that at the Dallas, the flux profiles from aircraft show that the BL behaves in a similar way to those reported previously near “horizontally homogeneous” conditions. Moisture and energy budgets performed for this day show the expected convergence of sensible and latent heat in the boundary layer but in a shallower layer than expected.

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Margaret A. LeMone, Gary M. Barnes, James C. Fankhauser, and Lesley F. Tarleton

Abstract

Perturbation pressure fields are measured by aircraft around the cloud base updrafts of seven clouds ranging in size from weak cumulus congestus to intense cumulonimbus during CCOPE (1981). The fields are characterized by a high-low pressure couplet of similar size to the updraft, but a quarter-wavelength out of Phase, with the minimum pressure downshear of the updraft maximum. An estimate of the terms in the Poisson equation for pressure show that the pressure perturbation results chiefly from the interaction of the updraft with the vertical shear of the environmental horizontal wind. The behavior of the pressure oscillation is well predicted by inserting sinusoidal functions in the corresponding terms in the Poisson equation. The amplitude of the pressure oscillation is proportional to the wavelengths of the pressure and vertical-velocity fields, the amplitude of the vertical-velocity oscillation, and the vertical shear of the horizontal environmental wind through cloud base, measured in the direction of the maximum pressure gradient.

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William Blumen, Nimal Gamage, Robert L. Grossman, Margaret A. LeMone, and L. Jay Miller

Abstract

This investigation examines the meso- and microscale aspects of the 9 March 1992 cold front that passed through Kansas during the daylight hours. The principal feature of this front is the relatively rapid frontogenesis that occurred. The total change in the cross-frontal temperature is about 6 K, with most of the change occurring between about 0820 and 1400 local time and over a relatively small subsection of the total frontal width. The surface data are able to resolve a sharp horizontal transition zone of 1–2 km. The principal physical processes that produce this frontogenesis are shown to be the cross-frontal differential sensible heating, associated with differential cloud cover, and the convergence of warm and cold air toward the front. The former process is responsible for an increase in the magnitude of the differential temperature change across the front; the latter process concentrates the existing temperature differential across an ever-decreasing transitional zone until a near discontinuity in the horizontal temperature distribution is essentially established during the period of a few hours. Two approaches are taken to demonstrate that these processes control the observed frontogenesis. First, surface data from an enhanced array, set up during the Storm-scale Operational and Research Meteorology Fronts Experiment System Test, are used to evaluate the terms that contribute to the time rate of change of the gradient of potential temperature, d|∇θ| / dt, following the motion of the front. Then, the processes of differential sensible heating and convergence are incorporated into a simple two-dimensional nonlinear model that serves to provide a forecast of the surface temperature and velocity fields from given initial conditions that are appropriate at the onset of the surface heating. Verification of the model predictions by observed data confirms that both processes contribute to the observed daytime frontogenesis on 9 March 1992. A critique of the model does. however, suggest that the accuracy of some quantitative evaluations could be improved.

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Margaret A. LeMone, Robert L. Grossman, Fei Chen, Kyoko Ikeda, and David Yates

Abstract

Data from the April–May 1997 Cooperative Atmosphere Surface Exchange Study (CASES-97) are used to illustrate a holistic way to select an averaging interval for comparing horizontal variations in sensible heat (H) and latent heat (LE) fluxes from low-level aircraft flights to those from land surface models (LSMs). The ideal filter can be defined by considering the degree to which filtered aircraft fluxes 1) replicate the observed pattern followed by H and LE at the surface, 2) are statically robust, and 3) retain the heterogeneity to be modeled. Spatial variability and temporal variability are computed for different filtering wavelengths to assess spatial variability sacrificed by filtering and how much temporal variability can be eliminated; ideally, spatial variability should approach or exceed temporal variability. The surface pattern to be replicated is a negative slope when H is plotted against LE for a given time. This is required for surface energy balance if H or LE vary horizontally more than their sum, R nG, the difference between the net radiation and heat flux into the ground. Statistical confidence is estimated using conventional techniques. The same factors can be used to examine comparisons already done, or to estimate the number of flight legs needed to measure heterogeneity at a given scale in future field programs.

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Margaret A. LeMone, Kyoko Ikeda, Robert L. Grossman, and Mathias W. Rotach

Abstract

Surface-station, radiosonde, and Doppler minisodar data from the Cooperative Atmosphere–Surface Exchange Study-1997 (CASES-97) field project, collected in a 60-km-wide array in the lower Walnut River watershed (terrain variation ∼150 m) southeast of Wichita, Kansas, are used to study the relationship of the change of the 2-m potential temperature Θ2m with station elevation z e, ∂Θ2m/∂z e ≡ Θ,ze to the ambient wind and thermal stratification ∂Θ/∂z ≡ Θ,z during fair-weather nights. As in many previous studies, predawn Θ2m varies linearly with z e, and Θ,ze ∼ Θ,z over a depth h that represents the maximum elevation range of the stations. Departures from the linear Θ2m–elevation relationship (Θ,ze line) are related to vegetation (cool for vegetation, warm for bare ground), local terrain (drainage flows from nearby hills, although a causal relationship is not established), and the formation of a cold pool at lower elevations on some days.

The near-surface flow and its evolution are functions of the Froude number Fr = S/(Nh), where S is the mean wind speed from the surface to h, and N is the corresponding Brunt–Väisälä frequency. The near-surface wind is coupled to the ambient flow for Fr = 3.3, based on where the straight line relating Θ,ze to ln Fr intersects the ln Fr axis. Under these conditions, Θ2m is constant horizontally even though Θ,z > 0, suggesting that near-surface air moves up- and downslope dry adiabatically. However, Θ2m cools (or warms) everywhere at the same rate. The lowest Froude numbers are associated with drainage flows, while intermediate values characterize regimes with intermediate behavior. The evolution of Θ2m horizontal variability σ Θ through the night is also a function of the predawn Froude number. For the nights with the lowest Fr, the σ Θ maximum occurs in the last 1–3 h before sunrise. For nights with Fr ∼ 3.3 (Θ,ze ≈ 0) and for intermediate values, σ Θ peaks 2–3 h after sunset. The standard deviations relative to the Θ,ze line reach their lowest values in the last hours of darkness. Thus, it is not surprising that the relationships of Θ,ze to Fr and Θ,z based on data through the night show more scatter, and Θ,ze ∼ 0.5Θ,z in contrast to the predawn relationship. However, Θ,ze ≈ 0 for ln Fr = 3.7, a value similar to that just before sunrise.

A heuristic Lagrangian parcel model is used to explain the horizontal uniformity of time-evolving Θ2m when the surface flow is coupled with the ambient wind, as well as both the linear variation of Θ2m with elevation and the time required to reach maximum values of σ Θ under drainage-flow conditions.

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Alexandre O. Fierro, Edward J. Zipser, Margaret A. LeMone, Jerry M. Straka, and Joanne (Malkus) Simpson

Abstract

This paper addresses questions resulting from the authors’ earlier simulation of the 9 February 1993 Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Research Experiment (TOGA COARE) squall line, which used updraft trajectories to illustrate how updrafts deposit significant moist static energy (in terms of equivalent potential temperature θe) in the upper troposphere, despite dilution and a θe minimum in the midtroposphere. The major conclusion drawn from this earlier work was that the “hot towers” that Riehl and Malkus showed as necessary to maintain the Hadley circulation need not be undilute. It was not possible, however, to document how the energy (or θe) increased above the midtroposphere. To address this relevant scientific question, a high-resolution (300 m) simulation was carried out using a standard 3-ICE microphysics scheme (Lin–Farley–Orville).

Detailed along-trajectory information also allows more accurate examination of the forces affecting each parcel’s vertical velocity W, their displacement, and the processes impacting θe, with focus on parcels reaching the upper troposphere. Below 1 km, pressure gradient acceleration forces parcels upward against negative buoyancy acceleration associated with the sum of (positive) virtual temperature excess and (negative) condensate loading. Above 1 km, the situation reverses, with the buoyancy (and thermal buoyancy) acceleration becoming positive and nearly balancing a negative pressure gradient acceleration, slightly larger in magnitude, leading to a W minimum at midlevels. The W maximum above 8 km and concomitant θ e increase between 6 and 8 km are both due to release of latent heat resulting from the enthalpy of freezing of raindrops and riming onto graupel from 5 to 6.5 km and water vapor deposition onto small ice crystals and graupel pellets above that, between 7 and 10 km.

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Fei Chen, David N. Yates, Haruyasu Nagai, Margaret A. LeMone, Kyoko Ikeda, and Robert L. Grossman

Abstract

Land surface heterogeneity over an area of 71 km × 74 km in the lower Walnut River watershed, Kansas, was investigated using models and measurements from the 1997 Cooperative Atmosphere Surface Exchange Study (CASES-97) field experiment. As an alternative approach for studying heterogeneity, a multiscale atmospheric and surface dataset (1, 5, and 10 km) was developed, which was used to drive three land surface models, in uncoupled 1D mode, to simulate the evolution of surface heat fluxes and soil moisture for approximately a 1-month period (16 April–22 May 1997) during which the natural grassland experienced a rapid greening. Model validation using both surface and aircraft measurements showed that these modeled flux maps have reasonable skill in capturing the observed surface heterogeneity related to land-use cover and soil moisture. The results highlight the significance of rapid greening of grassland in shaping the surface heterogeneity for the area investigated. The treatment of soil hydraulic properties and canopy resistance in these land surface models appears to cause the majority of differences among their results. Several factors contributing to the discrepancy between modeled and aircraft measured heat fluxes in relation to their respective time–space integration were examined. When land surface heterogeneity is pronounced, modeled heat fluxes agree better with those measured by aircraft in terms of spatial variability along flight legs. When compared to Advanced Very High Resolution Radiometer/Normalized Difference Vegetation Index (AVHRR/NDVI) data, it is demonstrated that modeled heat flux maps with different spatial resolutions can be utilized to study their scaling properties at local or regional scales.

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Diane Strassberg, Margaret A. LeMone, Thomas T. Warner, and Joseph G. Alfieri

Abstract

Comparisons of 10-m above ground level (AGL) wind speeds from numerical weather prediction (NWP) models to point observations consistently show that model daytime wind speeds are slow compared to observations, even after improving model physics and going to smaller grid spacing. Previous authors have attributed the discrepancy to differences between the areas represented by model and observations, and the small surface roughness upstream of wind vanes compared with the corresponding model grid value. Using daytime fair-weather data from the May–June 2002 International H2O Experiment (IHOP_2002), the effect of wind-vane exposure is explored by comparing observed 10-m winds from nine surface-flux towers in well-exposed locations to modeled 10-m winds found by applying Monin–Obukhov (MO) similarity for unstable conditions to flight-track-averaged data collected by the University of Wyoming King Air over flat to rolling terrain with occasional trees and buildings. In the calculations, King Air winds and fluxes are supplemented with thermodynamic means and fluxes from the surface-flux towers. After exercising considerable care in characterizing and reducing biases in aircraft winds and fluxes, the authors found that MO-based surface winds averaged 0.5–0.7 ± 0.2 m s−1 less than those measured—about the same as the smaller reported discrepancies between NWP models and observed winds.

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L. Jay Miller, Margaret A. LeMone, William Blumen, Robert L. Grossman, Nimal Gamage, and Robert J. Zamora

Abstract

Observations taken over the period 8–10 March 1992 during the Storm-scale Operational and Research Meteorology Fronts Experiment Systems Test in the central United States are used to document the detailed low-level structure and evolution of a shallow, dry arctic front. The front was characterized by cloudy skies to its north side and clear skies to its south side. It was essentially two-dimensional in the zone of intense observations.

There was a significant diurnal cycle in the magnitude of the potential temperature gradient across both the subsynoptic and mesoscale frontal zones, but imposed upon an underlying, more gradual, increase over the three days. On the warm (cloudless) side., the temperature increased and decreased in response to the diurnal heating cycle, while on the cold (cloudy) side the shape of the temperature decrease from its warm-side value (first dropping rapidly and then slowly in an exponential-like manner) remained fairly steady. The authors attribute the strong diurnal variation in potential temperature gradient mostly to the effects of differential diabatic heating across the front due to differential cloud cover.

The front is described in terms of three scales: 1) a broad, subsynoptic frontal zone (∼250–300 km wide) of modest temperature and wind gradients; 2) a narrower mesoscale zone (∼15–20 km wide) with much larger gradients; and 3) a microscale zone of near-zero-order discontinuity (≤1–2 km wide). There was some narrowing (≲50 km) of the subsynoptic frontal zone, but the authors found no evidence for any significant contraction of this zone down to much smaller mesoscale sizes. In response to the differential diabatic heating, the strongest evolution occurred in the micro-mesoscale zone, where dual-Doppler radar and aircraft measurements revealed the development of a density-current-like structure in and behind the leading edge of cold air. Here the steepest gradients developed shortly after sunrise and then increased by an order of magnitude during the day, with leading-edge vorticity, divergence, and temperature gradients reaching maximum values of 10−2 s−1 and 8 K km−1. A narrow updraft, marked by cumulus clouds, grew in intensity above the leading edge through the day to a maximum of 5–8 m s−1. Stratus clouds lay in the cold air, their leading edge receding by noon to 10–20 km behind the cumulus line.

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