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David P. Jorgensen, Edward J. Zipser, and Margaret A. LeMone

Abstract

Hurricane vertical motion properties are studied using aircraft-measured 1 Hz time series of vertical velocity obtained during radial penetrations of four mature hurricanes. A total of 115 penetrations from nine flight sorties at altitudes from 0.5 to 6.1 km are included in the data set. Convective vertical motion events are classified as updrafts (or downdrafts) if the vertical velocity was continuously positive (or negative) for at least 500 m and exceed an absolute value of 0.5 m s−1. Over 3000 updrafts and nearly 2000 downdrafts are included in the data set. A second criteria was used to define stronger events, called cores. This criteria required that upward (or downward) vertical velocity be continuously greater than an absolute value of 1 m s−1 for at least 500 m.

The draft and core properties are summarized as distributions of average and maximum vertical velocity, diameter, and vertical mass transport in two regions: eyewall and rainband. In both regions updrafts dominated over downdrafts, both in number and mass transport. In the eyewall region, the draft and core strength distributions were similar to data collected by aircraft in GATE cumulonimbus clouds. Unlike GATE clouds, however, the largest updraft cores (larger than 90% of the distribution) were over twice as large and transported twice as much mass as did the corresponding GATE updraft cores. Eyewall ascent was highly organized in a channel several kilometers wide located a few kilometers radically inward from the radius of maximum tangential wind.

As in GATE, the strongest hurricane updraft cores were weak in comparison with the strongest updrafts observed in typical midlatitude thunderstorms. Mean eyewall profiles of radar reflectivity and cloud water content are discussed to illustrate the microphysical implications of the low updraft rates.

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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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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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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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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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Teddy R. Holt, Dev Niyogi, Fei Chen, Kevin Manning, Margaret A. LeMone, and Aneela Qureshi

Abstract

Numerical simulations are conducted using the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) to investigate the impact of land–vegetation processes on the prediction of mesoscale convection observed on 24–25 May 2002 during the International H2O Project (IHOP_2002). The control COAMPS configuration uses the Weather Research and Forecasting (WRF) model version of the Noah land surface model (LSM) initialized using a high-resolution land surface data assimilation system (HRLDAS). Physically consistent surface fields are ensured by an 18-month spinup time for HRLDAS, and physically consistent mesoscale fields are ensured by a 2-day data assimilation spinup for COAMPS. Sensitivity simulations are performed to assess the impact of land–vegetative processes by 1) replacing the Noah LSM with a simple slab soil model (SLAB), 2) adding a photosynthesis, canopy resistance/transpiration scheme [the gas exchange/photosynthesis-based evapotranspiration model (GEM)] to the Noah LSM, and 3) replacing the HRLDAS soil moisture with the National Centers for Environmental Prediction (NCEP) 40-km Eta Data Assimilation (EDAS) operational soil fields.

CONTROL, EDAS, and GEM develop convection along the dryline and frontal boundaries 2–3 h after observed, with synoptic-scale forcing determining the location and timing. SLAB convection along the boundaries is further delayed, indicating that detailed surface parameterization is necessary for a realistic model forecast. EDAS soils are generally drier and warmer than HRLDAS, resulting in more extensive development of convection along the dryline than for CONTROL. The inclusion of photosynthesis-based evapotranspiration (GEM) improves predictive skill for both air temperature and moisture. Biases in soil moisture and temperature (as well as air temperature and moisture during the prefrontal period) are larger for EDAS than HRLDAS, indicating land–vegetative processes in EDAS are forced by anomalously warmer and drier conditions than observed. Of the four simulations, the errors in SLAB predictions of these quantities are generally the largest.

By adding a sophisticated transpiration model, the atmospheric model is able to better respond to the more detailed representation of soil moisture and temperature. The sensitivity of the synoptically forced convection to soil and vegetative processes including transpiration indicates that detailed representation of land surface processes should be included in weather forecasting models, particularly for severe storm forecasting where local-scale information is important.

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Alexandre O. Fierro, Joanne Simpson, Margaret A. LeMone, Jerry M. Straka, and Bradley F. Smull

Abstract

An airflow trajectory analysis was carried out based on an idealized numerical simulation of the nocturnal 9 February 1993 equatorial oceanic squall line observed over the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) ship array. This simulation employed a nonhydrostatic numerical cloud model, which features a sophisticated 12-class bulk microphysics scheme. A second convective system that developed immediately south of the ship array a few hours later under similar environmental conditions was the subject of intensive airborne quad-Doppler radar observations, allowing observed airflow trajectories to be meaningfully compared to those from the model simulation. The results serve to refine the so-called hot tower hypothesis, which postulated the notion of undiluted ascent of boundary layer air to the high troposphere, which has for the first time been tested through coordinated comparisons with both model output and detailed observations.

For parcels originating ahead (north) of the system near or below cloud base in the boundary layer (BL), the model showed that a majority (>62%) of these trajectories were able to surmount the 10-km level in their lifetime, with about 5% exceeding 14-km altitude, which was near the modeled cloud top (15.5 km). These trajectories revealed that during ascent, most air parcels first experienced a quick decrease of equivalent potential temperature (θe) below 5-km MSL as a result of entrainment of lower ambient θe air. Above the freezing level, ascending parcels experienced an increase in θe with height attributable to latent heat release from ice processes consistent with previous hypotheses. Analogous trajectories derived from the evolving observed airflow during the mature stage of the airborne radar–observed system identified far fewer (∼5%) near-BL parcels reaching heights above 10 km than shown by the corresponding simulation. This is attributed to both the idealized nature of the simulation and to the limitations inherent to the radar observations of near-surface convergence in the subcloud layer.

This study shows that latent heat released above the freezing level can compensate for buoyancy reduction by mixing at lower levels, thus enabling air originating in the boundary layer to contribute to the maintenance of both local buoyancy and the large-scale Hadley cell despite acknowledged dilution by mixing along updraft trajectories. A tropical “hot tower” should thus be redefined as any deep convective cloud with a base in the boundary layer and reaching near the upper-tropospheric outflow layer.

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Margaret A. LeMone, Mukul Tewari, Fei Chen, Joseph G. Alfieri, and Dev Niyogi

Abstract

Sources of differences between observations and simulations for a case study using the Noah land surface model–based High-Resolution Land Data Assimilation System (HRLDAS) are examined for sensible and latent heat fluxes H and LE, respectively; surface temperature Ts; and vertical temperature difference T 0Ts, where T 0 is at 2 m. The observational data were collected on 29 May 2002, using the University of Wyoming King Air and four surface towers placed along a sparsely vegetated 60-km north–south flight track in the Oklahoma Panhandle. This day had nearly clear skies and a strong north–south soil-moisture gradient, with wet soils and widespread puddles at the south end of the track and drier soils to the north. Relative amplitudes of H and LE horizontal variation were estimated by taking the slope of the least squares best-fit straight line ΔLE/ΔH on plots of time-averaged LE as a function of time-averaged H for values along the track. It is argued that observed H and LE values departing significantly from their slope line are not associated with surface processes and, hence, need not be replicated by HRLDAS. Reasonable agreement between HRLDAS results and observed data was found only after adjusting the coefficient C in the Zilitinkevich equation relating the roughness lengths for momentum and heat in HRLDAS from its default value of 0.1 to a new value of 0.5. Using C = 0.1 and adjusting soil moisture to match the observed near-surface values increased horizontal variability in the right sense, raising LE and lowering H over the moist south end. However, both the magnitude of H and the amplitude of its horizontal variability relative to LE remained too large; adjustment of the green vegetation fraction had only a minor effect. With C = 0.5, model-input green vegetation fraction, and our best-estimate soil moisture, H, LE, ΔLE/ΔH, and T 0Ts, were all close to observed values. The remaining inconsistency between model and observations—too high a value of H and too low a value of LE over the wet southern end of the track—could be due to HRLDAS ignoring the effect of open water. Neglecting the effect of moist soils on the albedo could also have contributed.

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