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David P. Jorgensen, Margaret A. LeMone, and Ben Jong-Dao Jou

Abstract

The precipitation, thermodynamic, and kinematic structure of an oceanic mesoscale convective system is studied using airborne Doppler and in situ (flight-level) data collected by the NOAA P-3 aircraft. The system, a well-organized, stationary, north-south convective line, was located near the east coast of Taiwan. In Part I, the basic structure of the line is documented with both datasets, a procedure revealing the strengths and weakness of both approaches.

The Doppler data reveal that the warm, moist air feeding the line enters from the east side. Most updrafts associated with the leading edge of the convective line tilt westward below 5 km and then eastward above 5 km. This change of tilt corresponds to a change in the sign of the vertical flux of east-west momentum. To the east of the leading edge, a 10-km-wide zone of strong mesoscale descent is seen. The band is not a complete barrier to the low-level southeasterly flow, and at times and places along the line the inflowing air can move through the band with little or no upward acceleration. The minimum pressures at low levels lie east of the highest reflectivity and also underneath the tilted updraft at upper levels, in agreement with the tilt of the updraft, the buoyancy distribution, and the interaction of the updraft with the vertical shear of the horizontal wind. The Doppler data show very few convective-scale downdrafts and no low-level gust front that would organize the convection as in propagating squall lines, although lack of resolution in the pseudo-dual-Doppler data at the lowest levels may mask features with horizontal scales <5 km. Vertical incidence Doppler observations show only a few relatively weak convective-scale downdrafts within the heavy rainfall region of the convective line.

The in situ data confirm that warm, moist air feeds the convective line from the east side, but they show a larger fraction of air coming into the convection from the boundary layer than do the Doppler data. They confirm that the line is not an effective barrier to the flow: some air from the east of the line, including boundary-layer air, passes through the line without joining the updrafts. Again, some weak convective-scale downdrafts are evident, but a gust front was not detected. However, at low levels, a pool of low-θe, air lies 10–20 km to the west of the line, outside the dual-Doppler domain. This cool air apparently originated to the north (beneath an extensive stratiform area, but preexisting baroclinicity associated with a front may have also contributed to the cool air) and advected southward. Vertically incident Doppler data confirm the upper-level downdraft zone to the east of the updraft. Above 2 km, the pressure and vertical velocity fields are consistent, with low pressure lying beneath the tilting updrafts in both datasets. Below 2 km, the in situ data reveal a mesolow beneath the westward-tilting updraft that was not captured by the Doppler data, apparently because of contamination of the very lowest levels by ground clutter.

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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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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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Jennifer L. Davison, Robert M. Rauber, Larry Di Girolamo, and Margaret A. LeMone

Abstract

This paper investigates wintertime tropical marine boundary layer (TMBL) statistical characteristics over the western North Atlantic using the complete set of island-launched soundings from the Rain in Cumulus over the Ocean (RICO) experiment. The soundings are subdivided into undisturbed and disturbed classifications using two discriminators: 1) dates chosen by Global Energy and Water Cycle Experiment (GEWEX) Cloud System Studies (GCSS) investigators to construct the mean RICO sounding and 2) daily average rain rates.

A wide range of relative humidity (RH) values was observed between the surface and 8.0 km. At 2.0 km, half the RH values were within 56%–89%; at 4.0 km, half were within 13%–61%. The rain-rate method of separating disturbed and undisturbed soundings appears more meaningful than the GCSS method. The median RH for disturbed conditions using the rain-rate method showed moister conditions from the surface to 8.0 km, with maximum RH differences of 30%–40%. Moist air generally extended higher on disturbed than undisturbed days.

Based on equivalent potential temperature, wind direction, and RH analyses, the most common altitude marking the TMBL top was about 4.0 km. Temperature inversions (over both 50- and 350-m intervals) were observed at every altitude above 1.2 km; there were no dominant inversion heights and most of the inversions were weak. Wind direction analyses indicated that winds within the TMBL originated from more tropical latitudes on disturbed days.

The analyses herein suggest that the RICO profile used to initialize many model simulations of this environment represents only a small subset of the broad range of possible conditions characterizing the wintertime trades.

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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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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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Jennifer L. Davison, Robert M. Rauber, Larry Di Girolamo, and Margaret A. LeMone

Abstract

This paper examines the structure and variability of the moisture field in the tropical marine boundary layer (TMBL) as defined by Bragg scattering layers (BSLs) observed with S-band radar. Typically, four to five BSLs were present in the TMBL, including the transition layer at the top of the surface-based mixed layer. The transition-layer depth (~350 m) exhibited a weak diurnal cycle because of changes in the mixed-layer depth. BSLs and the “clear” layers between them each had a median thickness of about 350 m and a lifetime over the radar of 8.4 h, with about 25% having lifetimes longer than 20 h. More (fewer) BSLs were present when surface winds had a more southerly (northerly) component. Both BSLs and clear layers increased in depth with increasing rain rates, with the rainiest days producing layers that were about 100 m thicker than those on the driest days. The analyses imply that the relative humidity (RH) field in the TMBL exhibits layering on scales observable by radar. Satellite and wind profiler measurements show that the layered RH structure is related, at least in part, to detraining cloudy air.

Based on analyses in this series of papers, a revised conceptual model of the TMBL is presented that emphasizes moisture variability and incorporates multiple moist and dry layers and a higher TMBL top. The model is supported by comparing BSL tops with satellite-derived cloud tops. This comparison suggests that the layered RH structure is related, in part, to cloud detrainment at preferred altitudes within the TMBL. The potential ramifications of this change in TMBL conceptualization on modeling of the TMBL are discussed.

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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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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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Monica Górska, Jordi Vilà-Guerau de Arellano, Margaret A. LeMone, and Chiel C. van Heerwaarden

Abstract

The effects of the horizontal variability of surface properties on the turbulent fluxes of virtual potential temperature, moisture, and carbon dioxide are investigated by combining aircraft observations with large-eddy simulations (LESs). Daytime fair-weather aircraft measurements from the 2002 International H2O Project’s 45-km Eastern Track over mixed grassland and winter wheat in southeast Kansas reveal that the western part of the atmospheric boundary layer was warmer and drier than the eastern part, with higher values of carbon dioxide to the east. The temperature and specific humidity patterns are consistent with the pattern of surface fluxes produced by the High-Resolution Land Data Assimilation System. However, the observed turbulent fluxes of virtual potential temperature, moisture, and carbon dioxide, computed as a function of longitude along the flight track, do not show a clear east–west trend. Rather, the fluxes at 70 m above ground level related better to the surface variability quantified in terms of the normalized differential vegetation index (NDVI), with strong correlation between carbon dioxide fluxes and NDVI.

A first attempt is made to estimate the ratios of the flux at the entrainment zone to the surface flux (entrainment ratios) as a function of longitude. The entrainment ratios averaged from these observations (β θυ ≈ 0.10, βq ≈ −2.4, and β CO2 ≈ −0.58) are similar to the values found from the homogeneous LES experiment with initial and boundary conditions similar to observations.

To understand how surface flux heterogeneity influences turbulent fluxes higher up, a heterogeneous LES experiment is performed in a domain with higher sensible and lower latent heat fluxes in the western half compared to the eastern half. In contrast to the aircraft measurements, the LES turbulent fluxes show a difference in magnitude between the eastern and western halves at 70 and 700 m above ground level. Possible reasons for these differences between results from LES and aircraft measurements are discussed.

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