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Edward I. Tollerud, Fernando Caracena, Steven E. Koch, Brian D. Jamison, R. Michael Hardesty, Brandi J. McCarty, Christoph Kiemle, Randall S. Collander, Diana L. Bartels, Steven Albers, Brent Shaw, Daniel L. Birkenheuer, and W. Alan Brewer

Abstract

Previous studies of the low-level jet (LLJ) over the central Great Plains of the United States have been unable to determine the role that mesoscale and smaller circulations play in the transport of moisture. To address this issue, two aircraft missions during the International H2O Project (IHOP_2002) were designed to observe closely a well-developed LLJ over the Great Plains (primarily Oklahoma and Kansas) with multiple observation platforms. In addition to standard operational platforms (most important, radiosondes and profilers) to provide the large-scale setting, dropsondes released from the aircraft at 55-km intervals and a pair of onboard lidar instruments—High Resolution Doppler Lidar (HRDL) for wind and differential absorption lidar (DIAL) for moisture—observed the moisture transport in the LLJ at greater resolution. Using these observations, the authors describe the multiscalar structure of the LLJ and then focus attention on the bulk properties and effects of scales of motion by computing moisture fluxes through cross sections that bracket the LLJ. From these computations, the Reynolds averages within the cross sections can be computed. This allow an estimate to be made of the bulk effect of integrated estimates of the contribution of small-scale (mesoscale to convective scale) circulations to the overall transport. The performance of the Weather Research and Forecasting (WRF) Model in forecasting the intensity and evolution of the LLJ for this case is briefly examined.

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Robin L. Tanamachi, Wayne F. Feltz, and Ming Xue

Abstract

On the morning of 12 June 2002, a series of upper boundary layer (UBL) rapid drying and moistening events (RDEs and RMEs, respectively) occurred at the “Homestead” site of the International H2O Project (IHOP_2002). Over a period of 10 h, atmospheric water vapor in the UBL decreased or increased within a matter of minutes four separate times. High-temporal-resolution data of the RDEs and RMEs collected by numerous instruments deployed for this intensive observation period are presented. The results of an Advanced Regional Prediction System (ARPS) simulation of the weather conditions around the time period reproduced one of the two RDE–RME pairs with reasonably accurate amplitude and timing. Both the observational data and ARPS numerical model output indicate that the second RDE–RME pair resulted from the interaction between a dry air mass descending from the Rocky Mountains and a cold pool–internal undular bore couplet propagating over the Homestead site from a mesoscale convective complex to the north. The RDEs and RMEs, which were rarely observed during IHOP_2002, are believed to be an indirect indicator of such bores.

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Robert J. Conzemius and Evgeni Fedorovich

Abstract

Results are presented from a combined numerical and observational study of the convective boundary layer (CBL) diurnal evolution on a day of the International H2O Project (IHOP_2002) experiment that was marked by the passage of a dryline across part of the Oklahoma and Texas Panhandles. The initial numerical setup was based on observational data obtained from IHOP_2002 measurement platforms and supplementary datasets from surrounding locations. The initial goals of the study were as follows: (i) numerical investigation of the structure and evolution of the relatively shallow and homogeneous CBL east of the dryline by means of large-eddy simulation (LES), (ii) evaluation of LES predictions of the sheared CBL growth against lidar observations of the CBL depth evolution, and (iii) comparison of the simulated turbulence structures with those observed by lidar and vertically pointing radar during the CBL evolution. In the process of meeting these goals, complications associated with comparisons between LES predictions and atmospheric observations of sheared CBLs were encountered, adding an additional purpose to this study, namely, to convey and analyze these issues.

For a period during mid- to late morning, the simulated CBL evolution was found to be in fair agreement with atmospheric lidar and radar observations, and the simulated entrainment dynamics were consistent with those from previous studies. However, CBL depths, determined from lidar data, increased at a faster rate than in the simulations during the afternoon, and the wind direction veered in the simulations more than in the observations. The CBL depth discrepancy can be explained by a dryline solenoidal circulation reported in other studies of the 22 May 2002 case. The discrepancy in winds can be explained by time variation of the large-scale pressure gradient, which was not included in LES.

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S. B. Trier, F. Chen, K. W. Manning, M. A. LeMone, and C. A. Davis

Abstract

A coupled land surface–atmospheric model that permits grid-resolved deep convection is used to examine linkages between land surface conditions, the planetary boundary layer (PBL), and precipitation during a 12-day warm-season period over the central United States. The period of study (9–21 June 2002) coincided with an extensive dry soil moisture anomaly over the western United States and adjacent high plains and wetter-than-normal soil conditions over parts of the Midwest. A range of possible atmospheric responses to soil wetness is diagnosed from a set of simulations that use land surface models (LSMs) of varying sophistication and initial land surface conditions of varying resolution and specificity to the period of study.

Results suggest that the choice of LSM [Noah or the less sophisticated simple slab soil model (SLAB)] significantly influences the diurnal cycle of near-surface potential temperature and water vapor mixing ratio. The initial soil wetness also has a major impact on these thermodynamic variables, particularly during and immediately following the most intense phase of daytime surface heating. The soil wetness influences the daytime PBL evolution through both local and upstream surface evaporation and sensible heat fluxes, and through differences in the mesoscale vertical circulation that develops in response to horizontal gradients of the latter. Resulting differences in late afternoon PBL moist static energy and stability near the PBL top are associated with differences in subsequent late afternoon and evening precipitation in locations where the initial soil wetness differs among simulations. In contrast to the initial soil wetness, soil moisture evolution has negligible effects on the mean regional-scale thermodynamic conditions and precipitation during the 12-day period.

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Steven E. Koch, Wayne Feltz, Frédéric Fabry, Mariusz Pagowski, Bart Geerts, Kristopher M. Bedka, David O. Miller, and James W. Wilson

Abstract

Families of solitary waves (“solitons”) associated with two atmospheric bores on the same day were observed by an unprecedented number of ground-based and airborne profiling systems during the International H2O Project (IHOP). In addition, a very high-resolution numerical weather prediction model initialized with real data was used with success to simulate one of the bores and the evolving soliton. The predicted wave amplitude, phase speed, wavelength, and structure compared well to these extraordinarily detailed observations. The observations suggest that during the active phase (when turbulent mixing was active, which was prior to bore collapse), the bores and waves vigorously mixed dry air from above a nocturnal boundary layer down to the surface. Refractivity computed from near-surface radar observations showed pronounced decreases due to sudden drying during the passage of the bores in this phase, but refractivity increases appeared during the period of bore collapse. During both phases, the bores wafted aerosol-laden moist air up to the middle troposphere and weakened the capping inversion, thus reducing inhibition to deep convection development. The model results indicate that the refractivity decreases near the surface were due to drying caused by downward turbulent mixing of air by the wave circulations. Turbulent kinetic energy was generated immediately behind the bore head, then advected rearward and downward by the solitary waves. During the dissipation stage, the lifting by the bore head produced adiabatic cooling aloft and distributed the very moist air near the surface upward through the bore depth, but without any drying due to the absence of vigorous mixing. Thus, this study shows that the moist thermodynamic effects caused by atmospheric bores and solitons strongly depend upon the life cycle of these phenomena.

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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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