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Todd M. Crawford
and
Howard B. Bluestein

Abstract

The characteristics of dryline passage are documented through an analysis of data from an instrumented surface mesonetwork in the Texas panhandle, and western and central Oklahoma during the Cooperative Oklahoma Profiler Studies field program. Some eastward-moving drylines at the surface during the day were characterized by monotonic drops in dewpoint after dryline passage; others were marked by a series of rapid drops punctuated by periods of no change after dryline passage, which suggests that the dryline often progresses in discrete steps, rather than continuously. The dryline during the daytime was not always collocated with a pressure trough, although the strongest dryline observed was. Analyses of surface pressure traces indicated that westward-moving drylines during the evening did not display behavior characteristic of strong, intense density currents, as had been found in other studies. Evidence is presented, in one case, of 90-min oscillations in water vapor and wind behind the dryline, which may have been associated with the downward transport of momentum associated with gravity waves aloft.

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Howard B. Bluestein
and
Todd M. Crawford

Abstract

A technique is introduced using surface data from triangular networks of adjacent mesonet stations to estimate the terms in the horizontal equation of motion at anemometer level (10 m) on either side of the dryline in the southern plains of the United States. Data from an instrumented surface mesonetwork in the Texas panhandle and western and central Oklahoma during the Cooperative Oklahoma Profiler Studies (COPS-91) field program were employed for this purpose. East of the dryline and surface pressure trough the vertical-mixing term had a component normal and to the right of the surface wind in accord with Ekman theory; west of the dryline and surface pressure trough the vertical-mixing term had a component normal and to the left of the surface wind in disagreement with Ekman theory. It is suggested that disagreements with Ekman theory may be due to baroclinic effects in the boundary layer. It is also shown that during the day both the westward component of the pressure gradient force and the easterly component of the surface wind increased east of the dryline, in accord with the “inland sea breeze” hypothesis, and that the maximum easterly wind component usually lagged the maximum westward component of the pressure gradient force by several hours.

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Todd M. Crawford
and
Claude E. Duchon

Abstract

An improved parameterization is presented for estimating effective atmospheric emissivity for use in calculating downwelling longwave radiation based on temperature, humidity, pressure, and solar radiation observations. The first improvement is the incorporation of an annual sinusoidal variation in effective clear-sky atmospheric emissivity, based on typical climatological variations in near-surface vapor pressure. The second is the continuous estimation of fractional cloudiness by taking the ratio of observed solar radiation to a modeled clear-sky solar radiation. Previous methods employed observer-estimated fractional cloudiness. Data from the Atmospheric Radiation Measurement (ARM) program were used to develop these improvements. The estimation of cloudiness was then used to modify the effective clear-sky atmospheric emissivity in order to calculate 30-min averages of downwelling longwave radiation. Monthly mean bias errors (mbe’s) of −9 to +4 W m−2 and root-mean-square errors (rmse’s) of 11–22 W m−2 were calculated based on ARM data over a 1-yr period. These mbe’s were smaller overall than any of the six previous methods tested, while the rmse’s were similar to the best previous methods. The improved parameterization was then tested on FIFE data from the summer of 1987. Although the monthly mbe’s were larger, the rmse’s were smaller.

It is also shown that data from upper-air soundings can be used to calculate the effective atmospheric emissivity rather than specifying the aforementioned sinusoidal variation. Using ARM upper-air soundings, this method resulted in larger mbe’s, −7 to +11 W m−2, especially during the summer months, and similar rmse’s. The success of the method suggests that it has application at any observing site within reasonable proximity of an upper-air sounding, while removing the empiricism used to specify the annual sinusoidal variation in emissivity.

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Todd M. Crawford
and
Howard B. Bluestein

Abstract

In recent years, there has been a growing appreciation of the importance of land–atmosphere interactions in determining the state of the boundary layer. To examine this phenomenon in more detail, a new technique has been developed to evaluate the surface energy budget during the daytime from standard meteorological observations. Using only Oklahoma Mesonetwork (Mesonet) data at 5- and 30-min intervals as input, the technique calculates net radiation (R n ), ground heat flux (G), and latent heat flux (LE). The sensible heat flux (H) is calculated as a residual. The R n term is calculated using observed values of downwelling shortwave radiation, an improved method of estimating downwelling longwave radiation, and simple parameterizations of upwelling shortwave and longwave radiation. The modeled values of R n are unbiased and are consistently within 25 W m−2 of observed values. Ground heat flux, which is the combination of a 5-cm soil flux term and a storage term, was difficult to verify without prior knowledge of vegetation height. Latent heat flux is calculated from the Penman–Monteith equation, in which surface resistance is estimated. Using data from the Atmospheric Radiation Measurement Program, simple parameterizations were developed (one each for eastern and western Oklahoma) for this term, based on observations of temperature, relative humidity, solar radiation, soil moisture, and estimates of leaf age.

Net radiation and G are calculated, and then their sum is partitioned into H and LE. Because there were no observations of LE at the Mesonet sites, the preexisting reliable estimates of H were used to verify the new estimates of both H and LE. Because there were problems with the soil moisture data from some of the sites, data from only two Mesonet sites were available for verification. The estimates of H were unbiased and within 60 W m−2 (rmse) at the sites in both eastern and western Oklahoma. Because of the limited verification data currently available, the model results are preliminary and in need of further testing.

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Jeffrey B. Basara
and
Todd M. Crawford

Abstract

The Oklahoma Mesonet, an automated network of 115 meteorological observing stations, includes soil moisture monitoring devices at 60 locations. The Campbell Scientific model 229-L matric potential (water potential) sensor was chosen for operational use based on its capability to perform as a fully automated soil water measuring device. Extensive laboratory calibrations were performed on each sensor to ensure the quality of the matric potential measurements.

Examination of the data from the Norman site during July 1997 revealed significant inconsistencies between near-surface (5 and 25 cm) measurements of soil moisture and deep-layer (60 and 75 cm) measurements of soil moisture. In particular, a heavy precipitation event was followed by only a small increase in near-surface soil water potential values, while a much larger increase occurred in the deep-layer values. It is theorized that an installation flaw is the cause for these inconsistencies. A solution is proposed in the hope that future efforts to measure soil moisture will not be hindered by similar problems.

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Carl E. Hane
,
Howard B. Bluestein
,
Todd M. Crawford
,
Michael E. Baldwin
, and
Robert M. Rabin

Abstract

Long-lived thunderstorms were initiated during the afternoon of 26 May 1991 ahead of a dryline in northwestern Oklahoma. Various reasons for initiation in this particular along-dryline location are investigated through analysis of observations collected during the Cooperative Oklahoma Profiler Studies—1991 field program. Observing systems included in situ and radar instrumentation aboard a research aircraft, soundings from mobile laboratories, a mesonetwork of surface stations, meteorological satellites, and operational networks of surface and upper-air stations.

Elevated moistening east of the dryline revealed by soundings and aircraft observations in combination with thermal plume activity was apparently insufficient to promote sustained convection on this day without aid from an additional lifting mechanism. Satellite observations reveal scattered convection along the dryline by midafternoon and a convective cloud line intersecting the dryline at an angle in the area of most pronounced storm initiation, extending southwestward into the dry air. Another prominent feature on this day was a mesoscale bulge along the dryline extending northeastward into southwest Kansas. Deep convection was initiated along this bulge, but was in general short-lived.

Potential causes of the lifting associated with the cloud line that was apparently key to the preferred location for storm development in northwest Oklahoma were investigated: (a) a mesoscale circulation resulting from horizontal differences in radiative (temperature) properties of the underlying surface and (b) upward motion induced by an upper-level mesoscale disturbance. Analysis of vegetative and surface temperature distributions from satellite observations suggests a potential (more research is needed) link between surface characteristics and the development of the dryline bulge and observed cloud line through horizontal differences in vertical momentum transport. A run of the currently operational eta model indicates some skill in predicting dryline location and motion and predicts upward motion in the northern part of the region that was generally more convectively active, but shows no indication of upper-level support in the vicinity of the observed cloud line.

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Carl E. Hane
,
Robert M. Rabin
,
Todd M. Crawford
,
Howard B. Bluestein
, and
Michael E. Baldwin

Abstract

A dryline that occurred on 16 May 1991 within a synoptically active environment is examined in detail using research aircraft, radar, surface, satellite, and upper air observations. The work focuses on multiple boundaries in the dryline environment and initiation of tornadic storms in two along-line areas.

Aircraft measurements in the boundary layer reveal that both the east–west extent of moisture gradients and the number of regions containing large moisture gradients vary in the along-dryline direction. Aircraft penetrations of thinlines observed in clear air return from radar reveal that all thinlines are associated with convergence and a moisture gradient, and that more distinct thinlines are associated with stronger convergence. However, significant moisture gradients are not always associated with either thinlines or convergent signatures.

Convective clouds on this day formed at the dryline rather than significantly east of the dryline. The three thunderstorm cells that occurred in east-central Oklahoma developed along a 20-km section of the dryline south of a dryline bulge and within a 30-min period. The storms appear to have developed in this location owing to enhanced convergence resulting from backed winds in the moist air in response to lowered pressure in the warm air to the northwest. Aircraft measurements in the boundary layer and satellite-sensed surface temperature both indicate localized warming in this area to the northwest.

Farther north there was a 70–100-km segment along the dryline where few convective clouds formed during the afternoon. This coincided with a swath of cooler boundary layer air that resulted from reduced surface heating over an area that received significant thunderstorm rainfall during the previous night.

A severe thunderstorm complex that developed along the Kansas–Oklahoma border was initiated at the intersection of the dryline and a cloud line that extended into the dry air. An aircraft penetration of the cloud line about 12 km from its intersection with the dryline shows convergence and deepened low-level moisture at the cloud line. The cloud field that evolved into the cloud line over a period of several hours developed over the area that had received the heaviest rainfall during the previous night.

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Carl E. Hane
,
Michael E. Baldwin
,
Howard B. Bluestein
,
Todd M. Crawford
, and
Robert M. Rabin

Abstract

Through a case study approach the motion of a dryline (on 16 May 1991) within a synoptically active environment in the southern plains, along which severe storms ultimately developed, is examined in detail. Observations from research aircraft, surface mesonetwork stations, mobile ballooning vehicles, radar, wind profilers, and operational surface and upper air networks are examined and combined. Additionally, output from the operational mesoscale Eta Model is examined to compare predictions of dryline motion with observations and to aid in interpretation of observations.

The dryline on this day advanced rapidly eastward and included formation of a bulge; additionally, in at least two instances it exhibited redevelopment (loss of definition at one location and gain at another). Aircraft observations revealed that an eastward redevelopment occurred in the early afternoon and was characterized by a series of four “steps” along the western edge of the boundary layer moisture. The westernmost and easternmost steps coincide with the locations of the dryline before and after redevelopment, respectively. The retreat of the dryline in the central and southern portion of the analysis domain in the late afternoon included both continuous motion and redevelopment toward the west-northwest. This dual-mode retreat of the dryline was accompanied by gradual backing of the winds and moistening in low levels.

The Eta Model forecast initialized at 1200 UTC produced dryline features that were qualitatively similar to observed fields. The eastward motion of a broad area of enhanced moisture gradient agreed well with observations following an initial spinup period. A north–south moisture convergence axis preceded the rapid eastward motion of the dryline by several hours. Lack of subsidence in the air behind the modeled dryline leads to the conclusion that processes other than downward transfer of horizontal momentum by larger-scale motions (that would support eastward advection) produced the rapid dryline motion and observed eastward dryline bulge. Results of diagnosing physical processes affecting model dryline motion point toward boundary layer vertical mixing coupled with advection of dry air aloft as vital components in rapid advance of the dryline eastward in this synoptically active case.

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Todd M. Crawford
,
David J. Stensrud
,
Toby N. Carlson
, and
William J. Capehart

Abstract

The Soil Hydrology Model (SHM) was modified, and daily simulations of soil volumetric water content were made at 38 Oklahoma Mesonet sites for July 1997. These model results were compared with soil moisture observations made at the mesonet sites at depths of 5, 25, 60, and 75 cm. This work is believed to be the first time that a hydrological model has been evaluated with in situ soil moisture measurements over such an extensive area spanning several climate zones.

Comparisons of time series between the observed and modeled domain-averaged volumetric water content at 5 cm revealed similar phase and amplitude changes, a coefficient of determination (R 2) of 0.64, and small mean bias and root-mean-square errors (MBE and rmse) of 0.03 and 0.09, respectively. At 25, 60, and 75 cm, the model performance was slightly worse, with R 2 values between 0.27 and 0.40, MBE between −0.01 and 0.02, and rmse between 0.11 and 0.13. The model response to changes in soil water at these levels was sluggish, possibly because of, among other things, a lack of ability to model preferential downward water flow through cracks in the soil.

The results of this study suggest that SHM can be used effectively to initialize 5-cm soil moisture values in numerical prediction models. At deeper soil levels, however, the relatively small R 2 values and negligible MBE suggest that the model may be better suited for initializing a regionally averaged soil moisture value rather than unique gridbox values. These results illustrate the difficulty in using point measurements to validate a hydrological model, especially deeper in the soil where moisture values are more dependent on soil properties (which can vary sharply over small distances) and are less dependent on recent rainfall.

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Todd M. Crawford
,
David J. Stensrud
,
Franz Mora
,
James W. Merchant
, and
Peter J. Wetzel

Abstract

The Parameterization for Land–Atmosphere–Cloud Exchange (PLACE) module is used within the Fifth-Generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) to determine the importance of individual land surface parameters in simulating surface temperatures. Sensitivity tests indicate that soil moisture and the coverage and thickness of green vegetation [as manifested by the values of fractional green vegetation coverage (fVEG) and leaf area index (LAI)] have a large effect on the magnitudes of surface sensible heat fluxes. The combined influence of LAI and fVEG is larger than the influence of soil moisture on the partitioning of the surface energy budget. Values for fVEG, albedo, and LAI, derived from 1-km-resolution Advanced Very High Resolution Radiometer data, are inserted into PLACE, and changes in model-simulated 1.5-m air temperatures in Oklahoma during July of 1997 are documented. Use of the land cover data provides a clear improvement in afternoon temperature forecasts when compared with model runs with monthly climatological values for each land cover type. However, temperature forecasts from MM5 without PLACE are significantly more accurate than those with PLACE, even when the land cover data are incorporated into the model. When only the temperature observations above 37°C are analyzed, however, the simulations from the high-resolution land cover dataset with PLACE significantly outperform MM5 without PLACE. Previous land surface models have simply used (at best) climatological values of these crucial land cover parameters. The ability to improve model simulations of surface energy fluxes and the resultant temperatures in a diagnostic sense provides promise for future attempts at ingesting satellite-derived land cover data into numerical models. These model improvements would likely be most helpful in predictions of extreme temperature events (during drought or extremely wet conditions) for which current numerical weather prediction models often perform poorly. The potential value of real-time land cover information for model initialization is substantial.

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