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R. Paul Lawson and Alfred R. Rodi

Abstract

A new airborne thermometer has been designed using results from numerical simulators of airflow and particle (drop) trajectories. Initial flight tests with the NCAR King Air show that the new thermometer, which uses a fine-wire thermocouple for the sensor and lacks a probe housing, has a response time that is significantly faster than thermometers currently in use. An example of heat-flux calculations in a convective boundary layer shows that, compared to measurements using the Rosemount thermometer and NCAR K probes, the turbulent heat flux is greater by about 20% when using measurements from the new thermometer. Theoretical calculations of time response support the claim that the improved response is due to the absence of a probe housing.

The new thermometer was designed to inertially separate cloud drops from the airflow, and flights in warm clouds suggest that the thermocouple sensor stays dry except in clouds that contain high concentrations of drizzle-size drops. In small cumulus clouds with approximately 1 g m−3 of liquid water that contained low concentrations (∼10 l−1) of drizzle drops, the new thermocouple probe consistently measured warmer temperatures than the reverse-flow and Rosemount thermometers, suggesting that in these clouds the thermocouple probe may not have been affected by errors from sensor wetting. Thus, static temperature measured by the new thermometer in clouds with continental drop spectra should be reliable. An example of data collected in a mixed region of a small cumulus cloud shows that there may be more temperature structure at scales of 2–50 m than previously observed.

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Alfred R. Rodi and Thomas R. Parish

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Determination of the horizontal pressure gradient force over distance scales less than 100 km is possible using airborne altimetry and detailed maps of the underlying terrain. To detect the very small isobaric slopes, instrumentation must perform up to specification and aircraft position must be known within about 250 m in order to achieve an adequate matching of altimeter height and terrain height. Numerous test flights were conducted to study the stability of the technique. Results indicate the system is capable of resolving pressure gradients with equivalent geostrophic wind errors of approximately ± 1 m s−1 over a 100 km horizontal scale.

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Arlen W. Huggins and Alfred R. Rodi

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William P. Mahoney III and Alfred R. Rodi

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During the Joint Airport Weather Studies (JAWS) project in 1982, the University of Wyoming's King Air research aircraft made observations of raindrop size distributions, vertical and horizontal air motions, and the temperature and moisture variables in and near precipitation shafts. This research examines the kinematic, thermodynamic, and microphysical characteristics of microburst-producing showers. Four precipitation showers with radar reflectivities of <35 dBZ were selected for study, three of which produced microbursts.

An equivalent potential temperature (θ e) analysis, as well as vertical velocity measurements at cloud base, showed no strong evidence that the downdrafts were originating well above cloud base.

A simple evaporation and downdraft model was used to examine the role of hydrometeor evaporation below cloud base as a microburst forcing mechanism. The one-dimensional model without entrainment provided the conceptual basis for microburst development by means of microphysical forcing alone. Cooling rates, caused by the evaporation of precipitation below cloud base, were calculated from the observed hydrometeor spectra and humidity profiles. The vertical profiles of the cooling rates were used to estimate downdraft magnitudes. The calculated downdraft speeds were in reasonable agreement with the observed speeds suggesting that, at least in these weak systems, subcloud evaporation was the predominant microburst forcing mechanism.

The conditions favorable to microburst development were found to be consistent with previous studies. They included: 1) a deep, dry adiabatic layer below cloud base, 2) a high concentration of hydrometeors at or below cloud base, and 3) low humidity values in the descending parcel.

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Wendy Schreiber-Abshire and Alfred R. Rodi

Abstract

In July 1987 during the CINDE project, three similar mesoscale planetary boundary layer convergence zones were observed to form in northeastern Colorado near Denver under synoptic-scale southwesterly flow. A number of recent studies have documented the importance of such convergence zones on the local weather in the Denver area. The three case studies presented in this paper are the boundary type previously classified by Wilson and Schreiber (1986) to be of unknown origin.

The analysis of mesonet, radar, and sounding data indicates that during periods of southwesterly flow at mountaintop levels over Colorado, the ridgetop winds may intrude into the Denver basin once the nocturnal temperature inversion has been eroded, provided that no other dominant synoptic-scale surface feature is affecting northeastern Colorado. When such an intrusion occurs, the southwest flow progresses northeastward until it reaches the frequently observed cold pool of air over the Platte River valley, which forms as the result of the nighttime drainage flow from the surrounding elevated terrain. It is at the leading edge of this cold pool that a surface-based convergence zone forms and remains until the cold pool is dissipated by insolation and mixing.

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Arlen W. Huggins and Alfred R. Rodi

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The effect of seeding convective clouds with dry ice was studied using simultaneous aircraft and radar observations. Clouds that were initially ice-free with supercooled liquid water contents of 0.5 g m−3 when the tops reached the −10°C level had similar responses to seeding, although significant natural variability existed. Aircraft particle probes detected sharp increases of small crystals (<100 μm) in 3–6 min followed by > 1 mm aggregates about 10 min after seeding. Observations supported the expectation that riming growth should not be important at these liquid water contents. Initial radar echoes formed in 7 win with distinctive time-height profiles of reflectivity.

Most radar echoes forming downwind of the seeding line were small and relatively weak compared with the natural echoes forming further downwind over the mountains. The impact of the seeding was shown to be observable but relatively small. It was found that unseeded clouds formed radar echoes later, and produced reflectivity time-height profiles that were significantly different from the seeded ones. The difference are considered in part to be due to variability in the initial cloud properties as well as the obvious and well-documented effects of injection of the seeding material early in the cloud lifetime. While the meteorological impact was small, documentation of the evolution of the seeding effect from cloud to ground is a prerequisite to further experimentation.

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H. Gerber, Glendon Frick, and Alfred R. Rodi

Abstract

Recently published ground-based measurements of liquid water content (LWC) measured in fogs by two microphysical instruments, the FSSP-100 and PVM-100, are evaluated. These publications had suggested that the PVM-100 underestimated LWC significantly in comparison to the FSSP-100 when the fog droplets were large. The present evaluation suggests just the opposite: The FSSP-100 overestimates LWC for large droplets because these droplets are unable to follow the curved streamlines of the flow generated by drawing air into the FSSP-100’s sensitive volume at 25 m s−1. This inertial effect causes droplets to accumulate near the active volume of the instrument’s laser beam and to produce large and spurious droplet concentration and LWC values for the largest droplets. Model calculations estimate the magnitude of this error for the FSSP-100.

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Jeffrey C. Weil, R. Paul Lawson, and Alfred R. Rodi

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Relative dispersion of ice crystals was measured in 30 seeded cumulus clouds. A quasi-instantaneous, vertical area source of ice was generated by releasing dry-ice pellets from an airplane. The ice concentration distribution and relative dispersion were measured normal to the source and were complemented by cloud turbulence measurements, namely, velocity variances and the energy dissipation rate ε. The clouds were selected based on an objective set of criteria and were treated as members of the same ensemble.

The observed mean relative dispersion σrx agreed well with predictions from a Lagrangian stochastic two-particle model, which reproduces Batchelor's theoretical results for σrx. For short times t after the seeding time ts, the predictions and observations suggested a growth like σrxtts rather than Batchelor's “intermediate” time prediction, σrx ∝ ε1/2 (tts)3/2. This difference was attributed to the rather large initial dispersion σ0 of ice crystals, 27–53 m, inferred from the measurements; Batchelor's result is only valid for σ0 ≪ σva 3/ε, where σva 2 is the average velocity variance. At long times, the predictions and observations approached the same asymptotic limit, σrx ∝ (tts)1/2.

In addition to the mean dispersion, probability density functions (pdfs) of the individual dispersion observations were constructed and showed an evolution from a highly skewed pdf at small times to a more symmetrical one at large times. This is one of the first reports of the σrx pdf, which is important for determining the variance and pdf of the randomly varying concentration in a small ice cloud or plume of material.

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Thomas R. Parish, Alfred R. Rodi, and Richard D. Clark

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A case study of the kinematical and dynamical evolution of the summertime Great Plains low level jet (LLJ) is presented. Airborne radar altimetry was used to discern the x and y components of the geostrophic wind at three levels in the lower atmosphere throughout the LLJ episode. Results appear to confirm previous theoretical and numerical studies regarding the importance of the diurnal cycle of heating over sloping terrain in producing an oscillating horizontal pressure gradient force. Inertial turning of the LLJ as a result of frictional decoupling was also documented. It is concluded that the inertial oscillation resulting from the sudden decrease in friction in the lower atmosphere during the early evening is the dominant mechanism in forcing this example of a summertime Great Plains LLJ.

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Thomas R. Parish, Matthew D. Burkhart, and Alfred R. Rodi

Abstract

The horizontal pressure gradient force is the single most important dynamical term in the equation of motion that governs the forcing of the atmosphere. It is well known that the slope of an isobaric surface is a measure of the horizontal pressure gradient force. Measurement of this force over mesoscale distances using an airborne platform has been attempted for over two decades in order to understand the dynamics of various wind systems. The most common technique has been to use a radar altimeter to measure the absolute height of an isobaric surface above sea level. Typical values of the horizontal pressure gradient force in the atmosphere are quite small, amounting to an isobaric surface slope of 0.0001 for a 10 m s−1 geostrophic wind at middle latitudes. Detecting the horizontal pressure gradient over irregular terrain using an instrumented aircraft has proven to be especially difficult since correction for the underlying terrain features must be made. Use of the global positioning system (GPS) is proposed here as a means to infer the horizontal pressure gradient force without the need for altimetry and terrain registration over irregular surface topography. Differential kinematic processing of data from dual-frequency, carrier phase tracking receivers on research aircraft with similar static base station receivers enables the heights of an isobaric surface to be determined with an accuracy estimated to be a few decimeters. Comparison of results obtained by conventional altimetry-based methods over the ocean and Lake Michigan with GPS reveals the potential of the GPS method at determining the horizontal pressure gradient force, even over complex terrain.

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