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  • Author or Editor: Alfred R. Rodi x
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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

Abstract

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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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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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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Alfred R. Rodi, James C. Fankhauser, and Robin L. Vaughan

Abstract

Aircraft distance-measuring-equipment (DME) data are used to update position, velocity, and wind measurements from inertial navigation systems (INS) measurements. Data from conventional single-channel DME sets, suitably calibrated, are shown to be adequate to resolve the Schuler oscillation and correct INS positions to better than 1-km accuracy. The satellite-based NAVSTAR global position system (GPS) is rapidly superseding other systems for external position reference. However, DME is reliable and very accurate and has been recorded on many research datasets. The principal limitation of the DME is that it is restricted to land-based navigation. The regression technique used does not necessitate multiple DME receivers or station switching and involves few restrictions on the collection of the data. However, the results improve when more than one station is used. Comparisons with other navigation systems (interferometer and loran) demonstrate the method's skill in resolving INS errors. Intercomparisons among several research aircraft flying in close formation support the method's usefulness in correcting biases in INS data.

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