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Peter C. Sinclair

Abstract

The design and construction of a small, highly responsive pressure instrument with a sensitivity of approximately 0.1 mb is discussed. Two pressure sensors, or microbarophones, were used on a mobile tower to measure pressure variations near and within several dust devils. A pressure trace of a dust devil penetration is presented. The microtbarophone is considered to have sufficient merit that it may be of use in other applications involving micro-pressure measurements.

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Peter C. Sinclair

A number of authors have, in the past, been of the opinion that dust devil direction of rotation is controlled by the earth's rotation. While this contention can be easily attacked through theoretical arguments, actual observations become the deciding factor. The observations presented, believed to be the largest collection on record, show quite conclusively that dust devils in general have no preferred direction of rotation.

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Peter C. Sinclair

Abstract

Measurements from an extensive field program in the desert near Tucson, Ariz., are used to formulate a quantitative dust devil model. The model is based on measurements of temperature, pressure and wind velocity taken from a mobile instrumented tower near and within dust devils at levels of 6, 17 and 30 ft.

Three dust devil penetrations are analyzed with respect to the temperature, pressure and wind velocity profiles contained in a plane defined by the dust devil axis (or local vertical) and the direction of motion. These profiles corroborate earlier preliminary measurements by Sinclair and thus firmly establish the existence of the warm, low-pressure core of the dust devil. Maximum temperature and pressure variations within the dust devil varied from 4 to 8C and 2.5 to 4.5 mb.

The most significant result of the measurement program was the discovery of a downcurrent or a region of markedly reduced vertical motion along and surrounding the sloping dust devil axis. Immediately outside the downcurrent, the vertical velocity reaches positive peak values and then diminishes rapidly with respect to increasing radius. Superimposed on this motion is a strong rotary and radial motion which results in a combined flow pattern similar to that of a helical vortex.

The measurements are synthesized into a dust devil model. The observational model represents a two-cell structure which also appears to exist in other atmospheric vortices, such as the tornado and hurricane, and some laboratory and theoretical vortex models.

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PETER C. SINCLAIR

Abstract

The development of a mobile recording system for measuring temperature, pressure, and wind velocity in dust devils is discussed. Four dust devil penetrations are presented in which the above three quantities were measured simultaneously. All four cases show quantitatively for the first time the warm, low-pressure core of the dust devil. While the instrumentation in some cases lacked proper response and damping, the results were of sufficient accuracy to guide the development of a larger multi-level system.

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Peter C. Sinclair

Abstract

An extensive dust devil census taken near Tucson, Ariz., has provided, from two independent areas, quantitative information on the relation of environmental conditions to the spatial distribution, frequency and duration of a particular type of dry atmospheric thermal. The observations indicate that for these two areas, each approximately 100 mi2 in area, dust devils are initially sighted most frequently in the vicinity of dry river beds which lie in the lee of small hills. Once initiated, the visible lifetime of the observed dust devils ranged from a few seconds to approximately 20 rain. These data show that dust devil duration increases with dust devil size. The results indicate that the energy for the dust devil is the warm boundary layer air which is continually being fed into the visible vortex as it moves along with essentially the same speed and direction as the environmental wind.

The highest dust devil frequency occurs with the lowest atmospheric stability and not necessarily with the highest shelter air temperature. The diurnal variation of dust devil occurrence follows a Gaussian type distribution with the maximum dust devil activity between 1300-1400 LST, which is approximately at the time of maximum soil surface temperature and convective heat flux. The appearance of dust devils suggests that the smaller scale eddies may be unable to accomplish the required vertical heat transfer; hence, larger elements such as thermal plumes and dust devils are necessary to accomplish the transfer through a deep layer of the atmosphere.

The mean number of dust devil occurrences per day varied from 50-80 between the two areas investigated. While the dust devil frequency increased from June to July, the observations indicate that on days with cumulus cloud cover dust devil activity is suppressed or terminated.

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Peter C. Sinclair and Peter M. Kuhn

Abstract

There is now considerable evidence to substantiate the causal relationship between low altitude wind shear (LAWS) and the recent increase in low-altitude aircraft accidents. The National Research Council has found that for the period 1964 to 1982, LAWS was involved in nearly all the weather-related air carrier fatalities. However, at present, there is no acceptable method, technique, or hardware system that provides the necessary safety margins, for spatial and timely detection of LAWS from an aircraft during the critical phases of landing and takeoff. The Federal Aviation Administration (FAA) has addressed this matter and supports the development of an airborne system for detecting hazardous LAWS with at least a one minute warning of the potential hazard to the pilot. One of the purposes of this paper is to show from some of our preliminary flight measurement research that a forward looking infrared radiometer (FLIR) system can be used to successfully detect the cool downdraft of downbursts [microbursts/macrobursts (MB)] and thunderstorm gust front outflows that are responsible for most of the LAWS events. The FLIR system provides a much greater safety margin for the pilot than that provided by reactive designs such as inertial-air speed systems that require the actual penetration of the MB before a pilot warning can be initiated. Our preliminary results indicate that an advanced airborne FLIR system could provide the pilot with remote indication of MB threat, location, movement, and predicted MB hazards along the flight path ahead of the aircraft.

In a proof-of-concept experiment, we have flight tested a prototype FLIR system (nonscanning, fixed range) near and within Colorado MBs with excellent detectability. The results show that a minimum warning time of one-four minutes (5×10 km), depending on aircraft speed, is available to the pilot prior to a MB encounter. Analysis of the flight data with respect to a modified “hazard index” indicates the severe hazard that the apparently weak and innocuous MBs present to both commercial transport pilots as well as the much larger number of pilots who fly the smaller general aviation and executive aircraft.

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Paul T. Scott, William G. Finnegan, and Peter C. Sinclair

Abstract

A new ice nucleant aerosol was produced by combustion of a 2% AgI-0.5 mole % Bil3-NH4I-acetone-water solution. The ice nucleating effectiveness of this aerosol is an order of magnitude greater than AgI alone at −10°C. An X-ray powder analysis identified the aerosol as the hexagonal crystal form of AgI having the closest match to ice ever reported for a nucleant of this type.

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Verne H. Leverson, Peter C. Sinclair, and Joseph H. Golden

Abstract

During September 1974 in the Lower Florida Keys, the first successful penetrations of mature waterspouts were accomplished by a specially instrumented research aircraft. Throughout the course of each penetration, the measurement system recorded the temperature, the pressure and the three-dimensional velocity field near and within the visible funnel. Multiple penetrations of both cyclonic and anticyclonic waterspouts in various life-cycle stages were achieved. The results indicate that the waterspout funnel structure exhibits 1) a warm central core region, 2) positive vertical velocities of 5–10 m s−1 outside of the warm core, and 3) tangential velocities and horizontal pressure gradients with characteristics similar to but with magnitudes greater than those of the dust devil. A scale analysis of each term in the governing equations of motion suggests a simplified set of modeling equations. The simple Rankine-combined vortex model with cyclostrophic flow explains approximately 75% of the total measured pressure deficit. This compares favorably with Sinclair's (1966, 1973) earlier results for the dust devil vortex.

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Sarah C. Jones, Patrick A. Harr, Jim Abraham, Lance F. Bosart, Peter J. Bowyer, Jenni L. Evans, Deborah E. Hanley, Barry N. Hanstrum, Robert E. Hart, François Lalaurette, Mark R. Sinclair, Roger K. Smith, and Chris Thorncroft

Abstract

A significant number of tropical cyclones move into the midlatitudes and transform into extratropical cyclones. This process is generally referred to as extratropical transition (ET). During ET a cyclone frequently produces intense rainfall and strong winds and has increased forward motion, so that such systems pose a serious threat to land and maritime activities. Changes in the structure of a system as it evolves from a tropical to an extratropical cyclone during ET necessitate changes in forecast strategies. In this paper a brief climatology of ET is given and the challenges associated with forecasting extratropical transition are described in terms of the forecast variables (track, intensity, surface winds, precipitation) and their impacts (flooding, bush fires, ocean response). The problems associated with the numerical prediction of ET are discussed. A comprehensive review of the current understanding of the processes involved in ET is presented. Classifications of extratropical transition are described and potential vorticity thinking is presented as an aid to understanding ET. Further sections discuss the interaction between a tropical cyclone and the midlatitude environment, the role of latent heat release, convection and the underlying surface in ET, the structural changes due to frontogenesis, the mechanisms responsible for precipitation, and the energy budget during ET. Finally, a summary of the future directions for research into ET is given.

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