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J. E. Tillman

Abstract

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J. E. Tillman

Abstract

Turbulence data from two sites have been analyzed to obtain an indirect relation between the Obukhov stability parameter, z/L, and temperature statistics during dry unstable conditions. The skewness of a single scalar variable, temperature, at a single height is used to obtain z/L, thereby demonstrating the value and importance of the no-Gaussian properties of the temperature statistics.

A relation between σθ/T* and z/L has been obtained for the stability range −60< z/L≤0.5 Explicit forms for heat flux and shear stress have been obtained for the unstable portion of this range in terms of z/L,k, σθ, z, θ and constants. Combining the above techniques provides a method of obtaining z/L, H/pc p and u* from temperature statistics during dry unstable conditions. Within the limitations of the observational uncertainties, the functions appear to be consistent for one or more different sites.

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W. J. Shaw
and
J. E. Tillman

Abstract

Fast-responding thermocouple psychrometers are often used in atmospheric boundary-layer turbulence measurements for the computation of heat and moisture fluxes. Small size, low cost, ease of interchange-ability and the use of the familiar psychrometric equations make this an ideal sensor for many applications at temperatures above freezing. However, a feature of these instruments that is frequently disregarded is that, due to wicking, the wet-bulb sensor has a frequency response that is an order of magnitude slower than the dry-bulb sensor. This difference in response time between the wet and dry sensors causes errors in the variances of humidity in one set of data as large as a factor of 5 and major errors in the shape of the humidity spectrum at high frequencies. We present a known but infrequently applied solution to this problem of sensor response differences in the hope that its simplicity, together with the reminder that a problem exists, will serve to encourage its use in the computation of humidity from fast-response psychrometric sensors.

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J. M. Wilczak
and
J. E. Tillman

Abstract

During April 1978, a field experiment was undertaken at the Boulder Atmospheric Observatory (BAO), near Boulder, Colorado, to investigate convective plumes in the atmospheric surface layer.

The plume translational velocities are determined for a wide range of stabilities, using an array of 16 temperature sensors, spanning a 100 m × 40 m area at a height of 4 m, and a three-dimensional sonic anemometer. The translational velocities are calculated from the phase information of the temperature cross spectrum, and by measuring the transit times of the plumes between sensors aligned in a direction parallel to the wind. Velocities obtained by the two methods are shown to be in rough agreement. Individual plume velocities are found to vary in proportion to the plume height.

The three-dimensional plume structure is investigated using both the horizontal array and the 300 m BAO tower. Under slightly unstable, high wind speed conditions, the majority of the plumes are distinctly elongated in the mean wind direction, with typical longitudinal and lateral dimensions of several hundreds and several tens of meters, respectively. As the instability increases the plume dimensions are found to decrease in the downwind direction, and to increase in the crosswind direction.

For very unstable, low wind speed conditions, the plumes are most often observed to be elongated in the lateral direction, although occasionally the isotherm patterns display a meandering behavior, with the trailing edge of one plume becoming the leading edge of the next.

The plume tilt is found to become more nearly vertical as −z/L increases, due to the surface shear layer becoming small in comparison with the height of the measurements. In addition, the plumes are found to have greatly varying vertical extents, and often several small plumes will merge together to form a larger plume at a higher level.

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M. Segal
,
R. W. Arritt
, and
J. E. Tillman

Abstract

The Martian daytime soil surface temperature is governed primarily by the net irradiance balance and surface soil heat flux. Thus the outbreak of a cold air mass generates increased sensible heat flux that is conducive to daytime dissipation of the cold air mass thermal characteristics. Conceptual and scaling evaluations of this dissipation are provided while comparison is made with similar situations on Earth. It is estimated that sensible heat flux contribution to the dissipation of the original thermal structure of the cold air could be three times larger than the corresponding situation on Earth. Illustrative numerical model simulations provide scaling of the potential impact on the dissipation of cold air masses for various combinations of background wind speed and latitudes.

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T. E. Chamberlain
,
H. L. Cole
,
R. G. Dutton
,
G. C. Greene
, and
J. E. Tillman

The Viking Meteorology Experiment is one of nine experiments to be carried out on the surface of Mars by each of two Viking Landers positioned at different latitudes and longitudes in the Northern Hemisphere. The meteorology experiment will measure pressure, temperature, wind speed, and wind direction at 1½ h intervals throughout the Martian day. The duration of each measurement period, the interval between data samples for a measurement period, and the time at which the measurement period is started will be varied throughout the mission. The scientific investigation and the sensors and electronics used for making the atmospheric measurement are discussed.

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