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Eric Nelson
,
Roland Stull
, and
Edwin Eloranta

Abstract

The thickness of the entrainment zone at the top of the atmospheric mixed layer is analyzed using measurements made with a ground-based lidar during the BLX83 and CIRCE field programs. When the entrainment-zone depth normalized by mixed-layer depth is plotted as a function of the entrainment rate normalized by the convective velocity scale, with time as a parameter, a hysteresis curve results. Although portions of the curve can be approximated by diagnostic relationships, the complete hysteresis behavior is better described with a prognostic relationship. A simple thermodynamic model that maps the surface-layer frequency distribution of temperature into a corresponding entrainment zone distribution is shown to approximate the hysteresis evolution to first order.

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William P. Hooper
and
Edwin W. Eloranta

Abstract

During the Central Illinois Rainfall Chemistry Experiment (CIRCE), the University of Wisconsin lidar measured wind and turbulence profiles through the planetary boundary layer for a 32-h period in conjunction with surface observations, radiosonde soundings and kytoon profiles made by Argonne National Laboratory. The lidar profiles were made using an advection model for aerosol inhomogeneities as described by Sroga et al. We discuss improvements to this model and explore the accuracy of the lidar wind and boundary layer depth measurements. In addition, the temporal variation of lidar data was utilized to measure boundary layer depth objectively. Cross sections of the speed, direction and rms variation of the wind for the 32-h period show the daytime convective layer, nocturnal stable layer and transitional periods.

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Shane D. Mayor
and
Edwin W. Eloranta

Abstract

Spatially resolved wind fields are derived by cross correlation of aerosol backscatter data from horizontal and vertical scans of the University of Wisconsin volume imaging lidar during the 1997/98 Lake-Induced Convection Experiment. Data from three cases are analyzed. The first two cases occurred on 10 and 13 January 1998 during cold-air outbreaks. Horizontal scans at 5 m above the lake reveal cellular structure of the steam fog. Vector winds are derived with 250-m spatial resolution over 60 and 36 km2 areas. These wind fields show acceleration and veering of offshore flow in the convective internal boundary layer along the upwind edge of Lake Michigan. The wind fields are used to compute divergence and vorticity. Effects of shoreline shape and topography are evident in the data. Horizontal wind speeds derived from vertical scans show the effects of convection on the vertical distribution of momentum. In the third case, 21 December 1997, a well-defined, shallow density current flowing offshore at ≈1 m s−1 is observed in the presence of larger-scale (3–4 m s−1) onshore flow. Winds on both sides of the land-breeze boundary as well as the three-dimensional structure of the event were recorded and analyzed.

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Roland B. Stull
and
Edwin W. Eloranta

Interactions between fair-weather cumulus clouds and mixed-layer thermals were the focus of a one-month field experiment in Oklahoma. This experiment, called Boundary Layer Experiment—1983 (BLX83), combined remote sensors, surface observations, balloon platforms, and aircraft measurements to study the kinematics at the top of the daytime convective boundary layer. Emphasis was placed on the study of the entrainment zone, and on the relationship between individual thermals as identified by lidar and turbulent motions and fluxes as measured by aircraft and sodar.

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Donald Wylie
,
Paivi Piironen
,
Walter Wolf
, and
Edwin Eloranta

Abstract

Optical depth measurements of transmissive cirrus clouds were made using coincident lidar and satellite data to improve our interpretation of satellite cloud climatologies. The University of Wisconsin High Spectral Resolution Lidar was used to measure the optical depth of clouds at a wavelength of 532 nm, while the GOES and AVHRR window channel imagers provided measurements at a wavelength of 10.8 µm. In single-layer cirrus clouds with a visible optical depth greater than 0.3, the ratio of the visible to the IR optical depth was consistent with the approximate 2:1 ratio expected in clouds comprised of large ice crystals.

For clouds with visible optical depths <0.3, the visible/IR ratios were nearly always <2. It is likely that this reflects a measurement bias rather than a difference in cloud properties.

Most cirrus clouds observed in this study were more than 1 km thick and were often comprised of multiple layers. Supercooled liquid water layers coexisted with the cirrus in 32% of the cases examined. In many of these cases the presence of water was not evident from the satellite images. Thus, it must be concluded that “cirrus” climatologies contain significant contributions from coexisting scattered and/or optically thin water cloud elements.

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Shane D. Mayor
,
Gregory J. Tripoli
, and
Edwin W. Eloranta

Abstract

The authors apply data analysis techniques that demonstrate the power of using volume imaging lidar observations to evaluate several aspects of large-eddy simulations (LESs). They present observations and simulations of an intense and spatially evolving convective boundary layer on 13 January 1998 during the Lake-Induced Convection Experiment (Lake-ICE). To enable comparison of observed and simulated eddy structure, aerosol scattering was estimated from LES output of relative humidity, a passive tracer, and liquid water. Spatial and temporal correlation functions of aerosol structure from horizontal planes reveal the mean and turbulent convective structure. The correlation functions of the observed and simulated aerosol backscatter are presented as a function of altitude and offshore distance. Best-fit ellipses to the closed contours encircling the origin of the correlation functions are used to obtain the mean ellipticity and orientation of the structures. The authors demonstrate that these two parameters are not sensitive to minor changes in the functional relationship between humidity and optical scattering. The lidar-derived mean wind field is used as a reference for evaluating the LES mean flow.

The ellipses from lidar data indicate that structures near the surface tend to be aligned with the mean wind direction, while in the entrainment zone they are aligned perpendicular to the mean wind direction. In the middle of the mixed layer, convective plumes tended to be circular and, therefore, had no preferred orientation at small lags. At longer lags, however, the correlation functions from the middle of the mixed layer show that the observed convective plumes were organized into linear bands oriented perpendicular to the mean wind direction. The perpendicular bands suggest the important role of gravity waves in organizing convective structures. The study shows that the model generates reasonable coherent structures (open cells) where the LES technique is expected to perform poorly (near the surface) and fails to capture the wind-perpendicular organization of closed cells in the middle of the mixed layer where the LES technique is expected to be robust. The authors attribute this failure to the boundary conditions that limited the growth of waves above the mixed layer.

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Nicholas P. Wilde
,
Roland B. Stull
, and
Edwin W. Eloranta

Abstract

Variations of the lifting condensation level (LCL) of surface layer air are documented based on data from the BLX83 field experiment in Oklahoma. For example, within a 25 km long region near Chickasha, the local LCL height was found to vary by 15–30% of its average height. This zone of variation, centered on the mean LCL height, is identified as the “LCL zone”. It is analogous to the entrainment zone for the local mixed layer depth. Cumulus clouds first form when the top of the entrainment zone reaches the bottom of the LCL zone. As more of the entrainment zone overlaps and reaches above the LCL zone, the cloud cover increases. Two case studies are presented to demonstrate the diagnosis of cumulus onset time and cloud cover amount using this overlap technique. Combined radar, aircraft, rawinsonde, and surface observations indicate that some of the air observed at cloud base has the same low LCL as that of the mean surface layer air. This leads us to speculate that some surface layer air is rising up to cloud base with relatively little dilution, perhaps within the cores of thermals.

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Timothy D. Crum
,
Roland B. Stull
, and
Edwin W. Eloranta

Abstract

Coincident observations of the daytime convective boundary layer over Oklahoma were made with the NCAR Queen Air aircraft and the University of Wisconsin ground-based lidar. The two data sets have been merged to provide a unique visual representation of the temperature, moisture, vertical velocity, turbulent kinetic energy and the momentum fluxes in a field of thermals. These data show that horizontal moisture profiles observed in thermals penetrating the entrainment zone tend to exhibit more of a top-hat profile than the corresponding temperature or vertical velocity profiles. The specific humidities observed at various heights including cloud base 1) are frequently nearly constant along the horizontal tracks within each thermal; 2) show thermal-to-thermal variability; and 3) have values nearly the same as found in the surface layer. This paper also proposes the concept of an “intromission zone” describing the zone of lateral entrainment at the edges of active thermals. For the data studied here, a lateral entrainment velocity of 0.3 m s−1 was observed.

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Jeffery T. Sroga
,
Edwin W. Eloranta
, and
Ted Barber

Abstract

A lidar technique for measuring wind in the atmospheric boundary is presented. Inhomogeneities in ambient aerosol content are used as tracers of the wind. This technique yields both horizontal components of the wind and the wind velocity variance. These results are achieved using a model which assumes an isotropic Gaussian distribution of turbulent velocities. Experimental results comparing lidar wind measurements with winds derived from radar-tracked pilot balloons and tower-mounted anemometers show good agreement. Wind measurements have been obtained at slant range distances up to 6.5 km.

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Edwin W. Eloranta
,
Roland B. Stull
, and
Elizabeth E. Ebert

Abstract

A calibration device was designed to fit over the Lyman-α (LA) probes on the NCAR King Air aircraft to allow the introduction of pure nitrogen, oxygen, and carbon dioxide gases into the probe's radiation path. With these three gases, it was possible to calculate three of the most important terms in the LA humidity equation: path length, reference voltage (radiation) and oxygen absorption. This calibration device was tested in France during the HAPEX-MOBILHY field program, and was found to perform successfully.

As a result of the calibration, it was found that the effective LA path lengths during HAPEX were significantly different from the “nominal” path length physically set at the start of the experiment. Also, the oxygen absorption cross section was over twice as large as the published values, suggesting that the emission spectra of the lamps used in the LA probes are contaminated with other emission lines. The measured LA probe output reference voltages for no absorption were found to be slowly varying in time, suggesting that inflight “floating” calibrations against another reference hygrometer are necessary, in addition to the pre- and post-flight calibrations on the ground using the test device.

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