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C. J. Grund and E. W. Eloranta

Abstract

During the FIRE cirrus IFO, the High Spectral Resolution Lidar (HSRL) was operated from a roof top site on the University of Wisconsin–Madison campus. Because the HSRL technique separately measures the molecular and cloud particle backscatter components of the lidar return, the optical thickness is determined independent of particle backscatter. This is accomplished by comparing the known molecular density distribution to the observed decrease in molecular backscatter signal with altitude. The particle to molecular backscatter ratio yields calibrated measurements of backscatter cross section that can be plotted to reveal cloud morphology without distortion due to attenuation. Changes in cloud particle size shape and phase affect the backscatter to extinction ratio (backscatter-phase function). The HSRL independently measures cloud particle backscatter phase function. This paper presents a quantitative analysis of the HSRL cirrus cloud data acquired over an ∼33 hour period of continuous near-zenith observations. Correlations between small-scale wind structure and cirrus cloud morphology have been observed. These correlations can bias the mite averaging inherent in wind profiling lidars of modest vertical resolution, leading to increased measurement errors at cirrus altitudes. Extended periods of low intensity backscatter were noted between more strongly organized cirrus cloud activity. Optical thicknesses ranging from 0.01–1.4, backscatter-phase functions between 0.02–0.065 sr−1, and backscatter cross sections spanning 4 orders of magnitude were observed. The altitude relationship between cloud top and bottom boundaries and the cloud optical center altitude was dependent on the type of formation observed. Cirrus features were observed with characteristic wind drift estimated horizontal sizes of 5 km–400 km. The clouds frequently exhibited cellular structure with vertical to horizontal dimension ratios of 1:5–1:1.

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K. E. Kunkel, E. W. Eloranta, and J. A. Weinman

Abstract

Procedures are described for the analysis of lidar data to remotely measure 1) spectra of aerosol density fluctuations, 2) radial and transverse components of the mean wind and turbulent fluctuations of the transverse component of the wind velocity in the convective boundary layer, and 3) the kinetic energy dissipation rate. Results were compared with independent data obtained with a bivane anemometer installed at the 70 m level on a tower within the scanning sector of the lidar. Good agreement was obtained whenever the lidar data had adequate signal-to-noise characteristics (i.e., S/ greater than unity).

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K. E. Kunkel, E. W. Eloranta, and S. T. Shipley

Abstract

A scanning lidar system has been used to observe convection in the atmospheric boundary layer. In particular, cell sizes and geometry have been determined and circulation patterns in and around the cells have been measured.

The lidar data show that the preferred form of convective cells are plumes with roots near the surface. The majority of these plumes have aspects ratios between 0.5 and 1.5. The measurements of circulation patterns show the strongest rising motion on the upwind side of the cell with sinking motion on the downwind side. These observations show that lidar is a powerful tool for observing convection.

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R. Boers, E. W. Eloranta, and R. L. Coulter

Abstract

Ground based lidar measurements of the atmospheric mixed layer depth, the entrainment zone depth and the wind speed and wind direction were used to test various parameterized entrainment models of mixed layer growth rate. Six case studies under clear air convective conditions over flat terrain in central Illinois are presented. It is shown that surface heating alone accounts for a major portion of the rise of the mixed layer on all days. A new set of entrainment model constants was determined which optimized height predictions for the dataset. Under convective conditions, the shape of the mixed layer height prediction curves closely resembled the observed shapes. Under conditions when significant wind shear was present, the shape of the height prediction curve departed from the data suggesting deficiencies in the parameterization of shear production. Development of small cumulus clouds on top of the layer is shown to affect mixed layer depths in the afternoon growth phase.

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S. T. Shipley, E. W. Eloranta, and J. A. Weinman

Abstract

Monostatic lidar is explored as a means for determining the rainfall rate over an extended atmospheric path with a spatial resolution comparable to that of rain gages. An empirical relationship is established between the optical extinction coefficient of rain β r (km−1) and the rainfall rate R (mm hr−1). Correlation of lidar-derived rainfall extinction and gage rainfall rates at Madison gives
βrR0.74
.

The β r-R relations obtained from the work of other authors compare well with this relationship.

A lidar equation which accounts for the multiple scattering of light in rain is presented. A numerical procedure which derives estimates of β r as a function of range from lidar returns is developed. Examples of lidar-derived rainfall rate range profiles in spatially inhomogeneous thunderstorms are given.

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E. W. Eloranta, J. M. King, and J. A. Weinman

Abstract

Vertical profiles of the horizontal radial wind component in the lowest kilometer of the atmosphere have been measured remotely with lidar. Wind speed determinations were made by observing the motion of naturally occurring aerosol density inhomogeneities. Lidar wind measurements compare favorably with simultaneous pilot balloon observations of the wind.

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Y. J. Kaufman, T. W. Brakke, and E. Eloranta

Abstract

Theoretical two-and three-dimensional solutions to the radiative transfer equation have been applied to the earth-atmosphere system. A field experiment was conducted to test this theory. in the experiment the upward radiance was measured above and below a haze layer during simultaneous measurements of the haze characteristics. The measurements were conducted at a narrow near-IR channel (773±22 nm) which represents the visible and near-IR spectral region. The aerosol vertical optical thickness at eight wavelengths, as well as the vertical and horizontal profiles of the scattering coefficient, the temperature and dew point were measured at several locations. These measurements quantified the vertical and spatial structure of the atmospheric haze and the atmospheric radiation. The result was a well-defined radiative transfer experiment. The experimental dataset is used to quantify the haze effect on upward radiance, including the adjacency effect (the effect of a bright area on the upward radiance measured above a dark adjacent area), and to test radiative transfer models for a plane parallel atmosphere above a nonuniform surface. A comparison is given between the theoretical prediction of upward radiance above the haze and the measurement. Agreement between theory and the experiment is discussed.

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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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S. A. Ackerman, R. E. Holz, R. Frey, E. W. Eloranta, B. C. Maddux, and M. McGill

Abstract

An assessment of the performance of the Moderate Resolution Imaging Spectroradiometer (MODIS) cloud mask algorithm for Terra and Aqua satellites is presented. The MODIS cloud mask algorithm output is compared with lidar observations from ground [Arctic High-Spectral Resolution Lidar (AHSRL)], aircraft [Cloud Physics Lidar (CPL)], and satellite-borne [Geoscience Laser Altimeter System (GLAS)] platforms. The comparison with 3 yr of coincident observations of MODIS and combined radar and lidar cloud product from the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program Southern Great Plains (SGP) site in Lamont, Oklahoma, indicates that the MODIS algorithm agrees with the lidar about 85% of the time. A comparison with the CPL and AHSRL indicates that the optical depth limitation of the MODIS cloud mask is approximately 0.4. While MODIS algorithm flags scenes with a cloud optical depth of 0.4 as cloudy, approximately 90% of the mislabeled scenes have optical depths less than 0.4. A comparison with the GLAS cloud dataset indicates that cloud detection in polar regions at night remains challenging with the passive infrared imager approach.

In anticipation of comparisons with other satellite instruments, the sensitivity of the cloud mask algorithm to instrument characteristics (e.g., instantaneous field of view and viewing geometry) and thresholds is demonstrated. As expected, cloud amount generally increases with scan angle and instantaneous field of view (IFOV). Nadir sampling represents zonal monthly mean cloud amounts but can have large differences for regional studies when compared to full-swath-width analysis.

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R. A. Ferrare, J. L. Schols, E. W. Eloranta, and R. Coulter

Abstract

Lidar observations of clear-air convection during the 1983 Boundary Layer Experiment (BLX83) reveal the presence of elongated, parallel regions of updrafts marked by enhanced aerosol backscattering. These linear (banded) aerosol structures were observed over a two-hour period during a cloud-free morning. During this period, the depth of the convective boundary layer (CBL) increased from 100 to 1300 m. Wind speeds averaged over the depth of the CBL varied between 0 and 2 m s−1, while the wind direction varied over a range of 160 deg. The CBL instability parameter, −Zi/L, increased from approximately 25 (weakly unstable) to 250 (strongly unstable). The spacings of the elongated, parallel plumes scaled with the CBL height. These findings suggest that secondary circulations in the form of horizontal roll vortices were present under conditions not normally associated with roll vortices. The lines of aerosol structures aligned much more closely (within 15 deg) with the direction of the vertical shear of the horizontal wind through the depth of the CBL than with either the surface wind, mean CBL wind, or the wind at an altitude of 1.1Zi.

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