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  • Author or Editor: Edwin Eloranta x
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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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Roni Avissar
,
Edwin W. Eloranta
,
Kemal Gürer
, and
Gregory J. Tripoli

Abstract

A large-eddy simulation (LES) model was used to simulate the convective boundary layer (CBL) that developed on 1 July 1987, over the domain of the First International Satellite Land Surface Climatology Project Field Experiment (FIFE). Three simulations were produced using different boundary conditions at the ground surface, namely, (i) spatial distribution of topography and spatial distribution of surface heat fluxes; (ii) spatial distribution of topography but mean surface heat fluxes; and (iii) no topography and mean surface heat fluxes. The diurnal variation of mean surface fluxes and their spatial distribution were derived from the FIFE network of observations. In all cases, the model was initialized with the atmospheric sounding observed in this domain at 0700, and run until 1500 local time. The resulting mean profiles of temperature and specific humidity were compared to those observed with atmospheric soundings at 0900, 1030, and 1230 local time. The simulated structure of turbulence was qualitatively compared with that obtained from a volume-imaging lidar (VIL) scanning the CBL over the simulated domain during that day. Power spectra and autocorrelations of mixing ratio were calculated from the model outputs and were compared to those obtained from the VIL.

Overall, the model performed quite well. Observed atmospheric soundings were within 1 K and 1 g kg−1 of the simulated mean profiles of temperature and specific humidity, respectively, and indicated that the model correctly predicts the CBL height. Similarities in the structure of the eddies obtained from the model and the VIL were clearly identified. Spectral analysis indicated that resolved eddies (i.e., eddies larger than 200 m) are relatively well simulated with the model, but that the energy cascade is not well represented by the Deardorff 1.5-order-of-closure subgrid-scale parameterization. Autocorrelation analysis indicated that the model correctly simulates the characteristic size of the eddies, but that their mean lifetime is longer than that observed with the VIL, indicating a too weak dissipation of the eddies by the subgrid-scale scheme. Thus, this study emphasized the need to develop better subgrid-scale parameterizations for LES models. The different simulations also indicated that topographical features of the order of 100 m and microβ-scale heterogeneity of surface heat fluxes had only a minor to modest impact on the CBL developing over a relatively humid surface.

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Gijs de Boer
,
Edwin W. Eloranta
, and
Matthew D. Shupe

Abstract

Macro- and microphysical properties of single-layer stratiform mixed-phase clouds are derived from multiple years of lidar, radar, and radiosonde observations. Measurements were made as part of the Mixed-Phase Arctic Clouds Experiment (MPACE) and the Study of Environmental Arctic Change (SEARCH) in Barrow, Alaska, and Eureka, Nunavut, Canada, respectively. Single-layer mixed-phase clouds occurred between 4% and 26% of the total time observed, varying with season and location. They had mean cloud-base heights between ∼700 and 2100 m and thicknesses between ∼200 and 700 m. Seasonal mean cloud optical depths ranged from 2.2 up. The clouds existed at temperatures of ∼242–271 K and occurred under different wind conditions, depending on season. Utilizing retrievals from a combination of lidar, radar, and microwave radiometer, mean cloud microphysical properties were derived, with mean liquid effective diameters estimated from 16 to 49 μm, mean liquid number densities on the order of 104–105 L−1, and mean water contents estimated between 0.07 and 0.28 g m−3. Ice precipitation was shown to have mean ice effective diameters of 50–125 μm, mean ice number densities on the order of 10 L−1, and mean water contents estimated between 0.012 and 0.031 g m−3. Mean cloud liquid water paths ranged from 25 to 100 g m−2. All results are compared to previous studies, and potential retrieval errors are discussed. Additionally, seasonal variation in macro- and microphysical properties was highlighted. Finally, fraction of liquid water to ice mass was shown to decrease with decreasing temperature.

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Timothy J. Wagner
,
Alan C. Czarnetzki
,
Megan Christiansen
,
R. Bradley Pierce
,
Charles O. Stanier
,
Angela F. Dickens
, and
Edwin W. Eloranta

Abstract

Ground-based thermodynamic and kinematic profilers were placed adjacent to the western shore of Lake Michigan at two sites as part of the 2017 Lake Michigan Ozone Study. The southern site near Zion, Illinois, hosted a microwave radiometer (MWR) and a sodar wind profiler, while the northern site in Sheboygan, Wisconsin, featured an Atmospheric Emitted Radiance Interferometer (AERI), a Doppler lidar, and a High Spectral Resolution Lidar (HSRL). Each site experienced several lake-breeze events during the experiment. Composite time series and time–height cross sections were constructed relative to the lake-breeze arrival time so that commonalities across events could be explored. The composited surface observations indicate that the wind direction of the lake breeze was consistently southeasterly at both sites regardless of its direction before the arrival of the lake-breeze front. Surface relative humidity increased with the arriving lake breeze, though this was due to cooler air temperatures as absolute moisture content stayed the same or decreased. The profiler observations show that the lake breeze penetrated deeper when the local environment was unstable and preexisting flow was weak. The cold air associated with the lake breeze remained confined to the lowest 200 m of the troposphere even if the wind shift was observed at higher altitudes. The evolution of the lake breeze corresponded well to observed changes in baroclinicity and calculated changes in circulation. Collocated observations of aerosols showed increases in number and mass concentrations after the passage of the lake-breeze front.

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