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O. Danne, M. Quante, D. Milferstädt, H. Lemke, and E. Raschke

Abstract

In this study, data obtained from measurements with a ground-based vertical-pointing 95-GHz polarimetric cloud radar are analyzed. The investigations concentrate on the relationships between the Doppler spectral moments observed in different regions of nonprecipitating cirrostratus and altostratus decks connected with warm fronts approaching the radar site. In some of these cases, a remarkably well-defined relationship between the radar reflectivity and the spectral width is found. It is demonstrated how this relationship can be used to obtain information on the size distributions and the fallspeeds of the particles in the investigated cloud sections. It is found that if single parameters of the size distributions, for example, are parameterized by a lognormal distribution, they cannot be determined with an acceptable accuracy. However, at least the changes of these parameters, such as mean particle diameter and particle concentration, with changing reflectivity as well as the behavior of the corresponding particle fallspeeds, can be described with the help of empirical relations between the Doppler moments. A main result is that significant changes in reflectivity within a cloud section (e.g., of 10 dBZ e) must correspond with a change in the relation between particle size and fallspeed, most commonly described by empirical power laws, and, therefore, probably with changes in particle shapes. This kind of radar data analysis will help to come to a better understanding of the microphysical and dynamical properties of the investigated cloud types, especially if further information from simultaneous measurements with other remote sensors is available.

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Kenneth Sassen, Likun Wang, David O’C. Starr, Jennifer M. Comstock, and Markus Quante

Abstract

In this fifth of a series of papers describing the extended-time high cloud observation program from the University of Utah Facility for Atmospheric Remote Sensing, the structural properties of cirrus clouds over Salt Lake City, Utah, are examined. Wavelet analysis is applied to a 10-yr record of cirrus cloud ruby (0.694 μm) lidar backscatter data as a function of cloud height in order to study the presence of periodic cloud structures, such as the signatures of Kelvin–Helmholtz instabilities, cirrus mammata, and uncinus cells (all with length scales of ∼1–10 km), as well as mesoscale cloud organizations generally believed to be induced by gravity waves. About 8.4% of the data display structures after passing a 95% confidence level test, but an 80% confidence level, which seems better able to resolve structures spread over long periods, yields 16.4%. The amount of identified cloud structures does not change significantly with length scale from 0.2 to 200 km, although the frequency of mesoscale cloud structures tends to increase as length scales increase. The middle-to-lower portion of cirrus clouds contains the most identified cloud structures, which seems related to the mesoscale organization of fall streaks from cloud-top-generating cells. The variability of cirrus cloud optical depth τ (defined by the standard deviation over mean τ) derived from a combined lidar and infrared radiometer (LIRAD) analysis is shown to be largely independent of τ. Because visual examination of the lidar displays also indicates that few cirrus layers can be considered horizontally homogeneous over our typical 3-h lidar data collection period, the authors conclude that the clouds in their sample are inherently inhomogeneous even though most cirrus structures are not revealed as periodic by wavelet analysis.

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J. Ström, R. Busen, M. Quante, B. Guillemet, P. R. A. Brown, and J. Heintzenberg

Abstract

During the pre-EUCREX (European Cloud and Radiation Experiment) intercomparison of airborne instrumentation in January 1992, nine hygrometers mounted on three different aircraft were compared. Although the different instruments are based on completely different principles and the three aircraft have very different flying characteristics, humidity data from both vertical profiles as well as horizontal flight legs showed good agreement. Despite the different aircraft limitations the intercomparison was done with the aircraft in close formation. In terms of relative difference in mixing ratio, most instruments agreed to within ±5% for values down to about 0.1 g kg−1. For mixing ratios between 0.03 and 0.1 g kg−1 most instruments agreed to within ±15%. Systematic differences between the instruments suggest that in joint experiments where data will be shared, the same algorithms for evaluating and converting humidity parameters should be used whenever possible.

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S. Crewell, H. Bloemink, A. Feijt, S. G. García, D. Jolivet, O. A. Krasnov, A. van Lammeren, U. Löhnert, E. van Meijgaard, J. Meywerk, M. Quante, K. Pfeilsticker, S. Schmidt, T. Scholl, C. Simmer, M. Schröder, T. Trautmann, V. Venema, M. Wendisch, and U. Willén

Clouds cause uncertainties in the determination of climate sensitivity to either natural or anthropogenic changes. Furthermore, clouds dominate our perception of the weather, and the relatively poor forecast of cloud and precipitation parameters in numerical weather prediction (NWP) models is striking. In order to improve modeling and forecasting of clouds in climate and NWP models the BALTEX BRIDGE Campaign (BBC) was conducted in the Netherlands in August/September 2001 as a contribution to the main field experiment of the Baltic Sea Experiment (BALTEX) from April 1999 to March 2001 (BRIDGE). The complex cloud processes, which involve spatial scales from less than 1 mm (condensation nuclei) to 1000 km (frontal systems) require an integrated measurement approach. Advanced remote sensing instruments were operated at the central facility in Cabauw, Netherlands, to derive the vertical cloud structure. A regional network of stations was operated within a 100 km × 100 km domain to observe solar radiation, cloud liquid water path, cloud-base temperature, and height. Aircraft and tethered balloon measurements were used to measure cloud microphysical parameters and solar radiation below, in, and above the cloud. Satellite measurements complemented the cloud observations by providing the spatial structure from above. In order to better understand the effect of cloud inhomogeneities on the radiation field, three-dimensional radiative transfer modeling was closely linked to the measurement activities. To evaluate the performance of dynamic atmospheric models for the cloudy atmosphere four operational climate and NWP models were compared to the observations. As a first outcome of BBC we demonstrate that increased vertical resolution can improve the representation of clouds in these models.

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