Universität zu Köln

Van Tricht, K., Gorodetskaya, I. V., Lhermitte, S., Turner, D. D., Schween, J. H. and Van Lipzig, N. P. M. (2014). An improved algorithm for polar cloud-base detection by ceilometer over the ice sheets. Atmos. Meas. Tech., 7 (5). S. 1153 - 1168. GOTTINGEN: COPERNICUS GESELLSCHAFT MBH. ISSN 1867-8548

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Abstract

Optically thin ice and mixed-phase clouds play an important role in polar regions due to their effect on cloud radiative impact and precipitation. Cloud-base heights can be detected by ceilometers, low-power backscatter lidars that run continuously and therefore have the potential to provide basic cloud statistics including cloud frequency, base height and vertical structure. The standard cloud-base detection algorithms of ceilometers are designed to detect optically thick liquid-containing clouds, while the detection of thin ice clouds requires an alternative approach. This paper presents the polar threshold (PT) algorithm that was developed to be sensitive to optically thin hydrometeor layers (minimum optical depth tau epsilon 0.01). The PT algorithm detects the first hydrometeor layer in a vertical attenuated backscatter profile exceeding a predefined threshold in combination with noise reduction and averaging procedures. The optimal backscatter threshold of 3 x 10(-4) km(-1) sr(-1) for cloud-base detection near the surface was derived based on a sensitivity analysis using data from Princess Elisabeth, Antarctica and Summit, Greenland. At higher altitudes where the average noise level is higher than the backscatter threshold, the PT algorithm becomes signal-to-noise ratio driven. The algorithm defines cloudy conditions as any atmospheric profile containing a hydrometeor layer at least 90 m thick. A comparison with relative humidity measurements from radiosondes at Summit illustrates the algorithm's ability to significantly discriminate between clear-sky and cloudy conditions. Analysis of the cloud statistics derived from the PT algorithm indicates a year-round monthly mean cloud cover fraction of 72% (+/- 10%) at Summit without a seasonal cycle. The occurrence of optically thick layers, indicating the presence of supercooled liquid water droplets, shows a seasonal cycle at Summit with a monthly mean summer peak of 40 % (+/- 4%). The monthly mean cloud occurrence frequency in summer at Princess Elisabeth is 46% (+/- 5%), which reduces to 12% (+/- 2.5%) for supercooled liquid cloud layers. Our analyses furthermore illustrate the importance of optically thin hydrometeor layers located near the surface for both sites, with 87% of all detections below 500 m for Summit and 80% below 2 km for Princess Elisabeth. These results have implications for using satellite-based remotely sensed cloud observations, like CloudSat that may be insensitive for hydrometeors near the surface. The decrease of sensitivity with height, which is an inherent limitation of the ceilometer, does not have a significant impact on our results. This study highlights the potential of the PT algorithm to extract information in polar regions from various hydrometeor layers using measurements by the robust and relatively low-cost ceilometer instrument.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Van Tricht, K. UNSPECIFIED UNSPECIFIED UNSPECIFIED Gorodetskaya, I. V. UNSPECIFIED UNSPECIFIED UNSPECIFIED Lhermitte, S. UNSPECIFIED UNSPECIFIED UNSPECIFIED Turner, D. D. UNSPECIFIED UNSPECIFIED UNSPECIFIED Schween, J. H. UNSPECIFIED UNSPECIFIED UNSPECIFIED Van Lipzig, N. P. M. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-450165
DOI:	10.5194/amt-7-1153-2014
Journal or Publication Title:	Atmos. Meas. Tech.
Volume:	7
Number:	5
Page Range:	S. 1153 - 1168
Date:	2014
Publisher:	COPERNICUS GESELLSCHAFT MBH
Place of Publication:	GOTTINGEN
ISSN:	1867-8548
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language RADIATION DRY BIAS; RELATIVE-HUMIDITY; ARCTIC SURFACE; BOUNDARY-LAYER; LIDAR; RADIOSONDES; ANTARCTICA; GREENLAND; CLIMATE; BUDGET Multiple languages Meteorology & Atmospheric Sciences Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/45016