Acquistapace, Claudia ORCID: 0000-0002-1144-4753, Kneifel, Stefan ORCID: 0000-0003-2220-2968, Lohnert, Ulrich ORCID: 0000-0002-9023-0269, Kollias, Pavlos ORCID: 0000-0002-5984-7869, Maahn, Maximilian ORCID: 0000-0002-2580-9100 and Bauer-Pfundstein, Matthias (2017). Optimizing observations of drizzle onset with millimeter-wavelength radars. Atmos. Meas. Tech., 10 (5). S. 1783 - 1803. GOTTINGEN: COPERNICUS GESELLSCHAFT MBH. ISSN 1867-8548

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Cloud Doppler radars are increasingly used to study cloud and precipitation microphysical processes. Typical bulk cloud properties such as liquid or ice content are usually derived using the first three standard moments of the radar Doppler spectrum. Recent studies demonstrated the value of higher moments for the reduction of retrieval uncertainties and for providing additional insights into microphysical processes. Large effort has been undertaken, e.g., within the Atmospheric Radiation Measurement (ARM) program to ensure high quality of radar Doppler spectra. However, a systematic approach concerning the accuracy of higher moment estimates and sensitivity to basic radar system settings, such as spectral resolution, integration time and beam width, are still missing. In this study, we present an approach on how to optimize radar settings for radar Doppler spectra moments in the specific context of drizzle detection. The process of drizzle development has shown to be particularly sensitive to higher radar moments such as skewness. We collected radar raw data (I/Q time series) from consecutive zenith-pointing observations for two liquid cloud cases observed at the cloud observatory JOYCE in Germany. The I/Q data allowed us to process Doppler spectra and derive their moments using different spectral resolutions and integration times during identical time intervals. This enabled us to study the sensitivity of the spatiotemporal structure of the derived moments to the different radar settings. The observed signatures were further investigated using a radar Doppler forward model which allowed us to compare observed and simulated sensitivities and also to study the impact of additional hardware-dependent parameters such as antenna beam width. For the observed cloud with drizzle onset we found that longer integration times mainly modify spectral width (S-w) and skewness (S-k), leaving other moments mostly unaffected. An integration time of 2 s seems to be an optimal compromise: both observations and simulations revealed that a 10 s integration time - as it is widely used for European cloud radars - leads to a significant turbulence-induced increase of S-w and reduction of S-k compared to 2 s integration time. This can lead to significantly different microphysical interpretations with respect to drizzle water content and effective radius. A change from 2 s to even shorter integration times (0.4 s) has much smaller effects on S-w and S-k. We also find that spectral resolution has a small impact on the moment estimations, and thus on the microphysical interpretation of the drizzle signal. Even the coarsest spectral resolution studied, 0.08 ms(-1), seems to be appropriate for calculation moments of drizzling clouds. Moreover, simulations provided additional insight into the microphysical interpretation of the skewness signatures observed: in low (high)-turbulence conditions, only drizzle larger than 20 mu m (40 mu m) can generate S-k values above the S-k noise level (in our case 0.4). Higher S-k values are also obtained in simulations when smaller beam widths are adopted.

Item Type: Journal Article
CreatorsEmailORCIDORCID Put Code
URN: urn:nbn:de:hbz:38-231181
DOI: 10.5194/amt-10-1783-2017
Journal or Publication Title: Atmos. Meas. Tech.
Volume: 10
Number: 5
Page Range: S. 1783 - 1803
Date: 2017
Place of Publication: GOTTINGEN
ISSN: 1867-8548
Language: English
Faculty: Faculty of Mathematics and Natural Sciences
Divisions: Faculty of Mathematics and Natural Sciences > Department of Geosciences > Institute for Geophysics and Meteorology
Subjects: no entry
Uncontrolled Keywords:
Meteorology & Atmospheric SciencesMultiple languages
Refereed: Yes


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