Drop shapes versus fall velocities in rain: 2 contrasting examples

Rainfall retrievals from polarimetric radar measurements require the knowledge of four fundamental rain microstructure parameters, namely, drop size distribution, drop shape distribution, canting angles and drop fall velocities. Some recent measurements of all four parameters in natural rain are summarized in [1]. In this paper, we perform an in-depth analysis of two events, using two co-located 2D video disdrometers (2DVD; see [2]) both with high calibration accuracy, and a C-band polarimetric radar [3], located 15 km away. The two events, which occurred 7 days apart (on the 18th and the 25th of Dec 2009), had moderate-to-intense rainfall rates, but the second event had an embedded convection line within the storm. The line had passed over the 2DVD site, thus enabling the shapes and fall velocities to be determined as the line crossed the site. The first event was also captured in a similar manner by both the 2DVDs as well as the C-band radar.

Drop fall velocity measurements for, say, the 3 mm drops show noticeable differences between the two events. Whereas for the first event, the velocity distribution showed a narrow and symmetric distribution, with a mode at the expected value (7.95 m/s, as given by the formula in [4]), the second event produced a wider distribution with a significant skewness towards lower velocities (although its mode too was close to the expected value). Moreover, the ‘slower' 3 mm drops in the second event occurred when the convection line was directly over the 2DVD site (03:35-03:45 utc), and not before nor after. A similar trend was observed in terms of the horizontal dimensions of the 3 mm drops, i.e. large fluctuations during the same time period, but not outside the period. Vertical dimensions of the drops also fluctuated but not to the same extent. Interestingly, the horizontal dimensions tended towards larger values during the 10-minute period, implying an increase in drop oblateness, which in turn indicates the possibility of the ‘horizontal' mode oscillation, one of the three fundamental modes of drop oscillations [5], albeit the most difficult one to excite.

In order to dismiss the possibility of surface layer effects, the C-band polarimetric radar data were processed for this event, and analyzed in terms of the ‘effective beta' method [6]. This method, although susceptible to radar calibration errors, can be used qualitatively to identify areas which do not conform to the polarimetric radar algorithms built on standard bulk assumptions on drop shapes. Application of this method to the PPI scans taken over a one-hour time period related to the second event has shown higher than normal beta-values in some areas of the convection line, the high values corroborating the notion of horizontal mode oscillations occurring in those regions.

We will present the fall velocity measurements and drop shapes for both events from the two 2DVDs, as well as results of the corresponding C-band data analyses.

References:

[1] Thurai, M., and Bringi, V. N.: Rain microstructure from polarimetric radar and advanced disdrometers, Chapter 10 in “Precipitation: Advances in Measurement, Estimation and Prediction”, Michaelides, Silas. (Ed.), Springer, ISBN: 978-3-540-77654-3, 2008.

[2] Schönhuber, M., Lammer, G. and Randeu, W. L.: The 2D-video-vistrometer, Chapter 1 in “Precipitation: Advances in Measurement, Estimation and Prediction”, Michaelides, Silas. (Ed.), Springer, ISBN: 978-3-540-77654-3, 2008.

[3] Petersen, W. A., Knupp, K. R., Cecil, D. J., and Mecikalski, J. R.: The University of Alabama Huntsville THOR Center instrumentation: Research and operational collaboration, 33rd Int. Conf. on Radar Meteorology, AMS, Cairns, Australia, 2007.

[4] Atlas D., Srivastava, R. C., Sekkon, R. S.: Doppler radar characteristics of precipitation at vertical incidence. Rev Geophys Space GE 2:1-35, 1973.

[5] Beard, K.V., Bringi, V.N. and Thurai, M.: A new understanding of raindrop shape, Atmos. Res. , Atmos Res., vol. 97, 396-415, 2010.

[6] Gorgucci, E., G. Scarchilli, V. Chandrasekar, and V. Bringi: Measurement of mean raindrop shape from polarimetric radar observations. J. Atmos. Sci., 57, 3406-3413, 2000.