Atmospheric Measurement Techniques Discussions www.atmos-meas-tech-discuss.net 1867-8610 3 2 2010 10.5194/amtd-3-891-2010 http://www.atmos-meas-tech-discuss.net/3/891/2010/ http://www.atmos-meas-tech-discuss.net/3/891/2010/amtd-3-891-2010.html http://www.atmos-meas-tech-discuss.net/3/891/2010/amtd-3-891-2010.pdf 891 921 2010-03-05 Estimating drizzle drop size and precipitation rate using two-colour lidar measurements C. D. Westbrook c.d.westbrook@reading.ac.uk R. J. Hogan E. J. O'Connor A. J. Illingworth University of Reading, Department of Meteorology, Reading, UK Finnish Meteorological Institute, Helsinki, Finland A method to estimate the size and liquid water content of drizzle drops using lidar measurements at two wavelengths is described. The method exploits the differential absorption of infrared light by liquid water at 905 nm and 1.5 µm, which leads to a different backscatter cross section for water drops larger than &asymp;50 µm. The ratio of backscatter measured from drizzle samples below cloud base at these two wavelengths (the colour ratio) provides a measure of the median volume drop diameter <i>D</i>₀. This is a strong effect: for <i>D</i>₀=200 µm, a colour ratio of &asymp;6 dB is predicted. Once <i>D</i>₀ is known, the measured backscatter at 905 nm can be used to calculate the liquid water content (LWC) and other moments of the drizzle drop distribution. <br><br> The method is applied to observations of drizzle falling from stratocumulus and stratus clouds. High resolution (32 s, 36 m) profiles of <i>D</i>₀, LWC and precipitation rate <i>R</i> are derived. The main sources of error in the technique are the need to assume a value for the dispersion parameter <i>μ</i> in the drop size spectrum (leading to at most a 35% error in <i>R</i>) and the influence of aerosol returns on the retrieval (&asymp;10% error in <i>R</i> for the cases considered here). Radar reflectivities are also computed from the lidar data, and compared to independent measurements from a colocated cloud radar, offering independent validation of the derived drop size distributions. Beard, K. V.: Terminal velocity and shape of cloud and precipitation drops aloft, J. Atmos. Sci., 33, 851–864, 1976. Bohren, C. F. and Huffman, D. R.: Absorption and scattering of light by small particles, John Wiley & Sons, New York, USA, 530~pp., 1983. Comstock, K. K., Wood, R., Yuter, S. E., and Bretherton, C. S.: Reflectivity and rain rate in and below drizzling stratocumulus, Q. J. R. Meteorol. Soc., 130, 2891–2918, 2004. Fox, N. I. and Illingworth, A. J.: The retrieval of stratocumulus cloud properties by ground-based cloud radar, J. Appl. Met., 36, 485–492, 1997. Hale, G. M. and Querry, M. R.: Optical constants of water in the 200 nm to 200$ \unit\mu$m wavelength region, Appl. Opt., 12, 555–563, 1973. Illingworth, A. J. and Blackman, T. M.: The need to represent raindrop spectra as normalized gamma distributions for the interpretation of polarization radar observations, J. Appl. Met., 41, 287–297, 2002. Illingworth, A. J., Hogan, R. J., O'Connor, E. J., et al.: CloudNet: Continuous evaluation of cloud profiles in seven operational models using ground-based observations, Bull. Amer. Meteorol. Soc., 88, 883–898, 2007. Kou, L., Labrie, D., and Chylek, P.: Refractive indices of water and ice in the 0.65–2.5$ \unit\mu$m spectral range, Appl. Opt., 32, 3531–3540, 1993. Marshall, J. S. and Palmer, W. M.: The distribution of raindrops with size, J. Met., 5, 165–166, 1948. O'Connor, E. J., Illingworth, A. J., and Hogan, R. J.: A technique for autocalibration of cloud lidar, J Atmos. Ocean. Tech., 21, 777–786, 2004. O'Connor, E. J., Hogan, R. J. and Illingworth, A. J.: Retrieving stratocumulus drizzle parameters using Doppler radar and lidar, J. Appl. Meteorol., 44, 14–27, 2005. Slingo, A.: Sensitivity of the earth's radiation budget to changes in low clouds, Nature, 343, 49–51, 1990. Vali, G., Kelly, R. D., French, J., et al.: Finescale structure of coastal stratus, J. Atmos. Sci., 55, 3540–3564, 1998. Westbrook, C. D., Illingworth, A. J., O'Connor, E. J., and Hogan, R. J.: Doppler lidar measurements of oriented planar ice crystals falling from supercooled and glaciated cloud layers, Q. J. R. Meteorol. Soc., 136, 260–276, 2010. Wilson, D. R. and Ballard, S. P.: A microphysically based precipitation scheme for the UK Meteorological Office Unified Model., 125, 1607–1636, Q. J. R. Meteorol. Soc., 125, 1607–1636, 1999. Wolf, S. and Voshchinnikov, N. V.: Mie scattering by ensembles of particles with very large size parameters, Comp. Phys. Comm., 162, 113–123, 2004. Wood, R.: Drizzle in stratiform boundary layer clouds. Part I: vertical and horizontal structure, J. Atmos. Sci., 62, 3011–3033, 2005.