4/25/18. Precipitation and Radar GEF4310 Cloud Physics. Schedule, Spring Global precipitation patterns

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1 Precipitation and Radar GEF4310 Cloud Physics Trude Storelvmo, Dept. of Geoscience, U. of Oslo Spring 2018 Schedule, Spring 2018 Week # Monday (exercises) Wednesday (lectures) 3 (15/1) No class (17/1) Introduction and descriptive overview of clouds (Ch. 1) 4 (22/1) No class (24/1) Thermodynamics (Ch ) 5 (29/1) Thermodynamics exercises (31/1) No class 6 (5/2) No class (7/2) Thermodynamics (Ch ) 7 (12/2) Thermodynamics exercises (14/2) Stability & convection (Ch , 3.2, 3.3 and 4.2) 8 (19/2) Stability/convection exercises (21/2) Atmospheric aerosols (Ch. 5) 9 (26/2) Aerosols exercises (28/2) No class 10 (5/3) No class (7/3) Droplet formation (Ch. 6) 11 (12/3) Droplet formation exercises (14/3) Warm cloud microphysics (Ch. 7) 12 (19/3) Warm cloud microphysics exercises (21/3) No class 12 Friday 23/3 Mid-term exam (NOTE: extra Q&A class will be scheduled this week) 13 (26/3) No class (Påske) (28/3) No class (Påske) 14 (2/4) No class (Påske) (4/4) Ice nucleation (Ch. 8.1 and 8.2) 15 (9/4) No class (EGU) (11/4) No class (EGU) 16 (16/4) Ice nucleation exercises (18/4) Cold cloud microphysics (Ch ) 17 (23/4) Cold cloud microphysics exercises (25/4) Precipitation/radar (Ch ) 18 (30/4) No class (2/5) Aerosol/cloud radiative effects (Ch. 11) 19 (7/5) Precip./radar + radiative eff. exercises (9/5) Aerosol-cloud interactions (Ch and 12.2) 20 (14/5) Aerosol/cloud interaction exercises (16/5) Cloud/climate feedbacks (Ch. 12.3) 21 (21/5) No class (Pinse) (23/5) Climate engineering (Ch. 12.4) 22 Exam week of May 28 Jun 2 Global precipitation patterns JFM JJA Precipitation intensity (=precipitation rate, R) is measured as the flux of precipitation through a horizontal surface [m 3 m -2 s -1 = m/s], but is usually expressed in mm/h. Precipitation in cm, from a compilation of satellite observations, rain gauge data and numerical simulations (revised from Xie & Arkin, 1997). 1

2 Rain drop size distribution Irrespective of the phase of the cloud from which precipitation originates, the majority of it reaches the surface as rain. Raindrop size distributions show a rapid decrease in drop concentration with increasing size for R > 0:5mm Raindrop size distributions change with rainfall intensity such that the relative number of large drops increases with increasing rainfall rate The size distribution is approximately exponential, especially in rain that is fairly steady, as first described in Marshall & Palmer (MP, 1948) Marshall-Palmer "($) = " ' exp( Λ$) N(D) is the number of rain drops with diameter D, N 0 is the intercept parameter (unit: cm -4 ), and! represents the slope of the rain drop size distribution (unit: cm -1 ). Marshall and Palmer (1948) found (empirically) that the slope parameter depends on the rain rate (in mm/hour) as follows: Λ - = 41-0'.23 Measuring rain drop size distributions Size distributions of rain drops can be observed acoustically with the Joss- Waldvogel distrometer. The amplitude of the sound of a rain drop falling onto the membrane is a measure for its kinetic energy. There are different types of distrometers (i.e. instruments that measure the number and size of rain drops) another type uses video cameras to take 2D images of the drops at high temporal resolution. 2

3 Rain drop break-up Raindrop break-up provides a partial explanation for the exponential form of the drop-size distributions Raindrops are limited in size (~5 mm in diameter), because the chance of disruption increases with size. Breakup is due to: Aerodynamic induced circulation of water in the drop Collision between drops, because permanent coalescence becomes less likely for increasing values of drop size, relative velocity and impact parameter. Rain drop break up Neck or filament breakup: is caused by glancing collisions. Identities of the colliding drops are preserved. New satellite droplets are created by disintegration of the connecting neck (Fig. a). Sheet breakup: drops collide such that one side of the large drop is torn off. The bulk of the large drop then rotates about the point of impact, issuing a sheet of water that breaks apart into satellite drops. Only the identity of the large drop is preserved (Fig. b) Disk breakup: point of impact is close to the center of the bigger drop. Coalescence occurs temporarily, but the disk disintegrates into a large number of medium-sized drops. Original identities are lost (Fig. c). Snow crystal size distributions Snowakes rather than individual ice crystals account for most of the precipitation reaching the ground as snow. As the snowflakes are irregular aggregates of crystals or smaller snowflakes, there is no easy way to measure their dimensions. Thus, data on snowflakes are usually expressed in terms of particle mass, or equivalent diameter of the drop formed when the snow melts. The snow size distribution is also exponential, and on the same form as for rain drops. 3

4 Snow crystal size distributions Note that here the intercept parameter N 0 [cm -4 ] depends on precipitation rate R [mm/h] (expressed in water-equivalent depth of the accumulated snow). It is more uncertain than for rain, because of problems measuring size-segregated snow An additional complication is that fall speeds are not uniquely dependent on size, but also depend on density and likely the crystal forms that make up the snowflake. Snow fall speeds are generally much lower than for rain, and the crystals are therefore less likely to break up and fragment in collisions. This leads to a broader distribution. Precipitation rates! = # ' 6 % ( ) ) * + ),) & Use D=melted diameter and R=equivalent rainfall rate to apply this equation to both rain and snow. Values of R vary between trace amounts up to several hundred mm/h. Rainfall rates in excess of about 25 mm/h are always associated with convective clouds. At most locations snowfall rates tend to be at least an order of magnitude lower than rainfall rates Radar retrieval While rain gauges only measure precipitation rates at individual sites, radars can detect precipitation over a large area. Radar (=Radio Detection and Ranging) is an active remote sensing instrument, i.e. it measures precipitation rates remotely by emitting pulses of electromagnetic radiation and receiving the reradiation from the raindrops. In the so-called Rayleigh scattering regime (i.e. when the precipitation particles are much smaller than the radar wavelength of a few cm), the radar signal is proportional to the sixth power of the size of the object. Most radar images show the reflectivity, Z. 4

5 Radar reflectivities vs. rain rates Based on long-term comparisons between radar reflectivities and rain gage measurements, empirical relationships between precipitation rate and reflectivity have been established (e. g. drizzle: - 10dBZ, moderate rain: 10-30dBZ, heavy rain: 30-60dBZ) #!"# = 10'() ** + *,- 5