Rain rate. If the drop size distribu0on is n(d), and fall speeds v(d), net ver0cal flux of drops (m - 2 s - 1 )

Transcription

1 Rain rate If the drop size distribu0on is n(d), and fall speeds v(d), net ver0cal flux of drops (m - 2 s - 1 ) Φ = 0 (w v(d))n(d)dd The threshold diameter has v(d th ) = w. Smaller drops move up, larger ones move down. rain The larger that w is, the larger D th must be Large rain rates tend to have large drops The rain rate at the surface is this flux computed at the ground (w=0) Mass flux [kg m - 2 s - 1 ] of rain himng the ground:

2 Size distribu/ons of rain drops Modified gamma distribution Alternative form of gamma distribution: Setting β=0 yields an exponential with n 0 = m -3 mm -1 and Λ = 41 R (R in mm h -1 ), this is the Marshall-Palmer distribution only captures large tail

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4 Evolu/on of drop size distribu/on (star/ng with Marshall- Palmer) Collection and disruptions oscillate, but after 30 min get trimodal Largest break up And fragments end up here Smallest get collected quickly but some replenished as smaller grow in by condensation Important to note that very large drops not allowed to survive by physics 8 mm diam sometimes observed, but GCCN origin invoked

5 What are these? Reminder: Warm cloud (supersatura/on via adiaba/c expansion)

6 Exchange of energy with environment by virtue of a T difference Diaba/c condensa/on: entrainment effects An air parcel may rise adiaba0cally in the core of a cloud, but turbulent mo0ons eventually mix in dry environmental air This mixing smooths out gradients dry air moistens and cloudy air dries Compare the 0me scales: τ evap << τ mixing This means that the drops respond quickly to the drier air some drops evaporate un0l the air is resaturated (according to the new condi0ons) Overall, retain the original distribu0on but N D has decreased BUT: now N D decreased condensa0onal sink is REDUCED! supersatura0on RISES faster than in neighboring cloud elements growth rates increase those surviving drops get larger than neighbors who didn t see an entrainment event When the processed air gets mixed in with those neighboring drops later, the final drop distribu0on will be broader

7 Diaba/c condensa/on: radia/ve effects Consider droplets near the top of the cloud: if they see a cooler atmosphere above them, they can radiate away some of their energy their temperature DROPS So the vapor pressure at the drop surface is DECREASED drops grow faster Interes0ngly, the radia0ve cooling is propor0onal to the cross- sec0onal area of the droplet so large drops cool more Harrington et al. (2000) showed that in a marine Scu environment, when drops compete for a limited supply of water vapor, the larger drops grow so rapidly via this enhancement that drops with diameters < 20 µm evaporate bimodal drop size spectrum! Only effec0ve in clouds where drops can reside near cloud top for 12 min or more Cumulus clouds with vigorous overturning expose drops to space for too short a 0me Hartman and Harrington, 2005

8 Arc/c stratus cloud example Drops smaller than ~10 µm are prevented from growing! With radiative effects Without

9 Autoconversion Autoconversion is the process of collision- coalescence that leads to the forma0on of new small drizzle drops In models, we generally cannot represent this process explicitly The drizzle drop threshold is generally taken to be r = 20 µm (other choices exist) Parameteriza0ons express the autoconversion rate in terms of drop size distribu0on moments, such as liquid water content (LWC) [Kessler, 1969], cloud droplet number concentra0on, or spectral dispersion The computed autoconversion rate is used to transfer water mass from cloud drops into drizzle drops (sets 0me scale) in order to ini0ate precipita0on When autoconversion is ac0ve, an average collision frequency is assumed for all cloud droplets, resul0ng in an autoconversion rate that scales with LWC 7/3 On the representation of droplet coalescence and autoconversion: Evaluation using ambient cloud droplet size distributions, W. C. Hsieh et al., JGR, 2009 In this study, we evaluate eight autoconversion parameterizations against integration of the Kinetic Collection Equation (KCE) for cloud size distributions measured during the NASA CRYSTAL-FACE and CSTRIPE campaigns. KCE calculations are done using both the observed data and fits of these data to a gamma distribution function; it is found that the fitted distributions provide a good approximation for calculations of total coalescence but not for autoconversion because of fitting errors near the drop-drizzle separation size.

10 Measured DSD with gamma distribution fits

11 Genera/ng supersatura/ons to create clouds So far, we have focused on genera0on of supersatura0on in an air parcel (really, COOLING that results in supersatura0on genera0on) by adiaba0c expansion. In general, For ver0cal liking, Updraft speed And using mole frac0on of total water that is liquid (y L ): Heating due to condensation Cooling due to expansion But other cooling mechanisms exist, in addi0on to uplik: Radia0on Conduc0on Mixing

12 Isobaric, diaba/c cooling Typical case: Earth s surface radiates energy to space under clear skies at night Air in contact with surface loses thermal energy by conduc0on, and cools neighboring air parcels by mixing (dt/dt) If moist air cools below its dew point, a radia0on fog is created Advec0on fog: moist warmer air flows over cooler surface (e.g., cold lake) Isobaric, adiaba/c mixing Typical case: contrails (warm, moist engine exhaust + cold ambient air) Notice both starting points were undersaturated -- but mixing can produce supersaturations over wide range of mixing fraction

13 Cloud proper/es Table 2. Observed typical values for the properties of clouds. The values are merely modal-means. The range of observed values is quite large. The radius of cloud droplets is r (microns), the effective optical radius optically is r', N is the number of droplets per cubic centimetre, L is the liquid water content of the cloud (g/m3). For all clouds, the level of observation is just below the freezing level, except for fog and cirrus. Cirrus consists entirely of ice crystals, and the values shown in this table are liquid equivalents (Source: (3,4)).

14 Some Useful (Ballpark) Values (Table 15.3, Seinfeld & Pandis) Cloud Type Updraft velocity (m s -1 ) Maximum supersaturation (%) Continental cumulus ~ Maritime cumulus ~ Stratiform ~0 1 ~0.05 Fog -- ~0.1

15 Subsidence creates warming that caps the BL Marine boundary layer Radiative cooling (creates negative buoyancy at cloud top) and entrainment (grows against subsidence) force the circulations Middle: notice that strong drying means VPT in downdraft can be warmer than updraft cloud has to try to compensate Left case: Air is cooled, and condensate lost to entrainment of dry air downdraft cloud base slightly higher than updraft Right: effect of drizzle is similar to that of strong entrainment; can stabilize BL which slow circulation