METR215- Cloud Droplet Formation http://apollo.lsc.vsc.edu/classes/met130/notes/chapter7/ccn_drop_prec.html R&Y book, Chapter 6 S. Platnick notes

PHYS 622 - Clouds, spring ‘04, lect. 2, Platnick Water Cloud Formation Water clouds form when RH slightly greater than 100% (e.g., 0.3% supersaturation). This is a result of a subset of the atmospheric aerosol serving as nucleation sites (to be discussed later). Common ways for exceed saturation: Mixing of air masses (warm moist with cool air) Cooling via parcel expansion (adiabatic) Radiative cooling (e.g. ground fog, can lead to process 2) PHYS 622 - Clouds, spring ‘04, lect. 2, Platnick 2

PHYS 622 - Clouds, spring ‘04, lect. 2, Platnick Concepts T e es(T) (T1,e1) (T2,e2) unsaturated saturated Mixing Radiative Cooling Adiabatic expansion PHYS 622 - Clouds, spring ‘04, lect. 2, Platnick 3

Saturation Vapor Pressure (Clausius-Clapeyron equation) Water Air and water vapor T At equilibrium, evaporation and condensation have the same rate, and the air above the liquid is saturated with water vapor; the partial pressure of water vapor, or the Saturation Vapor Pressure (es) is: Where Ts=triple point temperature (273.16K), L is the latent heat of vaporization (2.5106 J/kg), es(Ttr) = 611Pa (or 6.11 mb). Rv is the specific gas constant for water vapor (461.5 J-kg1-K1). PHYS 622 - Clouds, spring ‘04, lect. 2, Platnick 4

Saturation Vapor Pressure An approximation for the saturation vapor pressure (Rogers & Yau): Over liquid water: L = latent heat of vaporization/condensation, A=2.53 x 108 kPa, B = 5.42 x 103 K. Over ice: L = latent heat of sublimation, A=3.41 x 109 kPa, B = 6.13 x 103 K. Need a sketch of solid-liquid-vapor phase regions. PHYS 622 - Clouds, spring ‘04, lect. 2, Platnick 5

Example Microphysical Measurements in Marine Sc Clouds (ASTEX field campaign, near Azores, 1992) Data from U. Washington C-131 aircraft PHYS 622 - Clouds, spring ‘04, lect.2, Platnick 6

Rain Drops, Cloud Droplets, and CCN

relative sizes of rain drops, cloud drops, and CCN: raindrops - 2000 mm = 2 mm fall at a speed of 4-5 ms-1 cloud drops - 20 mm = 0.02 mm remain suspended in the air CCN - 0.2 mm = 0.0002 mm To get a droplet (20 mm) to grow to raindrop size (2000mm) it must increase in size by a factor of 100 (two orders of magnitude): 2000mm/20mm = 100 this occurs in about 30 minutes in a thunderstorm!!! this is like a 150 lb person growing in size to 15,000 lbs in half an hour!!! Q: How does this happen??

Processes for Cloud Droplet Growth How does this happen?? By: condensation collision/coalescence ice-crystal process

Condensation & Collision Water Droplet Growth Condensation & Collision Condensational growth: diffusion of vapor to droplet Collisional growth: collision and coalescence (accretion, coagulation) between droplets PHYS 622 - Clouds, spring ‘04, lect.4, Platnick

Water Droplet Growth - Condensation Flux of vapor to droplet (schematic shows “net flux” of vapor towards droplet, i.e., droplet grows) Need to consider: Vapor flux due to gradient between saturation vapor pressure at droplet surface and environment (at ∞). Effect of Latent heat effecting droplet saturation vapor pressure (equilibrium temperature accounting for heat flux away from droplet). From Bill Olsen PHYS 622 - Clouds, spring ‘04, lect.4, Platnick

Wang and Olsen slide. 12

Cloud Droplet Growth by Condensation Consider pure water in equilibrium with air above it C-C equation to calculate es

Cloud Droplet Growth by Condensation Consider pure water in equilibrium with air above it: then the RH = 100% evaporation = condensation vapor pressure (e) = saturation vapor pressure (es) if evaporation > condensation, water is _________ if evaporation < condensation, water is ________ Now, a droplet surface is not flat, instead, it has curvature..... Q: how does curvature affect the evaporation/condensation process?? Answer 1: Being added to Answer2: Being removed from

Equilibrium

Flat versus Curved Water Surfaces

Flat versus Curved Water Surfaces: curvature effect more energy is required to maintain the "curvature" of the drop therefore, the water molecules on the surface of the drop have more energy therefore, they evaporate more readily that from the flat water surface (compare the length of the red arrows) therefore: evaporation rate off curved surface > evaporation rate off of flat surface since air above both surfaces is saturated, then evaporation rate = condensation rate therefore, condensation rate onto droplet > condensation rate onto flat water surface therefore, esdrop > esflat therefore: if RHflat = 100%, then RHdrop > 100% the air surrounding the drop must be supersaturated!! This is called the curvature effect

Curvature Effect Curvature effect --> notice that for the droplet to be in equilibrium (evaporation off drop = condensation onto drop), the environment must be supersaturated also notice that the curvature effect is larger for smaller drops this makes sense since smaller drops have more curvature that larger drops

Class activity-Curvature Effect Q: what will happen to a drop 1.9 mm in size that is in a cloud where the RH is 100.05%? Q: what will happen to a drop 1.9 mm in size that is in a cloud where the RH is 100.15%?   

QUESTIONS FOR THOUGHT: 1. At what relative humidity will pure water droplets of the following sizes grow by condensation: a. 10 microns b. 4 microns c. 1 micron 2.  Explain why very small cloud droplets of pure water evaporate even when the relative humidity is 100%.

Solution Droplets Note that the previous discussion is valid for a pure water drop if a droplet is comprised of a solution - it can be in equilibrium with the environment at a much lower RH --> this explains the formation of haze This process of condensation will grow drops , but not to precipitation sizes š 2 mm Q: So, if a droplet grows to some size by condensation, how can it continue to grow to precipitation size???

QUESTION FOR THOUGHT: Haze particles can form when the relative humidity is less than 100%. Are these haze particles pure water droplets or solution droplets? Why?

Collision/Coalescence Collision/Coalescence - cloud droplet growth by collision this is a dominant process for precipitation formation in warm clouds (tops warmer than about -15°C) some cloud droplets will grow large enough and will start to fall in the cloud -->> since the bigger drops fall faster than the smaller drops, they will "collect" the smaller drops - the bigger drop grows droplet fall speed is called its terminal velocity need droplets of different sizes for this process to really work Q: what determines the droplets fall speed relative to the ground??

Droplet Fall Speeds and Droplet Growth Q: what determines the droplets fall speed relative to the ground?? A: droplet size and updraft strength --> given a growing cu with an updraft strength of 4 ms-1: if the particle terminal velocity is -2 ms-1, the particles fall speed is: ANSWER if the particle terminal velocity is -4 ms-1, the particles fall speed is: ANSWER if the particle terminal velocity is -6 ms-1, the particles fall speed is: 2ms-2 0ms-1 2ms-1 downward

Life cycle of a droplet Growth by collision the drop initially forms in the updraft of the cloud near cloud base it grows in size by collisions since Vg = w + Vt Vg = ground relative fall speed of the drop w = updraft velocity Vt = drop's terminal velocity then the drop will begin to fall when Vt > w

Factors promoting growth by collision/coalescence Different drop sizes --> thicker clouds stronger updrafts consider a shallow stratus deck....

Droplet Growth in a Shallow Stratus Deck Often, drops will evaporate from shallow stratus before reaching the ground or you may get drizzle if they are large enough

QUESTION FOR THOUGHT: 1.  Why is a warm, tropical cumulus cloud more likely to produce precipitation than a cold, stratus cloud? 2.  Clouds that form over water are usually more efficient in producing precipitation than clouds that form over land. Why?

Precipitation Growth in Cold Clouds - Warm versus Cold Clouds Our previous discussion regarding droplet growth by condensation and collisions is valid for warm clouds: warm clouds - have tops warmer than about 0°C comprised entirely of water

Cold Clouds old clouds are defined as those clouds with tops colder than 0°C can be comprised of: water super-cooled water - liquid droplets observed at temps less than 0°C ice Notice that super cooled water is found at altitudes where: -40°C < Temp < 0°C only ice is found at altitudes above -40°C Q: So how does frozen precipitation form in cold clouds?

Precipitation Types- Ice Habits

Precipitation Types - Snow Snow - often visible as fall streaks associated with high cirrus Snow Events: Flurries - weak, intermittent - produced from developing Cu Snow squalls - brief, heavy snow fall - produced from Cu Steady Snow - continuous for hours - produced from Nb Blizzard - low temperatures, strong winds, blowing snow... good stuff!!!!!