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
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
es(T) (T1,e1) Adiabatic expansion e Radiative Cooling saturated Mixing (T2,e2) unsaturated T Concepts PHYS 622 - Clouds, spring ‘04, lect. 2, Platnick
Air and water vapor T T Water Saturation Vapor Pressure (Clausius-Clapeyron equation) 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 (2.5106 J/kg), es(Ttr) = 611Pa (or 6.11 mb). Rv is the specific gas constant for water vapor (461.5 J-kg1-K1). PHYS 622 - Clouds, spring ‘04, lect. 2, Platnick
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. PHYS 622 - Clouds, spring ‘04, lect. 2, Platnick
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
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 • remain suspended in the air • 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
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). PHYS 622 - Clouds, spring ‘04, lect.4, Platnick
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??
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:
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 - 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!!!!!