Chapter 5: Cloud Development and Precipitation Atmospheric Stability Atmospheric Stability Determining stability Determining stability Cloud development and stability Cloud development and stability Precipitation processes Precipitation processes Precipitation types Precipitation types Measuring precipitation Measuring precipitation

Atmospheric Stability Q1: Why does the air rise on some occasions and not on others? Q2: Why does the size and shape of clouds vary so much when the air does rise?

Atmospheric Stability stable and unstable equilibria stable and unstable equilibria air parcels : rising air air parcels : rising air parcel expands and cools parcel expands and cools adiabatic process adiabatic process dry adiabatic lapse rate dry adiabatic lapse rate (9.8 C/km) (9.8 C/km) moist adiabatic lapse rate (less than 9.8 C/km) moist adiabatic lapse rate (less than 9.8 C/km) Stability does not control whether air will rise or sink. Rather, it controls whether rising air will continue to rise or whether sinking air will continue to sink.Stability does not control whether air will rise or sink. Rather, it controls whether rising air will continue to rise or whether sinking air will continue to sink.

Determining Stability Stable condition: If a rising parcel’s T is colder than its environment, it is denser and would sink back. Unstable condition: If the rising parcel’s T is warmer, it is less dense and will continue to rise

A Stable Atmosphere environmental lapse rate environmental lapse rate absolute stability absolute stability stabilizing processes stabilizing processes surface cooling; surface cooling; air aloft warming (e.g., subsidence inversions) air aloft warming (e.g., subsidence inversions) Stable air provides excellentStable air provides excellent conditions for high pollution levels. conditions for high pollution levels.

An Unstable Atmosphere absolute instability absolute instability destabilizin processes destabilizin processes warming of surface air warming of surface air superadiabatic lapse rates (> 9.8 C/km) superadiabatic lapse rates (> 9.8 C/km) Unstable air tends to be well-mixed.Unstable air tends to be well-mixed.

Conditionally Unstable Air dry and moist adiabatic lapse rates dry and moist adiabatic lapse rates Conditional instability: Conditional instability: environmental lapse rate environmental lapse rate between dry and moist between dry and moist lapse rates lapse rates Condensation level Condensation level cloud base cloud base

Cloud Development and Stability Layered clouds tend to form in a stable atmosphere; whereas cumuliform clouds tend to form in a Conditionally unstable atmosphere

Cloud Development and Stability surface heating and free convection surface heating and free convection uplift along topography uplift along topography widespread ascent widespread ascent lifting along weather fronts lifting along weather fronts

Convection and Clouds thermals thermals fair weather cumulus fair weather cumulus Fair weather cumulus provide a visual marker of thermals.Fair weather cumulus provide a visual marker of thermals. Bases of fair-weather cumulus clouds marks the lifting condensation level, the level at which rising air first becomes saturated.Bases of fair-weather cumulus clouds marks the lifting condensation level, the level at which rising air first becomes saturated.

Topography and Clouds orographic uplift orographic uplift rain shadow rain shadow The rain shadow works for snow too. Due to frequent westerly winds, the western slope of the Rocky Mountains receives much more precipitation than the eastern slope.The rain shadow works for snow too. Due to frequent westerly winds, the western slope of the Rocky Mountains receives much more precipitation than the eastern slope.

Precipitation Processes

Collision and Coalescence Process Warm clouds (above freezing T) Warm clouds (above freezing T) terminal velocity terminal velocity large drops fall faster than large drops fall faster than small drops small drops coalescence: coalescence: the merging of cloud the merging of cloud droplets by collision droplets by collision

Ice Crystal Process cold clouds (ice crystals and liquid drop coexist) cold clouds (ice crystals and liquid drop coexist) supercooled water droplets supercooled water droplets water drops below freezing T water drops below freezing T saturation vapor pressures over liquid water is higher than over ice saturation vapor pressures over liquid water is higher than over ice Accretion Accretion ice crystals grow by colliding with supercooled water droplets to form graupel (or snow pellets) ice crystals grow by colliding with supercooled water droplets to form graupel (or snow pellets)

Fig. 5-22, p. 124

Precipitation in Clouds accretion accretion ice crystal process ice crystal process

Cloud Seeding and Precipitation cloud seeding cloud seeding inject (or seed) a cloud with small particles that will act as inject (or seed) a cloud with small particles that will act as nuclei so that the cloud particles will grow large enough to nuclei so that the cloud particles will grow large enough to fall to the surface as precipitation fall to the surface as precipitation silver iodide: as cloud-seeding agent because it silver iodide: as cloud-seeding agent because it has a crystalline structure similar to an ice crystal has a crystalline structure similar to an ice crystal It is very difficult to determine whether a cloud seeding attempt is successful. How would you know whether the cloud would have resulted in precipitation if it hadn’t been seeded?It is very difficult to determine whether a cloud seeding attempt is successful. How would you know whether the cloud would have resulted in precipitation if it hadn’t been seeded?

Precipitation Types

Rain Rain: drop diameter of 0.5 ~ 6mm Rain: drop diameter of 0.5 ~ 6mm Drizzle: drop diameter < 0.5mm Drizzle: drop diameter < 0.5mm Virga: rainfall not reaching surface Virga: rainfall not reaching surface shower shower small to large medium droplet

Snow Snow Snow Fallstreaks: ice crystals and snowflakes from high cirrus clouds that usually do not reach surface Fallstreaks: ice crystals and snowflakes from high cirrus clouds that usually do not reach surface Dendrite: Snowflake shape depends on both temperature and relative humidity Dendrite: Snowflake shape depends on both temperature and relative humidity Blizzard: low T and strong wind bearing large amounts of snow, reducing visibility to 1 few meters Blizzard: low T and strong wind bearing large amounts of snow, reducing visibility to 1 few meters

Sleet and Freezing Rain Sleet: Sleet makes a ‘tap tap’ sound when falling on glass Sleet: Sleet makes a ‘tap tap’ sound when falling on glass freezing rain freezing rain rime rime

Snow Grains and Snow Pellets snow grains: snow equivalent of drizzle snow grains: snow equivalent of drizzle snow pellets: larger and bounce on surface; formed snow pellets: larger and bounce on surface; formed as ice crystals collide with supercooled water droplets as ice crystals collide with supercooled water droplets Graupel: Graupel: when snow pellets accumulate a heavy coating of rime, they are called graupel when snow pellets accumulate a heavy coating of rime, they are called graupel

Hail updraft cycles updraft cycles accretion accretion A hailstone can be sliced open to reveal accretion rings, one for each updraft cycle.A hailstone can be sliced open to reveal accretion rings, one for each updraft cycle.

Measuring Precipitation

Instruments standard rain gauge: 0.01 inch interval ; trace standard rain gauge: 0.01 inch interval ; trace tipping bucket rain gauge: used in ASOS tipping bucket rain gauge: used in ASOS It is difficult to capture rain in a bucket when the wind is blowing strongly.It is difficult to capture rain in a bucket when the wind is blowing strongly.

Doppler Radar and Precipitation Radar Radar radio detection radio detection and ranging and ranging Doppler radar Doppler radar provide precip. provide precip. area and intensity area and intensity as well as as well as horizontal speed of horizontal speed of falling rain falling rain

Fig. 5-40a,b, p. 136