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Clouds, Precipitation and Radar How do clouds and precipitation form? Cloud Formation Clouds: Visible products of condensation or deposition of water vapor within the atmosphere • Likely as air nears saturation, but occur in unsaturated air • In pristine air, cloud droplets will not form unless RH>340%!!! • But… Condensation § The process by which water vapor changes to a cloud droplet • Examples of cloud condensation nuclei (CCN)include: Dust Salt Smoke * § These CCN are solid matter (aerosols) in the atmosphere § Water vapor molecules may ‘stick’ to condensation nuclei and grow (billions) to eventually form cloud droplet. Terminal Velocity: speed at which gravitational and resistance forces balance Collision-‐Coalescence Process • Large droplets fall faster than small ones • Large droplets collide and consume small droplet and increase in size • Larger drop grows via coalescence (sticking) • Falls when terminal velocity exceeds updraft speed Bergeron Processes (cold clouds) • Saturation vapor pressure less over ice less than over water. • Growth of ice crystals at expense of water droplets. • Collision-‐Coalescence process further contribute Cloud Seeding Hygroscopic nuclei (water loving) Clouds form more readily in their presence (sub saturated air) • • Example: magnesium chloride in ocean spray can promote condensation at relative humidity as low as 70% Man-‐made nuclei for cloud seeding purposes?? Many sources exist in urban-‐industrial areas • Cloud/precipitation max downwind of urban areas (summer) Fog: Ground Hugging Cloud Fog: Ground Hugging Cloud Clouds as Story Tellers • Atmospheric moisture • Wind direction/shear • Stability Cloud Formation • Main process: expansional cooling due to uplift • Stable atmospheres: – gentle, large scale rising air motion-‐-‐stratiform • Unstable atmospheres: – vigorous rising air, concentrated updrafts surrounded by larger cloud-‐free areas of sinking air-‐-‐cumuliform • Fog: cooling process can be radiational cooling or advection across a cold surface—stratiform—lifting not required, but cooling is. Cloud Classification • Clouds classified according to: – Height—defined by altitude of cloud base. • High—Cirro • Middle—Alto • Low—Strato – Form—determined by stability and altitude • Cirriform—high altitude, ice crystals • Stratiform—stable atmosphere, flat, layered, featureless • Cumuliform—unstable, vertical development – Precipitating: “nimbus” Atmospheric Optics: Refraction Columnar Ice Crystals This type of refraction produces a 22-‐degree halo centered on the Sun or Moon. This type of refraction produces a 46-‐degree halo centered on the Sun or Moon Sundogs 22∘ Caused by cirrus clouds composed of large plate-‐like ice crystals Rainbow • Caused by a combination of refraction and reflection of sunlight by raindrops • Solar ray is refracted on entering a raindrop, reflected internally, then refracted when exiting • Sun must be shining, no higher than 42° above horizon and to your back Noctilucent clouds • • • • Form in the upper mesosphere above an altitude of about 80 km Seldom observed, only at high latitudes Key to formation is exceptionally low temperatures Water vapor from volcanic eruptions or chemical reactions involving methane Forms of Precipitation • Drizzle: Diameters <0.5mm, mainly stratus clouds and fog • Rain: 0.5-‐6mm, fall as flat sphere’s not raindrops, nimbostratus • Freezing Rain: Melts and refreezes upon impact falling through inversion • Sleet: Melts and refreezes before reaching ground • Snow: 5-‐10mm aggregation of ice crystals Weather Radar: Locating Precipitation • • • • Emits short pulses of microwave energy with wavelengths of 10-‐11.1 cm Radar pulses scattered readily by rain, snow, hail, not significantly by cloud droplets Radar knows the exact speed of transmission and after sending out radiation waits to see the fraction of energy reflected back to radar (measured in dBZ) Radar scans 360-‐degree every 5-‐minutes across different elevation angles Base Reflectivity • • • • • Lowest elevation scene Best for precipitation reaching the ground Limited in mountainous regions Not as good further away from radar No good beyond 200 miles from radar Composite Reflectivity • MAXIMUM of different elevation scans made at slightly different times • Can provide additional information about vertical development and locations of virga Radar holes > 3000 above ground level Measuring Precipitation Weighing-‐bucket rain gauge • Records accumulated weight over time • May melt frozen precipitation with antifreeze or heater Tipping-‐bucket rain gauge • Free-‐swinging container partitioned into 2 compartments like a see-‐ saw Standard Gauge 8” gauge Weighing-‐bucket Rule of thumb: 1% loss for every 1 mph increase in wind speed, or 2.2% for every 1 m/s increase Wind Shields Single alter Double alter Fencing Hot plate Measuring Snow Liquid water content ratios typically 10:1 Can be 30:1 in dry/cold conditions Dendrite Growth Zone 10:1 30:1 15:1 Thunderstorm Life Cycle © AMS Thunderstorm Life Cycle Towering Cumulus Stage • Produced by convection within the atmosphere – Free convection: diurnal solar heating of Earth’s surface (not usually enough for strong thunderstorms) – Forced convection: frontal/orographic lift or converging winds – Latent heat released during condensation adds to buoyancy Thunderstorm Life Cycle Towering Cumulus Stage Cumulus Congestus Updrafts still developing Cumulonimbus Downdrafts develop adjacent to updraft Entrained dry air at edge of cloud leads to evaporative cooling (entrainment) • Weakens the uplift and strengthens the downdraft • Magnitude of dewpoint depression in descending branch determines the strength of the downburst AND drop in temperature • Colder air is also more dense and adds to downburst strength Thunderstorm Life Cycle Mature Stage • Upward billowing cumulonimbus clouds reach the tropopause, spreading into a flat anvil top – Cloud tops during mature stage can build to altitudes above 60,000 ft Thunderstorm Life Cycle Dissipating Stage • Precipitation and the downdraft spread throughout the thunderstorm cell • Elimination of updraft cuts off the supply of moisture • Adiabatic compression warms the subsiding air, relative humidity below cloud base decreases Thunderstorms: Where and When Conditions for thunderstorms development – Humid air in low-‐ to mid-‐troposphere – Atmospheric instability – A source of uplift Where and When Average annual number of thunderstorm days in the U.S. Severe Thunderstorms • Criteria – Generates surface winds >58 mph and/or hailstones > 1” – May produce flash floods or tornadoes • Greater altitude of the thunderstorm top, more likely it will produce severe weather • Key ingredient: Vertical wind shear – Favors vigorous updrafts, great vertical cloud development – Best for sustaining influx of warm, humid air © AMS Thunderstorm Hazards Lightning • Electrical discharge due to large differences in charge • Lightning forges a path between oppositely charged regions • Updrafts bring water to top-‐cloud, freezes + develops positively charged cloud top, ground is typically negatively charged T<5F, graupel is -‐ charge, ice crystal is + charge T>5F, graupel is + charge, ice crystal is + charge Negative and Positive Strikes • Negative strikes (net transfer of negative charges from the cloud to the ground) account for 95% of strikes. • Positive charges tend to last longer + are reported to cause more damage (e.g., wildfire, electrical infrastructure) • Positive strikes preferentially occur severe t-‐storms and colder thunderstorms (thundersnow) Global Lightning Frequency Thunderstorm Hazards Hail • Develops when ice pellet travels vertically through cumulonimbus clouds coated by alternating layers of glaze and rime • Falls out of cloud base when too large and heavy for updrafts • Size of hail stone is proxy for updraft strength Tornadoes Ingredients: 1. Warm Moist Unstable Air 2. Strong Vertical Wind Shear 3. Additional uplift mechanism 4. Nearby trailer park Tornadoes • Violently rotating column of air in contact with the ground o 10% of the annual 10,000 U.S. severe thunderstorms produce tornadoes o Usually track from SW to NE o Strong horizontal air pressure gradient responsible © AMS Tornadoes ~1300/year in the US Peak activity in May-‐June • • Greater instability of the lower atmosphere Favorable synoptic weather conditions: well-‐defined polar front and intense cyclones Lack of warm moist air and topography limit tornado activity in the West Most Tornadoes in ID/ western WA/OR are late spring phenomena
