Class #4: Stability, cloud development, and precipitation Chapters 6 and 7 1Class #4 Monday, July 12, 2010

Stability & Cloud development Chapter 6 2Class #4 Monday, July 12, 2010

Fig. 6-CO, p. 140

Fig. 6-1, p. 142

Importance of Clouds Release heat to atmosphere Help regulate energy balance Indicate physical processes 5Class #4 Monday, July 12, 2010

Atmospheric Stability Clouds from as air rises and cools Adiabatic processes: change in temperature without giving or removing – Dry rate = 10°C/1000m – Moist rate = 6°C/1000m Stability is a state of equilibrium in terms atmospheric movement; no vertical movement occurs 6Class #4 Monday, July 12, 2010

Determining Stability Warm air rises or is unstable Cool air sinks or is stable Compare air parcel lapse rate to environmental lapse rate 7Class #4 Monday, July 12, 2010

8

Table 6-1, p. 143

Determining Stability Stable environment – Environmental lapse rate less than moist lapse rate – If an air parcel is forced it will spread horizontally and form stratus clouds – Usually a cool surface (radiation, advection) – Inversion: warm over cool. 10Class #4 Monday, July 12, 2010

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Fig. 6-6, p. 145

Determining Stability Special Topic: Subsidence Inversions – Strong subsidence exacerbates air pollution due to the lack of vertical motion. – Pollution is not diluted. 15Class #4 Monday, July 12, 2010

Determining Stability An Unstable Atmosphere – Environmental lapse rate greater than the dry adiabatic lapse rate – As air parcel rises it forms a vertical cloud – Convection, thunderstorms, severe weather 16Class #4 Monday, July 12, 2010

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Determining Stability A Conditionally Unstable Atmosphere – Moist adiabatic lapse rate is less than the environmental lapse rate which is less than the dry adiabatic lapse rate – Stable below cloud unstable above cloud base – Atmosphere usually in this state 18Class #4 Monday, July 12, 2010

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Fig. 6-13, p. 149

Fig. 6-14, p. 149

Fig. 1, p. 150

Determining Stability Causes of Instability – Cool air aloft (advection, radiation cooling in clouds) – Warming of surface (insolation, advection, warm surface) 27Class #4 Monday, July 12, 2010

Cloud Development Clouds develop as an air parcel rises and cools below the dew point. Usually a trigger or process is need to initiate the rise of an air parcel. 28Class #4 Monday, July 12, 2010

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Cloud Development Convection – Differential land surface heating creates areas of high surface temperature. – Air above warm land surface heats, forming a ‘bubble’ of warm air that rises or convection. – Cloud base forms at level of free convection. 30Class #4 Monday, July 12, 2010

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Stepped Art Fig. 6-16, p Class #4 Monday, July 12, 2010

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Fig. 2, p. 155

Cloud Development Topography – Orographic uplift – Orographic clouds – Windward, leeward, rain shadow – Lenticular clouds 37Class #4 Monday, July 12, 2010

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Cloud Development Topic: Adiabatic charts – Adiabatic charts show how various atmospheric variables change with height: pressure, temperature, humidity. 42Class #4 Monday, July 12, 2010

Fig. 3, p. 158

Fig. 4, p. 158

Fig. 5, p. 158

Fig. 6, p. 159

Fig. 7, p. 159

Cloud Development Changing cloud forms – Stratus clouds can change to cumulus clouds if the top of the cloud cools and the bottom of the cloud warms. – Alto cumulus castellanus: towers on alto stratus – If moist stable air without clouds is mixed or stirred it can form stratocumulus clouds. 48Class #4 Monday, July 12, 2010

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Precipitation Chapter 7 54Class #4 Monday, July 12, 2010

Fig. 7-1, p. 166

Fig. 7-2, p. 166

Fig. 7-3, p. 167

Table 7-1, p. 168

Precipitation Processes Precipitation is any form of water that falls from a cloud and reaches the ground. How do cloud drops grow? – When air is saturated with respect to a flat surface it is unsaturated with respect to a curved droplet of water. Super saturated 59Class #4 Monday, July 12, 2010

Precipitation Processes Collision & Coalescence – Droplets of different sizes collide and coalesce into larger droplets; warm cloud process – Ice-Crystal Process Cold clouds a mixture of ice & water Ice crystals grow at expense of surrounding water droplets Saturation vapor pressure greater over water as compared to ice. 60Class #4 Monday, July 12, 2010

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Stepped Art Fig. 7-5, p Class #4 Monday, July 12, 2010

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Fig. 1, p. 171

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Fig. 7-10, p. 173

Precipitation Processes Topic: Freezing of Cloud Droplets – Spontaneous or homogeneous freezing – Ice embryo 73Class #4 Monday, July 12, 2010

Precipitation Processes Cloud Seeding – Inject cloud with small particles that act as condensation nuclei, starting the precipitation process. – NEED CLOUDS: seeding does not generate clouds – Cold clouds with a low seed ration best – Dry ice, silver iodide 74Class #4 Monday, July 12, 2010

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Stepped Art Fig. 7-12, p Class #4 Monday, July 12, 2010

Precipitation in Clouds Starts quickly Most Precipitation formed through accretion Many times rain starts as ice 78Class #4 Monday, July 12, 2010

Fig. 7-12, p. 174

Stepped Art Fig. 7-12, p. 174

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Precipitation Types Rain: falling drop of liquid water – Drizzle less than 0.5 mm – Virga – Cloudburst Snow: frozen water falling from sky (crystal or flake) – Most precipitation starts as snow – Freezing level, snow & cloud appearance, fall streaks, drifting snow, blizzard – A blanket of snow is a good insulator 83Class #4 Monday, July 12, 2010

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Table 7-2, p. 176

Fig. 2, p. 177

Fig. 7-16, p. 178

Fig. 7-17, p. 178

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Table 7-3, p. 178

Fig. 3, p. 179

Fig. 7-18, p. 180

Table 7-4, p. 180

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Precipitation Types Topic: Tear Drops – Raindrops not tear shaped – Shape is size dependent Less than 2 mm = sphere Greater than 2 mm = flattened sphere 98Class #4 Monday, July 12, 2010

Precipitation Types Topics: Sounds and snow – A blanket of snow will act like an acoustic tile and absorb sound waves. Topics: Snow with Temperature above Freezing – Unsaturated wet bulb temperature below or equal to 0°C, rain cooled by evaporation forms snow despite environmental temperature above freezing. 99Class #4 Monday, July 12, 2010

Precipitation Types Sleet: air below freezing, then travels through a layer of air above freezing, begins to melt and then falls through a layer of air below freezing just above the ground surface. Freezing Rain: ground surface is freezing as rain hits the surface it freezes. 100Class #4 Monday, July 12, 2010

101Class #4 Monday, July 12, 2010

Fig. 7-21, p. 182

Fig. 7-22, p. 182

Fig. 7-23, p. 182

Precipitation Processes Precipitation is any form of water that falls from a cloud and reaches the ground. How do cloud drops grow? – When air is saturated with respect to a flat surface it is unsaturated with respect to a curved droplet of water. Super saturated 105Class #4 Monday, July 12, 2010

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Precipitation Types Observation: Aircraft Icing – Aviation hazard is created by the increase in weight as ice forms on the body of the airplane. – Spray plane with anti-freeze. 110Class #4 Monday, July 12, 2010

Precipitation Types Snow Grains: solid equivalent of drizzle, no bounce or shatter Snow Pellets: larger than grains, bounce, break, crunch underfoot Graupel: ice particle accumulation with rime Hail: graupel act as embryo in intense thunderstorm, grow through aggregation as pushed up by updraft. 111Class #4 Monday, July 12, 2010

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Stepped Art Fig. 7-29, p Class #4 Monday, July 12, 2010

Measuring Precipitation Instruments – Rain gauge: standard, tipping bucket, weighing Snow: average depth at 3 locations, 10:1 water equivalent Doppler Radar – Transmitter generates energy toward target, returned energy measured and displayed Brightness of echo = amount/intensity of rain – Doppler: measures speed of horizontal rain 118Class #4 Monday, July 12, 2010

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Stepped Art Fig. 7-33, p Class #4 Monday, July 12, 2010

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Measuring Precipitation Measuring from space – Specific satellites designed to assess clouds, atmospheric moisture, and rain TRMM CloudSat 125Class #4 Monday, July 12, 2010

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