These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages Precipitation Definitions: Vapor pressure - through motion and collisions every gas exerts pressure, called a partial pressure - - saturation vapor pressure (e sat ) is the maximum vapor pressure that is thermodynamically stable and is related solely to temperature: where T is in °C and e sat is in mb Exp is “exponential”…exp(x) is he same as e x where e in this case is the natural log

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These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages Absolute Humidity (also vapor density) in g/m3 Where Relative Humidity where e a is the actual vapor pressure Sling Psychrometer

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These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages Formation of Precipitation Need: 1. Creation of saturated conditions in the atmosphere Saturated conditions usually arise when air is lifted and cooled growth of small droplets by collision and coalescence until they become large enough to precipitate FOR SIGNIFICANT AMOUNTS OF PRECIPITATION TO OCCUR

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages How do we cool parcels of air?: 1.Radiation to cooler surroundings 2.Mixing 3.Conduction to a cool surface (e.g. cool ocean or snow) 4.Horizontal The above cause fog or drizzle. Significant precipitation storms require: 5. Cooling

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a. Convection b. Convergence thermals

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How does precip actually form? -Need cloud condensation nuclei (CCN) - air can be saturated up to 800% but without nuclei rain won’t happen - - water molecules are attracted to nuclei via hydrogren bonds Droplet Growth In order for droplets to form, droplets must grow so that their fall velocity exceeds uplift, and survive evaporation on the way down Droplet growth occurs by: 1)Collision 2)Ice crystal growth Collision Ice crystal growth -If air is saturated and temperature is <-40C, molecular vibration is so slow that clusters of molecules can spontaneously form ice crystals -Between -40 and 0C, ice cystals can form on CCN templates (CCN with molecular structure similar to ice…often clay minerals)

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages From: Dingman, 1994

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages - at these temperatures water is still nucleating in liquid form as well so that clouds at these temperatures are often a mixture of ice crystals and super-cooled water - because e sat of an ice surface is slightly less than e sat of a water surface for a given temperature, water molecules evaporate from water and condense on ice particles - - whether the precipitation hits the ground as snow or rain depends on the position of the zero degree isotherm Importation of water vapor - - for significant rain to occur there has to be a continual supply of water vapor imported into the cloud to replace that which falls. - this vapor is provided by winds that converge on precipitation producing clouds. -

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CCM3 T170 Cloud and Precipitation Simulation Supercomputer: NEC SX4/8 CPU Time: 55 hours Atmosphere Layer: Troposphere Horizontal Resolution: 75 km

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Measurement of Precipitation Point Measurements Basic goal - to measure the amount of precipitation that would fall on the ground if the instrument measuring precipitation was no there. Instead we try to come up with a number that can be related to reality in some way Point measurements usually measured in depth units (volume of catch divided by cross-sectional area of collector Point measurement devices of two general varieties: non-recording (storage) and recording gages

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages Storage gages simplest are wedge or cylinder-shaped vessels 8” NWS Type Standard Rain Gage

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages Recording gages Two basic types: weighing and tipping bucket weighing Tipping bucket

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages Selecting a gage…sources and reduction of errors: Orifice size and orientation - size should not be less than about 30 mm diameter, go big or stay home - Height and Wind Shielding -Wind eddies cause under-catch of 10% and 30% for unshielded rain and snow gages, respectfully - - pit gages much better than above ground gages for rain, but expensive, can’t be used for snow Distance from Obstructions - no surrounding objects should extend into a conical space defined by a 45 degree centered on the gage -isolated obstructions should not be closer to the gage than twice its height above the gage

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages Splash and Evaporation - if water surface in gage is to close to rim rain can splash out - can be prevented by deep gages, walls that slope out towards bottom, moving the water to a vessel, or emptying gage frequently - -evaporation can be reduced by moving water to a vessel, adding a Layer of oil, and/or removing catch frequently Instrument errors - tipping bucket gages often under record heavy rains and weighing buckets often get less sensitive as they get more full - failures are always possible so good practice to install a standard gage beside a recording gage

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages Observer errors - - can often be detected in networks - compare anomolous records against weather maps, streamfiow records, etc. Differences in observation time Errors due to occult precip - fog drip and rime can be as high as 30% of annual precip in some areas - high elevation areas especially susceptible Low intensity rains -e.g. in U.S. rain measured to nearest 0.25 mm and < 0.13 is trace -

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Precipitation-Gage networks - minimum number of gages per unit area recommended by WMO in U.S. NWS surface monitoring network includes about 300 primary and 8000 cooperative stations - - density can make a large difference in estimates of areal precipitation - density is a trade-off between the cost of adding a gage and the increase in information gained -while both are important, a long record is more critical to accurate determination of annual precip than is a dense network

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Radar -wavelengths between 5 and 10 cm used to image raindrop and snowflakes (hydrometeors) -radar reflectance is influenced by size distibution, number/unit volume, state, and shape of hydrometeors (as well as characteristics of radar system) - good relative tool, but not very accurate because of a) above factors, b) evaporation as rain falls, c) distortion of precipitation field by wind - - however with gage estimates can be used to improve areal estimates Satellite - visible band can tell where clouds are - I.R. can map cloud top heights and identify potential precip cells from this -