Precipitation Definitions: Vapor pressure

Precipitation Definitions: Vapor pressure - through motion and collisions every gas exerts pressure, called a partial pressure - - - saturation vapor pressure (esat) is the maximum vapor pressure that is thermodynamically stable and is related solely to temperature: where T is in °C and esat is in mb Exp is “exponential”…exp(x) is he same as ex where e in this case is the natural log 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

Saturation vapor pressure is slightly less over ice 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 ea is the actual vapor pressure Sling Psychrometer 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

Relative humidity temperature dependant. Warmed air as increased capacity to hold moisture 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

Dew Point Temperature to which the parcel has to be cooled to reach saturation Where 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. 2. 3. 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?: • Radiation to cooler surroundings • Mixing • Conduction to a cool surface (e.g. cool ocean or snow) • Horizontal • The above cause fog or drizzle. Significant precipitation storms require: • 5. Cooling 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

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a. Convection thermals b. Convergence 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

c. Frontal lifting 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

d. 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 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: • Collision • 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 esat of an ice surface is slightly less than esat 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. - 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

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

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 - 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-Gage networks • - minimum number of gages per unit area recommended by • WMO in 1974 • -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 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

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 • - 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