Transcript PowerPoint
Formation of Precipitation Requires • Cooling of air to dew point temperature (requires a lifting mechanism) • Condensation of water vapor onto nuclei (dust, ions) to form droplets • Growth of droplets so that – a) terminal velocity updraft velocity – b) sufficient mass of liquid to survive evaporation on way down • Importation of water vapor into cloud to replace precipitation and sustain process Lifting Mechanisms Three meteorological situations lead to vertical uplift of air masses: • uplift due to convergence – Nonfrontal convergence of air masses with equal temperature to a low pressure point (i.e. at ITCZ due to convergence of NE/SE tradewinds). Generates moderate rainfall over long duration. – Frontal convergence of air masses of different air temperature. Produces cold fronts/warm fronts. • uplift due to convection • uplift due to orography Uplift due to convergence • warm front - Occurs when warm air impinges on cold. Two air masses do not mix. Warm moist air is less dense, rises over cold air at relatively gentle slope. Warming occurs gradually resulting in more moderate storms which last longer. See high clouds first. • cold front - Cold air impinges on warm air. Again do not mix but cold air moves under warm forcing it upward. Get a steeper sloped interface. Rapid cooling, stronger storms of shorter duration. See low clouds first. Uplift due to convection • Convective cells are initiated by heating of lower air mass by ground surface. • Heating of the lower air mass causes instability of air column because of density differentials ( T, ). • Lower air density rises and cools and condenses (releases heat sustains process) leads to thunderstorms. High intensity, short duration events which occur mainly in the tropics. • Typically originate over land mass in central Florida during summer when ground heats rapidly during the day. Uplift due to orography • Occurs when air mass is forced to rise over air obstruction such as a mountain. • Pronounced on central west coast of N. America where moist winds off the Pacific hit series of mountain ranges parallel to coast. • In most regions of world mean precipitation increases with elevation. Condensation/Nucleation Mechanisms • Initiation of condensation typically requires a seed or condensation nucleus around which the water molecules can attach. • Impurities in the air (dust, salt, ions, ice crystals, volcanic material, smoke, clay) act like catalysts for condensation to occur (cloud condensation nuclei - CCN) • Without nuclei, condensation rates will be very low even for e 4es. Air usually contains lots of particles that can act as nuclei ( 10-4 mm, attract H2O via H bonds) so get condensation at e es. Droplet growth • Before falling, condensed droplets must grow to size and weight capable of overcoming – (1) updraft velocities in the cloud and – (2) evaporation. • Growth occurs by coalescence as raindrops collide on the way down. Big drops fall faster than little ones so they catch up, hit them and absorb them. Importation of Water Vapor • Concentration of liquid water and/or ice in most clouds is in the range of 0.1 to 1 g/m3. Even if all this water in a very tall cloud were to fall as rain the total depth of precipitation would be small. – (10,000 m tall cloud) * (0.5 g/m3) = 5000 g/m2 = 0.5 cm per unit area • Final requirement for occurrence of significant amount of rainfall is that a continuous supply of water vapor must be imported into cloud to replace what falls out. • Inflow of moisture is provided by winds that converge on rain producing clouds. Rainfall Measurement • Most accurate - Weighing recording gage which continuously collects rainfall and records weight over time. $$$$ • Least accurate - Standard rain gage. Measures accumulated depth at a point. Get only volume rain since last reading accuracy 1/10th inch evaporation problems. • Most common - Tipping bucket rain gage. Records number of tips of bucket with known volume over time. Intermediate cost and accuracy. Often under-records during heavy rainfall events. Analysis of Rainfall Data • Storms are classified by “exterior” and “interior” characteristics. • Exterior characteristics define general storm properties, i.e. – – – – a) total storm depth b) stormduration c) time between storms d) areal extent These characteristics are generally accepted to be probabilistic in nature. • Interior characteristics refer to time and space distribution of a particular storm, – a) hyetograph - plot of rainfall depth or intensity vs. time intensity or depth time – b) cumulative hyetograph - sum of rainfall depth vs. time % cumulative rainfall or inches cumulative rainfall time Max. intensity (depth) recorded in a given time interval is an index of storm intensity, as interval , max. intensity .. c) isohyetal maps - contour map showing lines of equal rainfall depth 4 6 8 4 2 An important design criteria for flood control: average depth of rainfall over an area. • To get areal measurements typically must interpolate rainfall measurements always taken at a point or several points in a drainage basin over the entire basin • Two accuracy problems: – how accurate are point measurements? – how accurately can point measurements be converted to areal estimates? • Long term studies have shown that errors due to evaporation, wind currents, obstructions, and reading errors in point rainfall measurements vary from 5% to 15% for long-term data and as high as 75% for individual storms: