Precipitation and Wind AOSC 200 Tim Canty Class Web Site: http://www.atmos.umd.edu/~tcanty/aosc200 Topics for today: Precipitation cont. RADAR Pressure Coriolis force Lecture 12 Oct 5 2017 1 Current Weather http://www.wpc.ncep.noaa.gov/#page=sfc 2
Today s Forecast http://www.hpc.ncep.noaa.gov/national_forecast/natfcst.php 3 Rain: Falling drops of water larger than 0.02 inch in diameter. In forecasts, "rain" usually implies that the rain will fall steadily over a period of time. (See "showers" below). Light rain: Falls at the rate of 0.10 inch or less an hour. Moderate rain: Falls at the rate of 0.11 to 0.30 inch an hour. Heavy rain: Falls at the rate of 0.30 inch an hour or more. Drizzle: Falling drops of water smaller than 0.02 inch in diameter. They appear to float in air currents, but unlike fog, do fall to the ground. Light drizzle: Drizzle with visibility of more than 5/8 of a mile. Moderate drizzle: Drizzle with visibility from 5/16 to 5/8 of a mile. Heavy drizzle: Drizzle with visibility of less than 5/16 of a mile. Showers: Rain that falls intermittently over a small area. The rain from an individual shower can be heavy or light, but doesn't cover a large area or last more than an hour or so. rain is steady and continuous, showers start and stop abruptly and can vary in intensity 4
Snow Flurries: light snow falling for short durations. Little to no accumulation. The most accumulation that can be expected is a light snow dusting. Snow Showers: Snow falling at varying intensities for brief periods of time. Some accumulation is possible, but not guaranteed. Snow Squalls: intense, but limited duration, period of moderate to heavy snowfall, accompanied by strong, gusty surface winds and possibly lightning (generally moderate to heavy snow showers). Snow accumulation may be significant. Blizzards: Winds over 35 mph. Visibility is often near zero. Significant accumulations of snow likely 5 RADAR: RAdio Detection and Ranging RADAR measures the time it takes for the signal to return Fig 5.45: Essentials of Meteorology 6
RADAR: RAdio Detection and Ranging http://www.wunderground.com/blog/jeffmasters/another-wild-night-in-tornado-alley 7 RADAR: RAdio Detection and Ranging Doppler Doppler RADAR measures changes in the return signal Fig 5-22 Meteorology: Understanding the Atmosphere 8
Understanding the Wind Fig 6-1 Meteorology: Understanding the Atmosphere 9 Understanding the Wind To understand the wind we need to understand forces... Isaac Newton started us off in the 17 th Century 10
Understanding Pressure Fig 6.2: Essentials of Meteorology 11 Understanding Pressure Fig 6.3: Essentials of Meteorology 12
Understanding Pressure Fig 6.4: Essentials of Meteorology 13 Understanding Pressure Altitude will influence surface pressure. Pressure decreases the higher you go in the atmosphere You can correct for altitude to determine sea-level pressure Roughly 10mb every 100 meters Fig 6.8: Essentials of Meteorology 14
Combination of Forces 5 major forces control wind: Gravity Pressure Gradient Centrifugal Coriolis Friction 15 Pressure Gradient Force Pressure Gradient Force: due to differences in pressure over a distance Always pushes from high pressure to low pressure Fig 6.10: Essentials of Meteorology 16
Pressure Gradient Force PGF Change in Pressure Distance Fig 6.11: Essentials of Meteorology 17 Pressure Gradient Force Lines of constant pressure are called isobars The closer the isobars the stronger the PGF Fig 6-4 Meteorology: Understanding the Atmosphere 18
Pressure Gradient Force Lines of constant pressure are called isobars The closer the isobars the stronger the PGF Fig 6.12: Essentials of Meteorology 19 Pressure Gradient Force: Isobaric Maps Measure the height where pressure is a specific value In this example, the 500mb height is shown Fig 6.12: Essentials of Meteorology 20
Coriolis Force Fig 6.14: Essentials of Meteorology 21 Coriolis Force When the ball leaves the hand of the thrower it travels in a straight line to the other person When the platform is rotating the ball follows exactly the same path The catcher sees the ball move away from him as it travels across the platform 22
Coriolis Force In the Northern Hemisphere, Coriolis forces always deflect to the RIGHT Depends on latitude. Coriolis forces are ZERO at the equator, maximum at the poles. 23 Coriolis Force Fig 6-11 Meteorology: Understanding the Atmosphere 24
Coriolis Force The closer to the pole, the stronger the coriolis force Also depends on how fast the object is moving Fig 6-15 Meteorology: Understanding the Atmosphere 25 Coriolis Force CF f V f is proportional to sin(latitude) Fig 6-11 Meteorology: Understanding the Atmosphere 26