Transcript Lecture 7
Meteo 3: Chapter 7 Analyzing weather above Earth’s surface Read: pp. 251-277 (ignore confluence) Why Care about Upper-Air? Talked about surface pressure and what winds we expect to see at the surface Lots of atmosphere not at the surface that affects weather conditions at the surface! Need to have ways to observe and analyze the atmosphere kilometers from the surface Maps of weather above the surface Upper-air weather conditions plotted on maps of constant pressure Data obtained from radiosondes: heights, temperatures, wind speeds and directions, humidity Constant Pressure Surfaces Simplifies math All upper-air maps plotted on constant pressure surfaces Mandatory pressure levels: pressures where radiosondes always take observations Pressure decreases faster with height closer to ground A 500 mb chart- Height contours, isohypses, or heights = isopleths of equal height Density decreases with height, so a 150 mb layer must be thicker higher in the atmosphere A volume of air will become larger when heated Pressure decreases more rapidly with height in cold air columns than warm air columns Warm columns of air taller than cold columns A 500 mb chart- Height contours, isohypses, or heights = isopleths of equal height Temperatures near the Surface Forming upper-level troughs/ridges Cold air masses move south, warm air masses move north…forming upper-level troughs/ridges Relationship between heights and pressure On a constant pressure map, a minimum in height corresponds to a low pressure center on a constant height surface on an altitude equal to that height – Treat a center of low heights on a constant pressure surface as if it were a center of low pressure On a constant pressure map, a maximum in height corresponds to a high pressure center on a constant height surface at an altitude equal to that height – Treat a center of high heights on a constant pressure surface as if it were a center of high pressure Height gradients α pressure gradients A contour map of pressure on a constant height surface looks the same as a contour map of heights on a constant pressure surface. Therefore, the relationship between the wind and pressure fields is the same as the relationship between the wind and height field. Upper-level winds: Geostrophic approximation Forces on parcel: pressure (height) gradient force….Coriolis force increasing in magnitude until it becomes equal in magnitude but opposite in direction to PGF…wind blows parallel to height contours with lower heights to left PGF 5400 m 5460 m COR 5520 m A 500 mb chart- with upper-air wind observations Geostrophic wind (wind aloft) Geostrophic wind: Wind that results due to a balance between the height gradient and Coriolis force – Good approximation to wind above the ground – There is some friction to throw this balance off, but its effects are minimal at high altitudes – Near the ground, friction causes wind to cross isobars toward lower pressure at ~30º angle Upper-level wind speed & Jet Stream Horizontal temperature gradients α height gradients AND the larger the height gradient, the faster the wind speed Wind speeds increase with altitude up to about 250 mb because height gradient on constant pressure surface increases with altitude Jet Stream When warm and cold air masses collide, the strongest winds occur just below the tropopause (the top of the troposphere, about 250 mb) This fast flowing river of air over mid-latitudes = midlatitude jet stream Subtropical jet? 250mb Heights and Wind Obs More on jet streams Embedded in mid-latitude “westerlies” Only several hundred kilometers wide, thousands of km long Discontinuous Sharp surface front underneath jet stream Moves south in winter, north in summer Stronger in winter than summer Jet Streak: Pocket of faster winds embedded in the jet stream – Located in regions with enhanced height gradients at ~ 250 mb Summer and winter jets high-amplitude pattern (major storms possible); “meridional pattern” low-amplitude pattern (no major storms); “zonal pattern”