Vertical Structure of Atmosphere

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1 ATMOS 3110 Introduction to Atmospheric Sciences Distribution of atmospheric mass and gaseous constituents Because of the earth s gravitational field, the atmosphere exerts a downward forces on the earth surface. Pressure = (Weight of the atmosphere) / (Area) Prof. Zhaoxia Pu Department of Atmospheric Sciences University of Utah August 26, 2009 The mean atmospheric pressure averaged over the earth surface: M g / 4πR 10 Pa M g R 0 E A A 0 = = 9. 8ms 2 5 E 2 18 kg 6 = km Total mass of the atmosphere The mean gravitational acceleration The mean radius of the earth Density = Mass/volume The standard sea level pressure value for atmospheric pressure is mb = hpa = in. Hg. (inches of mercury) Vertical profiles of pressure and density The vertical variability of pressure and density is much larger than the horizontal and temporal variability of these quantities. log[ p( z)] log[ p( 0)] Bz Standard atmosphere represents the horizontal and time averaged structure of the atmosphere as a funct ion of height only. Within the lowest 100km, Let H=1/(2.3B): p( z) P( 0) exp( z / H) ρ( z) ρ( 0) exp( z / H) H ~ 7km Scale height of the atmosphere Vertical Structure of Atmosphere Vertical temperature profile shows different layers in the lower atmosphere Troposphere: temperature decreases with height at about 6.5 o C per km of ascent, weather occurs in this layer, which extends from the surface to about 12 km Stratosphere: temperature is initially isothermal (i.e. constant) with height, then increases with height to about 50 km Mesosphere: temperature decreases with height, to about 80 km Thermosphere: above 80 km, temperature increases once again with height 1

2 Vertical Structure of Atmosphere Ionosphere Boundaries between layers are tropopause, stratopause and mesopause There are thus 3 regions of relative warmth: Earth's surface, stratopause and above 80 km Ionosphere is not actually a layer, but an electrified region where ions and free electrons exist Plays a major role on radio communications D region: reflects AM signals, thus enabling transmission over large distances on Earth Radio Wave Reflections Variability of vertical T profile Within the troposphere T decrease with latitude, with the latitudinal gradient being about twice as steep in the winter hemisphere as in the summer Hemisphere. The tropopause is considerably higher and colder over the tropics than over the polar regions. The summer hemisphere is characterized by a cold equator and warm pole, while the winter hemisphere displays a distinct temperature maximum over Middle latitude. Sudden warming of stratosphere 2

3 Zonal wind and temperature Global wind system The seasonal variation of temperature is strongly influenced by the distribution of land and sea. Temperature displays a pronounced diurnal variability in certain regions of the atmosphere. Much of the equator to pole temperature gradient tends to be concentrated in narrow bands called frontal zones. Longitudinally averaged zonal winds Time-height section of zonal wind in the vicinity of the equator based on monthly averaged data The longitudinally averaged zonal circulation of the tropical stratosphere is characterized by a strong semi-annual (half yearly) oscillation at upper levels and a more irregular quasibiennial (nearly, but not quite, two yearly) oscillation at lower levels. 3

4 Tropospheric winds at middle and high latitude Mid-latitude circulations: Geostrophic balance Mean sea level pressure, wind speed at 850 hpa and geopotential 500 hpa --- N. H. The winds at middle and high latitude tend to blow parallel to the isobars or height contours. At any given latitude the speed of the wind tends to be inversely proportional to the spacing of the isobars or height contour. To explain mid-latitude horizontal wind patterns at some arbitrary altitude z above 1 km, need 2 facts: 1. Pressure at a fixed altitude tends to decrease from equator to poles; 2. Winds obey the geostrophic approximation. In addition, surface temperatures and atmospheric temperatures at a fixed altitude decrease with increasing latitude Mean sea level pressure, wind speed at 850 hpa and geopotential 500 hpa --- S. H. Jan/Feb sea level pressure

5 Tropospheric winds at low latitudes Easterly wave : the low-level circulation over the Atlantic and Pacific Oceans is dominated by the easterly flow around the equatorward flank of the subtropical high-pressure belts near 30 degree North and 30 degree South. Tropical cyclones: The strongest sustained surface winds. Northeast and southeast trade wind: Within the lowest kilometer above the sea surface the easterly flow is particularly steady and has distinct equatorward component. The prevailing winds in the tropical North Atlantic and North Pacific are from the northeast, and those in the tropical South Atlantic and South Pacific are from the southeast. Intertropic convergence zone (ITCZ): a very narrow belt located several degrees north of the equator where the transition between the northeast and southeast trades usually takes place. It is characterized by strong upward motion and heavy rainfall. Monsoon: a strong seasonal-dependent wind system, with a tendency toward onshore (sea to land) flow during summer and offshore flow during winter. The seasonal reversal is particularly pronounced over southeast Asia and adjacent regions of the Indian Ocean where the previling winds blow from the southwest during the summer and northeast during winter. Over most of India, the summer monsoon is characterized by heavy rainfall while the winter monsoon is extremely dry. METEO 3110, Prof.Zhaoxia Pu Global rainfall Averaged annual precipitation (in mm) 5

6 Extratropical cyclones Tropical cyclones 6