EES215

Lecture 17

Energy flux from the sun:
Variables: distance from sun - elliptical orbit: distance during northern summer greater than during southern summer: consequence - should be greater extremes in southern hemisphere; other effects mask this consequence: Fig. 1
Angle to sun: equator receives more energy than poles; seasons Fig. 2
Length of day: constant at equator; between 0 and 24 hours at poles  Fig. 3
Effect of latitude: angle of solar radiation; high reflectivity of snow and ice in polar regions; maximum temperatures not at equator but at tropics
Energy budget: internal (heat) energy; latent energy (condensation of water); geopotential energy (gravity); kinetic energy
Net energy surplus at low latitudes is transferred by motion of atmosphere and oceans to high latitudes

Motion: energy transport from equator to poles
Pressure gradient: two components - horizontal gradient responsible for most motion: flow of air from high pressure to low pressure zones

Horizontal pressure gradient force:
Isobares - lines of constant pressure
Causes for horizontal pressure gradient: temperature differences (mechanical forces - topography)
Coriolis force (see discussion in ocean currents)

Geostrophic wind: wind blows in rigth angles to pressure gradient (disregarding friction) high pressure to the right, low pressure to the left looking downwind in northern hemisphere: pressure gradient force is balanced by Coriolis force  Fig. 4

Wind speed given as

u_g = 1/(f r) dp/dx = 1/(r 2W sinf) dp/dx ,

with f - Coriolis parameter; r - density of air; dp/dx - pressure gradient.
Wind speed function of latitude - due to latitude dependence of Coriolis force.

Formation of cyclonic and anticyclonic flow patterns around low and high pressure cells  Fig. 5,  Fig. 6
Friction with land surface causes Ekman spiral – similar to situation in oceans
All these forces assume constant distribution of pressure - constant wind - but pressure changes due to movement changes in wind speed and direction

Pressure and temperature changes in the atmosphere are given in Table 1

General circulation:  Hadley cells  Fig. 7
Upper winds: windshear - difference in velocity (and or direction) of movement between air masses at different heights: function of temperature structure in the air
Thermal wind: blows parallel to thickness with velocity proportional to gradient
Thermal wind blows with cold air (low thickness) to the left in northern hemisphere (viewed downwind) poleward decrease of temperature associated with a large westerly component in the upper winds. Because T gradient steepest in winter, zonal westerlies are most intense in winter
Jet stream at collison zone between cold and warm air masses; the best known one is the polar jet stream (usually just called the ‘jet stream’) between polar air and tropical air masses typically found in our latitudinal region, further to the north in the summer, to the south in the winter  Fig. 8
Meanders in jet streams and similar winds are called Rossby waves, generated by cyclonic an anticyclonic wind cells  Fig. 9
Upper geostrophic winds are westerly between subtropical high-pressure cells (around 15^oN/S) and polar low pressure center
Betwen subtropical high-pressure and equator they are easterly
Westerly circulation reaches maximum speeds of 45-67 m/s (100-150mph), twice that in winter. Maximum speeds are concentrated at around 30^o, between 9000 and 15000m - Global pattern - diagram

Local winds - influence of topography
Mountain and valley winds (katabatic and anabatic winds)
Winds over barriers – Fig.10