Lecture 9 Conceptual model of the global circulation Conservation of angular momentum Subtropical jetstream ITCZ Hadley circulation Upper-air midlatitude westerlies Poleward transport of energy (in midlats) Seasonal variations What are conceptual models A general description of a few important atmospheric features Allows understanding of the system Not to be taken too seriously Remember u=(u,v,w) and u~10, v~1, w~0.01 m/s. Atmospheric motions are dominated by horizontal wind, especially u Conceptual model should explain: General location of weather patterns Wind belts and associated pressure distributions Areas of precipitation, location of deserts Jet streams, existence and location Hadley circulation ITCZ, trades, horse latitudes Midlatitude westerlies Embedded low pressure systems Polar easterlies Water covered Earth (no continents), rotating Earth, spherical Earth The simple conceptual model Upward vertical motion near equator (ITCZ) Low-level flow toward equator (trades) Deflected toward the west in NH (by Coriolis), the North easterly trades in NH Upper-level flow away from equator Coriolis force acts on the N moving air, deflecting it toward the E. At 30N flowing W to E, the subtropical jet stream (westerly wind) Sinking motion near 30N, subtropical high Conservation of angular momentum mass x rotation velocity x distance to rot axis The skater has to Spin faster when She folds her arms A parcel of air moving N is at a decreased distance to rotation axis compared to that at Equator Conceptual model (continued) As parcel of air moves away from the equator at upper levels, it is cooled by radiative cooling. Tends to sink in the subtropical highs Sinking suppresses cloud development and precipitation. Subtropical highs are very dry (horse latitudes, deserts) The ITCZ is located where the trade winds converge, upward motion, precipitation Features of the global circulation Clouds are shown in grey and white 14 Oct Notice the ITCZ What is ITCZ breakdown? GOES-10, VS, SEP, 2000 Timescale ~ 3 weeks Air flowing N in the (upper branch of the Hadley circulation) converges at ~30N. High pressure at the surface Summary on conceptual model of Hadley circulation Hadley circulation is a direct circulation cell. Rising motion in the ITCZ (rainy), southerly motion at upper levels that gets turned toward the right by the Coriolis force into subtropical jet. Convergence at upper levels and sinking therefore dry. Horse lat, desert regions on continents, the subtropical highs (anticyclones). Associated with calm winds, dry weather. The return flow toward equator in trades that converge into the ITCZ doldrums. POLAR CELLS: At the pole there is sinking, N flow at surface = polar easterlies, polew of 60N. Eventually must have rising motion and return flow at upper levels. Polar cells are well developed in winter and over continents. Conceptual model of global circulation (cont) Between the Hadley and polar cells, the midlatitude westerlies flow. The midl westerlies encounter the polar easterlies at the surface at the polar front, involves rising motion and precip, weather At upper levels, this is where the polar jet is located. Indirect circulation cell, sometimes called the Ferrel cell. Continents disturb the conceptual model just described. Instead of W jets, get meandering jets that have waves superposed on them Troughs and ridges, Rossby waves in the westerlies Waves in the westerlies, Rossby waves geopotential height on 500mb surface Note the differences between Jan and July Zonal flow pattern, meridional flow pattern, split flow pattern Fig. 2.21 Poleward heat transport, total, atmospheric and oceanic. The max in atm is due to eddies