General Circulation + Midlatitudes Flashcards
CCV
positive radiation budget at tropics 440W/m2 and negative radiation budget at poles 160W/m2 = general circulation
(Hartmann, 2016; Atkinson, 1987)
Hadley Circulation -> driven by latent heat through water vapour
Midlatitude Circulation -> driven by potential gradients
OLR release = proportional to the fourth power of the temperature of body
cloud tops = low OLR -> at equator
ice = low OLR -> at poles
clear skies = high OLR -> subtropics
(Hartmann, 2016)
geostrophic wind -> forms from 30°N/S no cross-latitude movement = horizontal only
gradient wind -> geostrophic and centrifugal = divergence and convergence (Smithson et al., 2008).
Hadley Circulation rotation rate (Navarra and Boracletti, 2002)
72hr day = Hadley Circulation 40-50°N/S
240hr day = Hadley Circulation is global
Hadley Circulation (Garstang and Fitzjarrald, 1999; Hartmann, 2014)
- Water vapour -> ice via latent heat of vaporisation at the equatorial trough -> cools at dry adiabatic lapse rate but as sensible heat released cools at saturated adiabatic -> hot towers at speeds of 30m/s gaining potential energy.
- Strikes tropopause which is dense, precipitation leads to energy loss via latent heat of condensation -> cooling -> diverges and sinks.
- 30°N/S air sinks warming adiabatically 100°C/10km every few cm/day -> anticyclones and subsistence
- Trade winds -> undo some of the work moving along the p.g.
Convection and latent heat release (Liu et al., 2015)
> 12km = thunderstorms w/ intense latent heat release e.g. Congo Basin
7-8km - deep convection and high latent heat release w/ cumulonimbus e.g. ITCZ
4-5km - Amazon
2-3km - cumulous convection (shallow) e.g. Indian Ocean
Vertical velocity (omega)
positive = descent
negative = ascent
hemispherically = more positive omega in SH as more subsistence most symmetrical in Africa = land on either side of ITCZ
e.g. subtropical highs - Azores anticyclone (Atkinson, 1987)
idealised view of Hadley Circulation
ITCZ 350km from rainbelt in west africa = instead caused by MCCs, african easterly jet and tropical easterly jet (Nicholson, 2018)
standardised way to determine ITCZ position -> effective at highlighting seasonal shifts -> double ITCZ in Indian Ocean during seasons (Berry and Reeder, 2014)
Double ITCZ in Pacific Ocean
Hadley Circulation Size
latitudinal extent is increased in CMIP5 -> papers argued the cause was ozone (Adam et al., 2013; Waugh et al., 2015) but others suggest ghgs (Nguyen et al., 2015)
will increase poleward energy transfer by 1.28latitude/kelvin global temp increase (Lu et al., 2007)
Motion through the atmosphere (Hoskins and Pearce, 1983)
Froude number = inertial flow = U^2/gH where U = background flow 10m/s and H = 104m.
Rossby number = strength of rotational effects on inertial flow = U/fLwhere U = background flow 10m/s and f = Coriolis force, L = synoptic scale.
Large Rossby Number = Coriolis is small so little rotation or scale is small so rotational movement is reduced
Variation in the synoptic scale of the atmosphere
the Rossby number changes as due to pressure differences (dP/P) -> while the Froude number is the same globally.
Froude/Rossby =
Extratropics = 10^-2
Tropics on planetary scale = 10^-2
Tropics on synoptic scale = 10^-3
means p.g. maintains longer in midlatitudes than tropics on synoptic scale
Mesoscale Convective Systems (Maddox, 1980)
system of thunderstorms 100,000km2 -> hours/days
from squall lines at night as convective dissipation less likely and over oceans as warm SSTs destabilise atmosphere (Garstrang and Fitzjarrald, 1999)
hourly across Africa -> East African Rift Valley (Roca et al., 2015) and Congo Basin due to orographic forcing (Jackson et al., 2009)
Variability in MCSs (Roca et al., 2014)
12hr+ = 75% of precipitation across tropical regions
250km+ = 60% of precipitation across tropical regions
Modelling MCSs (Vellinga et al., 2014)
Models need fine grid resolution so struggle -> TRMM highlights how increase in resolution require for better simulations
CP4 in Africa operates on a fine resolution of 4.5km so is often used
Convection in the atmosphere
CAPE = convectively available potential energy -> measures conditional instability and a value > 0 indicates thunderstorms are likely (Nicholson et al., 2018)
MSE = moist static energy -> atmospheric content including sensible heat, water vapour, potential energy and a constant = higher at ITCZ and lower in high latitudes often used to describe H.C. (Heaviside and Czaja, 2013)
Equivalent potential temperature -> informs stability of air (Garstang and Fitzjarrald, 1999)
Conditional Instability of a different kind (CISK)
thunderstorm-feedback mechanism -> as air rises it releases energy through latent heat of condensation = warms more air parcels (Charney and Eliassen, 1964).
transient eddies/extra-tropical cyclones, tropical storms (Corby, 1969; Hartmann, 2016)
operate for several days, few thousand of km, operate around 45°N/S -> transfer 3pW/yr
formation of transient eddies is a result of baroclinic instability (Hoskins and Valdes, 1990)
perturbations within the resultant jetstreams lead to the formation of eddies as they aim to take the available potential energy and convert it to kinetic energy via advection (Hartmann, 2016)
westerly jets -> baroclinicity produced by two air fronts of different temperature -> alter the geopotential height (Held, 2019)
boreal winter = NH jet stronger
boreal winter = SH jet stronger
intensification of p,g,
but SH jet has less variability as less land over S.O.
Rossby waves -> produced within westerlies due to changes in vertical depth due to the conservation of vorticity
can be induced by orographic features like mountains or temp contrasts (Held, 2019)
Rossby waves within the Jetstream lead to the production of transient eddies
as they cause compressional flow via deceleration or extensional flow via acceleration leading to transient eddies (Corby, 1969)
storm tracks -> regions where a significant number of eddies are forming
influenced by easterlies, westerlies and baroclinicity (Mesquita et al., 2008)
e.g. North Atlantic and North Pacific Storm Tracks (Hoskins and Hodges, 2019)
Standing eddies -> no movement ->produced through fixed features e.g. mountain range of ocean current
Rossby wave in N Pacific where air crosses from very cold Siberia (-40C) over warm oceanic air temperatures from Kuroshio Current by Japan (~15C)
