Goal: Understand the dynamics and thermodynamics of the Hadley
circulation
• Description of the zonal mean tropical overturning (or Hadley) circulation
• Some simple dynamical and thermodynamic models of the Hadley circulation*
• The Hadley circulation in a global circulation context
*Held, Isaac M., Arthur Y. Hou, 1980: Nonlinear Axially Symmetric Circulations in a Nearly Inviscid Atmosphere. J. Atmos. Sci., 37, 515–533.
Overview of the Hadley circulation • The Hadley circulation
comprises two cells, characterized by rising motion near the equator, poleward flow aloft at roughly 10-15 km, descending motion over the subtropics, and equatorward flow near the surface
• The Hadley circulation is strongly tied to the trade winds, tropical convergence zones, subtropical deserts, and jet streams
Brief history • Edmond Halley (1656-1742): theorized that near-equatorial
solar heating generates upward motion that is replaced by inflowing air masses from adjacent latitudes and that the easterly component of the trades arises from differential heating of the atmosphere over the course of a day
• George Hadley (1685-1768): Earth’s rotation plays a role in the direction of air masses moving relative to the earth’s surface
• William Ferrel (1817-1891): angular momentum, rather than linear momentum, is conserved
Very wet: Intertropical Convergence Zone (ITCZ)
East Pacific ITCZ from the Geostationary Operational Environmental Satellite 11 (GOES-11) July 2000
And very dry: Sahara Desert
Trade winds
Herman Moll, 1736
The Horse Latitudes • The subsiding subtropical edges of the
Hadley circulation are regions of high surface pressure and light winds called the “horse latitudes”
• Why are they called “horse latitudes”? – Drifting ships in the calm wind region would toss
horses overboard to conserve resources. Unlikely.
– “Dead horse” ritual: a sailor’s receipt of a pay advance from shipmaster resulted in a period of debt (“dead horse” time), at the end of which a straw-stuffed horse effigy was thrown overboard—for a westbound ship from Europe, this usually occurred upon reaching the subtropics
Source unknown
The Doldrums • Near-equatorial zone of calm
winds
• Rime of the Ancient Mariner (1797-98) by Samuel Taylor Coleridge: !All in a hot and copper sky,!The bloody Sun, at noon,!Right up above the mast did stand,!No bigger than the Moon.!Day after day, day after day,!We stuck, nor breath nor motion;!As idle as a painted ship!Upon a painted ocean." “Canoes in the Doldrums” by Herb
Kawainui Kane
Streamfunction Consider the mass conservation (continuity) equation in φ, λ, and p:
!"v = 1Re cos!
"u!"
+1
Re cos!"(vcos!)
"!+"#" p
= 0
Integrating this equation over all longitudes gives:
!"v0
2!
# d! = u(2! )$u(0)Re cos!
+1
Re cos!"(vcos!)
"!+"#" p
%
&'
(
)*
0
2!
# d! = 0
!
… = (2" )#1 …d$0
2"
%
The 2D zonal-averaged flow in the latitude-pressure plane can be expressed in terms of a mass (Stokes) streamfunction, ψ, defined such that:
v =g
2!Re cos"#$# p
!
" = #g
2$Re cos%&'Re&%Thus:
!(", p) =!(", p0 )+ (2#Re cos")g!1 v d "p
p0
p
# !(", p) = (2#Re cos")g!1 v d "p
ptoa
p
#
Starting the integral at the top of the atmosphere where ψ≡0
Note from dimensional considerations, ψ has units of mass/time.
Streamfunction climatology • The annual mean
streamfunction is slightly asymmetric w.r.t. the equator
• Seasonally, the streamfunction experiences a dramatic reversal
– For solsticial seasons: a single dominant Hadley cell, circulating clockwise (solid contours) in January and counterclockwise in July
– For equinoctal seasons: two Hadley cells of comparable (but weaker) intensity
Dima and Wallace, 2003
An angular momentum argument for the zonal mean circulation
M (!) = Re cos![u(!)+!Re cos!]
Suppose this ring of mass originated at the equator with zero windspeed. If angular momentum per unit mass is conserved, then:
M (0) =M (!)!"Re2 ="Re
2 cos2 ! +u(!)Re cos!
At 20° from the equator, this implies a windspeed of 56 ms-1, while at 30°, the windspeed is 127 ms-1. The observed subtropical jet windspeed is roughly 35-40 ms-1: thus, while the zonal windspeed implied by angular momentum conservation at 20° is reasonable, the value at 30° is not.
The absolute angular momentum per unit mass of atmospheric flow flow, , is given by: !
M =!r ! !vinertial =
!r ! (!v +!"!!r )
Let’s assume a ring of air at a latitude λ with a zonal windspeed u(λ). Then the magnitude of is:
Ω
λ Re
u
!
" u(#) =$Re sin
2 #cos#
!M
!M
The Held-Hou Model (1980) The Held-Hou model of the Hadley circulation considers the set-up as shown.
Key Assumptions:
λ
Re
• Two layer zonally-symmetric flow
• The zonal flow is steady and “balanced”
• The upper poleward branch conserves angular momentum (no frictional dissipation)
• Surface winds are small (friction in the lower layer)
• Negligible thermal diffusion
The Held-Hou Model (1980) Let’s consider the hydrostatic and meridional momentum equations in pressure coordinates under the assumptions of the Held-Hou model:
!!! p
= "1"
We consider here a geostrophic or thermal wind balance. Differentiating the momentum equation with respect to pressure gives:
1Re!!!"
+ 2"usin"= 0
On the other hand, differentiating the hydrostatic equation with respect to latitude⎯and noting that, in doing so, p is fixed⎯gives:
Equating the mixed partial derivatives of geopotential yields:
!! p
!!!"
= "2#Re sin"!u! p
!!"
!!! p
= "!!"
1#
#
$%
&
'(p
= "!!"
RTp
#
$%
&
'(p
= "Rp!T!y#
$%
&
'(p
= "RpT$
!$!y#
$%
&
'(p
Here the definition of potential temperature is used, again noting p is constant.
!2"Re sin!"u" p
= !RpT#
"#"y#
$%
&
'(p
= !1$" ln#"!
!
2"sin# $u$z
= %g$ ln&$y
Convert to z using hydrostatic balance and let dy = Redλ.
The Held-Hou Model (1980) Using the expression for u derived from angular momentum considerations, and making small angle approximations for sine and cosine, gives:
Since the lower layer windspeed is ~0, the vertical shear of the zonal wind can be approximated as:
Thus:
!
"u"z
#$y 2
ReH!
u(") =#Re sin
2 "cos"
$#Re ("
2)1
=#(Re")
2
Re
% u(y) $ #y2
Re
!
" ln#"y
= $2%sin&
g"u"z
'1#0
"#"y
( $2%yReg
%y 2
ReH
)
* +
,
- .
and:
!
d"" M0
" M
# = $2%2"0Re
2gH& y 3d & y
0
y
#
!
"M = "M 0#
$2"02Re
2gHy 4
The subscripts M and M0 denote that this solution is consistent with angular momentum balance.
The Held-Hou Model (1980) We can also estimate the potential temperature profile as predicted by radiative constraints. To leading order, this profile has a form approximated by:
Here, θEG is an upper tropospheric global mean radiative equilibrium potential temperature, Δθ is the pole-to-equator radiative equilibrium potential temperature difference, and P2 is the “2nd Legendre polynomial”, defined such that P2(x)=1/2(3x2 - 1).
Considering again the small angle approximation, and converting to y, gives:
!
"E (#) = "EG $23%"P2(sin#)
!
"E #"E0 $%"y 2
Re2
The profiles on the right illustrate θM (red curve) and θE (blue curve). If the net atmospheric heating is assumed to be the difference θE - θM, then close to the equator, net warming is taking place, while at somewhat higher latitudes, net cooling occurs.
The Held-Hou Model (1980) If we assume that over the width of the Hadley cell the net warming cancels the net cooling, then
Performing the above integration gives:
We also note that
!
"E (y) #"M (y)0
Y
$ dy = 0
!
("E0 #"M 0) +
$2"010Re
2gHY 4 #
%"3Re
2 Y2 = 0 One root, Y = 0, is disregarded.
!
"E (Y ) = "M (Y )# ("E0 $"M 0) +
%2"02Re
2gHY 4 $
&"Re
2 Y2 = 0
Thus,
!
Y =5"#gH3$2#0
%
& '
(
) *
1/ 2
and #E0 +#M 0=5"# 2gH18Re
2$2#0
For earth’s upper atmosphere, with θ0 = 255K, Δθ = 40K, and H = 12 km, so Y = 2400 km (or roughly 22° latitude) and θE0 - θM0 = 0.9K.
We’re formally calculating:
!
d"dtdy
0
Y
# = 0 ; d"dt
="E $"%E
;where " = "M
The Held-Hou Model (1980) For y > Y, the difference θE - θM > 0, implying net warming of the atmosphere. However, this is not physically realistic. So, for this region,
Applying thermal wind balance here gives:
!
" = "E #"E0 $%"y 2
Re2
At 45°, the small angle approximation is only off by 10%.
!
u(y) =gH"#Re$#0
!
" ln#E"y
= $2%sin&
g"u"z
'$2(##0
yRe
2 ) $2%ygRe
uH
The Held-Hou Model (1980) We finally consider the vertical and meridional winds.
If the radiative relaxation time is 15 days and Brunt-Väisälä frequency N is 0.01 s-1, wE ≈ 0.00027 m s-1. If we assume that the vertical velocity vanishes at the z = 0 and z = H of the atmosphere and wE is the value at wE at H/2, then: !
wE"#"z
$#E0 %#M 0
&E' wE $
"#"z(
) *
+
, - %1 #E0 %#M 0
&E=g#0N%2 #E0 %#M 0
&E
!
w(z) " 4wEzH1# z
H$
% &
'
( )
By continuity of the flow:
!
"w"z y
+"v"y z
# 0$ v(y,z) # % 4wE
H1% 2z
H&
' (
)
* +
,
- .
/
0 1 Y ; y 2
Y2
At Y = Y/2 and z = H, v ≈ 2wEY/H ≈ 0.01 ms-1. In the real atmosphere, v is close to 1 ms-1, so the modeled Hadley circulation is much too weak.
Recall this balance in the thermodynamic equation from the tropical scaling analysis.
Extensions to the Held-Hou Model • e.g., Lindzen and Hou (1988): asymmetric heating about
the equator
