Cloud feedbacks on dynamics and SST in an equatorial mock-walker circulation

Transcription

1 Cloud feedbacks on dynamics and SST in an equatorial mock-walker circulation Equator (f=0) p W Pacific Warm SST x E Pacific Colder SST Ocean heat loss Very cold deep ocean Understand cloud feedbacks on: SST(x) Atmospheric circulation and rainfall distribution (Bretherton and Sobel 2002; Sobel 2003; Peters and Bretherton 2005) Trop Met: Walker Model, Slide 1

2 A one-mode WTG model of Walker circulation T ( x, p) = T ( p) + a( p) T 0 1 q( x, p) = q ( p) + b( p) q ( x) 0 1 u( x, p) = V ( p) u ( x)!( x, p) = "( p)! ( x) 1 1 H L Bretherton and Sobel (2003); Sobel, Bretherton, Peters and Gildor (2004) Trop Met: Walker Model, Slide 2

3 Bulk stability quantities Gross dry stability (DSE div per unit ω) Moisture stratification (Lq conv per unit ω) Gross moist stability (rainy regions; q 1 = T 1 MSE div per unit ω) H 1 p #s M s = "(p) 0 p s! p T #p + #a s $ #p c ' * pt 1 p T % & ( ) dp =! M + s0! M c s1 pt J kg! p M q =! "(p) L #q 0 p s! p T #p + #b s $ #p Lq ' * 1 p T % & ( ) dp = M! + M q0 q1! Lq J kg! p M = M s! M q =! "(p) #h s p s! p p T T $ dp = M 0 #p! +! M c 1 pt J kg!1!0.008 L Bretherton and Sobel (2003); Sobel, Bretherton, Peters and Gildor (2004) Trop Met: Walker Model, Slide 3

4 Low-latitude cloud-radiation interaction z Anvil greenhouse Cloud shading Warm SST Trop Met: Walker Model, Slide 4 Cold SST Over warm SSTs: Deep cloud systems shade surface, cooling ocean but cirrus greenhouse reduces atmospheric cooling nearly equally. Over cold SSTs: Boundary-layer stratocumulus layers shade the surface and cool the atmosphere. Tropics-wide, 20 W m -2 extra radiation to space due to clouds: Cloud refrigeration (compare with 2xCO 2 = 4 W m -2, insolation ~ 450 W m -2 ) x Longwave cooling Cloud shading

5 Satellite (ISCCP)-derived cloud-radiative heating atmospheric warming R cld ocean cooling -S cld Sc regions -R cld = 0.17 LP -S cld = LP (Peters and Bretherton 2005) Trop Met: Walker Model, Slide 5

6 Stratocumulus clouds over cold oceans LTS = θ(700 mb) SST -S cld = (LTS 14) R cld = 3.5(LTS 14) (Peters and Bretherton 2005) Trop Met: Walker Model, Slide 6

7 Walker model physical assumptions. (Peters and Bretherton 2005 J Clim) WTG: T 1 is x-independent. No rain where q 1 (x) < T 1 ; q 1 = T 1 in convecting regions. Surface moisture flux E = c q (γ s T s1 (x) b s T 1 ); negligible surface heat flux. Column energy inputs (with deep and low clouds) : 1. Solar radiation S(x) = S 0 r 1 P(x) r 2 (T 1 - T s1 (x)) (S 0 = 230 W m -2, r 1 = 0.2, r 2 = 7 W m -2 K -1 ) 2. Longwave radiative loss R(x) = R 0 + c R T 1 r 1 P(x) (R 0 = 150 W m -2, c R = 1.5 W m -2 K -1 ) 3. Ocean energy export (x/l)δs, ΔS = 100 W m -2. Atmospheric export ΔR = 30 W m -2 to extratropics. Radiative exchange between atm. and ocn. Cloud reduction in insolation felt 100%/50% in ocean for deep/low clouds. Trop Met: Walker Model, Slide 7

8 Model equations (drop 1 s) Atmospheric column mass budget (*) du/dx ω = 0, u(0,t) = u(l,t) = 0. Atmospheric column heat budget (1) A dt/dt = P(x) - M s ω/g - R clr - ΔR + r 1 P - 0.5r 2 (T 1 - T s1 (x)) Since LHS independent of x, RHS must also be. Atmospheric column moisture budget (2) B dq/dt + A q u dq/dx M q w = E(x) P(x) Ocean heat budget (3) C dt s /dt = S clr (x/l)δs - E - r 1 P - 0.5r 2 (T 1 - T s1 (x)) Integrate (1-3) to a steady state under constraint (*), noting P = 0 where q < T and (1) and (2) couple into one energy eqn if q = T. Steady state can also be analytically derived if no low cloud. Trop Met: Walker Model, Slide 8

9 Fixed SST solutions (Bretherton and Sobel 2002 J Clim) High cloud feedback concentrates the rainy region q 1 =T 1 q 1 <T 1 Trop Met: Walker Model, Slide 9

10 No-CRF and deep-cloud solutions Cloud shading Insolation-xΔS Max cld, rain No cld. Reduced by cloud shading Atmospheric temperature and circulation nearly unchanged because deep clouds do not affect column energy budget Warm-pool SST and air-sea temperature difference reduced by deep cloud shading Warm-pool SST flattened by deep cloud shading. Trop Met: Walker Model, Slide 10

11 Low cloud feedbacks Low cloud column radiative cooling reduces SST, especially in cold pool. Differential in column radiative energy input in warm vs. cold pool enhances the Walker circulation and narrows the rainy region. Trop Met: Walker Model, Slide 11

12 Take-home messages from simple model Cloud-radiation feedback works nonlocally and is important tropics-wide, corroborating prior diagnostic studies. Deep cloud shading flattens warm-pool SST gradients. Low clouds cool coldest SSTs (and the whole climate), enhance circulation, focus rainfall. Ocean dynamical feedbacks (e.g. coastal and equatorial upwelling) not included in simple model may further enhance low-cloud impact on SST. Narrow ITCZ Eq. upwelling Broad warm pool Trop Met: Walker Model, Slide 12