Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin...
-
Upload
victoria-hampton -
Category
Documents
-
view
212 -
download
0
description
Transcript of Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin...
![Page 1: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/1.jpg)
Axisymmetric QTCM2 prototype – a survey of recent results
Adam Sobel, Gilles Bellon, David Neelin
Convection Workshop, Oct 16 2009Harvard, Cambridge MA
![Page 2: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/2.jpg)
1. QTCM2 development, mean ITCZ
Nieto Ferreira & Schubert 1997
![Page 3: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/3.jpg)
What controls the location and intensity of the ITCZs?
Thermodynamic factors acting in a single column: SST, fluxes, temperature & humidity, etc.?
Or momentum dynamics of the boundary layer, by which SST gradients control low-level convergence?(Lindzen & Nigam 1987; Tomas & Webster 1997, etc.)
![Page 4: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/4.jpg)
We designed a model to study the interaction of boundary layer dynamics and tropospheric thermodynamics, based on the Neelin-Zeng (2000) quasi-equilibrium tropical circulation model (QTCM).
The first QTCM had a baroclinic and barotropic mode,v(x,y,p,t) = v1(x,y,t)V1(p) + v0(x,y,p,t)V0(p)
No boundary layer.
We added an explicit boundary layer, fully prognosticin temperature, humidity, momentum. This allows allthe boundary layer momentum mechanisms to operate.
At this point we only have an axisymmetric version fully developed. (3D underway: B. Lintner, G. Bellon)
![Page 5: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/5.jpg)
Vertical structure:
pt
pb
00 q
bb(z)
b1(z)
0 Tab(z)
a1(z)
0 vVb(z)
V0(z)V1(z)
v(t,y,z) = v0 (t,y)V0(z) + v1(t,y)V1(z) + vb(t,y)Vb(z) T(t,y,z) = Tref(z) + T1(t,y)a1(z) + sb (t,y)ab(z) q(t,y,z) = qref(z) + q1(t,y)b1(z) + qb (t,y)bb(z)
pt
pb
0
Mass conservation: (pt-pb) ∂yv0(t,y) =- pb ∂yvb(t,y)
Vertical structures: QTCM1-like, but chopped at bottom and placed on top of slab atmospheric BL. “Barotropic” & baroclinic modes orthogonal in free troposphere.
![Page 6: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/6.jpg)
ps
pe
pt
b
01
The baroclinic mode is top-heavy and has weakly positive gross moist stability, the ABL/barotropic mode is bottom-heavy and has very negative GMS.
vertical modestructures forverticalvelocity
![Page 7: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/7.jpg)
The ITCZ precipitation is very sensitive to the horizontal diffusivity of free-tropospheric humidity.
These are calculations over an idealized SST distributionon a beta plane, Newtonian cooling for radiation, no WISHE (Sobel & Neelin 2006)
![Page 8: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/8.jpg)
MSE budget over the whole troposphere shows a local balance in the ITCZ between diffusion and ABL import
![Page 9: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/9.jpg)
ABL momentum budget shows a balance near the ITCZbetween Lindzen-Nigam forcing and friction
![Page 10: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/10.jpg)
In this model, the ABL momentum dynamics, driven by SST gradients a la Lindzen-Nigam, pushes moist static energy into the ITCZ, where deep convection (and the associated circulation) tries to get rid of it, resulting in large rainfall. The circulation is ineffective at getting rid of the energy, so an horizontal diffusion of moisture is needed to keep rainfall at reasonable values.
Important parameters are horiz. moisture diffusivity, and those related to vertical structure function profiles (these strongly influence GMS), incl. PBL depth and assumed depth of convection (level to which temp. perturbations are assumed moist adiabatic)
![Page 11: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/11.jpg)
Some observational (& other) support for this model
• Negative (or near-zero) GMS in ITCZ (Back & Bretherton 2006)• Lindzen-Nigam picture relevant to time mean E. Pac ITCZ
(Raymond et al. 2006)• Thermodynamic influence of SST & dynamic influence of SST
gradient appear to be independent (Back & Bretherton 2009 semi-empirical models)
• Existence of “shallow circulation” shows that LN w/o some form of convective instability doesn’t lead to precip (Zhang, Nolan)
• Diffusive behavior of transient tropical eddies, with magnitude in ballpark of what we use in the model (Peters et al. 2008)
![Page 12: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/12.jpg)
Sikka and Gadgil 1980Time
latitude
2. Intraseasonal oscillations
![Page 13: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/13.jpg)
29
-2
SST
y (thousands of km)y0
-7-9 9
Gilles Bellon had the idea of using the axisymmetric QTCM2 to simulate the northern summer ISO – Indian monsoon active and break periods
The model is similar in some respects (but not all) to some others that have been used on this problem (B. Wang & colleagues)
![Page 14: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/14.jpg)
The model produces a nice intraseasonal northward propagating oscillation, robustly to parameters – remarkably easy (by MJO standards)
time
Latitude (1000’s km)
Precipitation (mm/d)
Bellon & Sobel 2008a,b
![Page 15: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/15.jpg)
obl dynamics is essential to northward propagation
growth rate period
Mixed layer depth
Wind-induced sfc fluxes are crucial to the model instability, hence dependence on ocean mixed layer depth is as we expect (e.g. S&Gildor 2003, S&Maloney 2004)
![Page 16: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/16.jpg)
3. Multiple equilibria
![Page 17: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/17.jpg)
SST = SSTEq - ΔSST [(1-k) sin2(lat) + k sin4(lat) ]
k = 0.6
k = 0
k = 1
We consider the axisymmetric model again on the equatorial ¯ plane, now with equatorially symmetric SST field of varying flatness, described by parameter k
![Page 18: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/18.jpg)
k = 1
k = 0
k = 0.2
k = 0.4
k = 0.6
latitude (1000s of km)
For sufficiently flat SST profiles, there are multiple stable equilibria: one symmetric, two asymmetric (mirror images)
![Page 19: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/19.jpg)
k = 0.6
Latit
ude
of m
axim
umpr
ecip
itatio
n
Sensitivity of the convection to free-tropospheric humidity
asymmetric equilibrium
symmetric equilibrium
control case
The multiple equilibria arise as long as convective parameterization is sufficiently sensitive to free-tropospheric humidity (large parameter ¸)
![Page 20: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/20.jpg)
λ = 0.73
WTG SCM for the same forcing
Latitude
Subtropical minimum of precipitation in the axisymmetric model
The bifurcation in the axisymmetric model corresponds to the emergence of multiple equilibria in the SCM
![Page 21: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/21.jpg)
Summary• In our simple model, PBL momentum dynamics
driven by SST gradients pump MSE into the ITCZ; horiz diffusion is necessary to vent it if we want a nice-looking solution
• If we allow WISHE, this model produces northward-propagating intraseasonal disturbances very robustly
• For sufficiently flat SST profiles symmetric about the equator, we get multiple equilibria, either symmetric or asymmetric ITCZ/Hadley cells
• These multiple equilibria map fairly directly onto SCM multiple equilibria
![Page 22: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/22.jpg)
The diffusivity may represent transient eddies?
Nieto Ferreira & Schubert 1997
Peters et al. 2008
![Page 23: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/23.jpg)
Multiple equilibria in the General Circulation Models
Hysteresis in the seasonal cycle [Chao, 2000; Chao and Chen, 2001]
Multiple regimes of the Hadley circulation for uniform, constant forcing[Chao and Chen, 2004; Barsugli et al., 2005]
[Barsugli et al. ,2005]
Solar forcing (Wm-2)
20°S
Eq
310 300 290
Latit
ude
of IT
CZ
Equatorial ITCZOff-equatorial ITCZ
![Page 24: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/24.jpg)
Multiple equilibria in the tropical atmosphere
Is there a link between SCM and GCM multiple equilibria?
Do we find them in models of intermediate complexity?
![Page 25: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/25.jpg)
Wang et al. 2006
There is both northward and eastward propagation
![Page 26: Axisymmetric QTCM2 prototype a survey of recent results Adam Sobel, Gilles Bellon, David Neelin Convection Workshop, Oct 16 2009 Harvard, Cambridge MA.](https://reader035.fdocuments.in/reader035/viewer/2022070606/5a4d1b6d7f8b9ab0599b3f0c/html5/thumbnails/26.jpg)
Mean states
Results: two limit cycles
CMAP July, 80E-90E
Limit Cycle 2
Limit Cycle 1