A Sample
The Zoo of Mechanisms for Tropical Rainfall
Variability and Change
J. D. Neelin*, C. Chou**, B. Lintner*, M. Munnich*, H. Su*,
J. Meyerson*, C. Holloway*, K. Hales*, & O. Peters*
*Dept.
of Atmospheric Sciences &
Inst. of Geophysics and Planetary Physics, U.C.L.A.
**Academica Sinica, Taiwan
Outline
• Illustration of model precipitation sensitivity
• Global warming/El Niño
• Sahel bafflement
• Some principles:
• Widespread warming/ local precipitation balances
• Moist static energy budget
• A few mechanisms:
upped-ante, rich-get-richer, ventilation
• Role of ventilation: Mid-Holocene example
• Prototype for convective margins
Precipitation Change under global warming
(CMIP3 a.k.a. IPCC 4th Assessment report models)
SRES A2 scenario (heterogeneous world, growing population,…)
for greenhouse gases, aerosol forcing
Precip change: HadCM3, June-Aug., 2070-2099 avg minus 1961-90 avg.
4 mm/day
model
climatology
black
contour for
reference
Neelin, Munnich, Su, Meyerson and Holloway , 2006, PNAS
mm/day
•Data: LLNL Prog. on Model Diagnostics & Intercomparison;
Thanks: Clivar Working Group on Coupled Modeling+groups
GFDL_CM2.0
JJA Prec. Anom.
NCAR_CCSM3
JJA Prec. Anom.
CCCMA
JJA Prec. Anom.
CNRM_CM3
JJA Prec. Anom.
GFDL_CM2.1
JJA Prec. Anom.
CSIRO_MK3
JJA Prec. Anom.
UKMO_HadCM3
JJA Prec. Anom.
MIROC_3.2
JJA Prec. Anom.
MRI_CGCM2
JJA Prec. Anom.
NCAR_PCM1
JJA Prec. Anom.
MPI_ECHAM5
JJA Prec. Anom.
GFDL_CM2.1
• Prec change (2070-99)-(1951-80); Clim wind 850-700hPa
MPI_ECHAM5
• Prec change (2070-99)-(1951-80); Clim wind 850-700hPa
MRI_CGCM2
• Prec change (2070-99)-(1951-80); Clim wind 850-700hPa
HadCM3
• Prec change (2070-99)-(1951-80); Clim wind 850-700hPa
NCAR_CCSM3
• Prec change (2070-99)-(1951-80); Clim wind 850-700hPa
MIROC_3.2_Med.Res.
• Prec change (2070-99)-(1951-80); Clim wind 850-700hPa
Precipitation change: measures at the local level
Trend of the 10-model ensemble median
> 99% significance (1979-2099)
Neelin, Munnich, Su, Meyerson and Holloway , 2006, PNAS
Inter-model precipitation agreement
Number of models (out of 10) with > 99% significant*
dry/wet trend (1979-2099) and exceeding 20% of the
median clim./century
*[Spearman-rho test]
Neelin, Munnich, Su, Meyerson and Holloway, 2006, PNAS
Hypothesis for analysis method:
• models have similar processes for precip increases and
decreases but the geographic location is sensitive
…to differences in
model clim. of wind,
precip; to variations
in the moistening
process (shallow
convection, moisture
closure,…)
Hypothesis for analysis method:
• models have similar processes for precip increases and
decreases but the geographic location is sensitive
•Check agreement on amplitude measure:
•Spatial projection of precip change for each model on that
model’s own characteristic pattern of change
Projection of JJA (30yr running mean) precip
pattern onto normalized positive & negative latecentury pattern for each model
Neelin, Munnich, Su, Meyerson and Holloway , 2006, PNAS
ENSO precip. anoms: obs. vs. atm. models
• Warm-cold composite for Xie-Arkin obs,
ECMWF-AMIP2, NCEP-AMIP2, QTCM
Observed & 3
models forced
by observed
sea surface
temperature
(AMIP2=Atm. Model
intercomparison
project)
See also Sperber
and Palmer 1996,
Giannini et al
2001; Saravanan
& Chang, 2000;
Joseph & Nigam
2006
(El Niño avg 1982-83, 87-88, 92-93, 95-96 – La Niña avg 1984-85, 89-90, 96-97)
ENSO teleconnections to regional precip. anomalies
Su & Neelin, 2002; Chiang & Sobel 2002
The “upped-ante” mechanism
1
Margin of
convective zone
Neelin, Chou & Su, 2003 GRL
The Rich-get-richer mechanism
Formerly M (anomalous Gross Moist Stability) mechanism1
Center of convergence zone:
incr. moisture 
lower gross moist stability
incr. convergence, precip
Descent region:
incr. descent
less precip.
Chou & Neelin, 2004; Held and Soden 2006
Temperature T and Moisture q equations
Moisture & moist static energy (MSE) budgets
Moisture budget for perturbations
P' = – <q' · `v >– <`v ·  q' > – <`q ·v' > – <v ' ·`q > + E' …
R.get.R
Upped-ante
Convgc Fb
1. Use MSE budget to obtain ·v' (Chou & Neelin 2004)
2. Neglect ·v' , v ' (Held and Soden 2006;
plausible for spatial avgs if ·v' at smaller scales than ·`v )
Yields precip anoms as T'  q'  q' , M'
Budget diagnostics for mechanisms
• Moist Static Energy transport by divergent flow M·v
• Gross Moist Stability M = Ms- Mq, (Mq inc. with moisture)
MSE budget for perturbations T' + ocean mixed layer / land
M ·v' = –M' ·v – (v·q)' – ctTs ' + Ftopnet' + (v·T)' …
Yields precip anoms as T'  q'  q' , M' ; v' , q'  E' etc.
Mq
'
P (1+)
M
[–(v·q)' + ·v(–M' ) – ctTs ' + … ]
Upped-ante Rich-get-richer
GMS multiplier effect
Rad cooling, (v·T)'
ocean transp, …
SST disequilibrium
QTCM 2xCO2 Expt. suppressing change in
moisture advection
(testing the upped-ante mechanism)
Suppression Expt
2xCO2 Precip. change
(mm/day)
Control
2xCO2 Precip. change
Neelin, Chou & Su, 2003 GRL
ECHAM4 + ocean mixed layer 2xCO2 equilib.
Precip. anom. rel.
to control
--- Clim. Precip.
(6 mm/day contour)
Moisture anom.
(1000-900 hPa)
Moisture anom.
(900-700 hPa)
Chou, Neelin, Tu & Chen (2006,
J. Clim.)
ECHAM4/OPYC3 2070-2099 IS92a (GHG only)
Precip. anom. rel.
to control
--- Clim. Precip.
(6 mm/day contour)
Moisture anom.
(1000-900 hPa)
Moisture anom.
(900-700 hPa)
Chou et al. (2006, J. Clim.)
ECHAM4 DJF
Contributions to the
moisture/MSE budget
Assoc. with upped ante
Assoc. with rich-get-richer
(M') mechanism
Convergence
feedback on both
Chou et al, 2006, J. Clim.
4panel
Observed Fnet climatology July
Net flux into
atmosphere
Solar, IR, sensible,
latent
(Net surface
flux=0 over land)
Shaded over/under
+/- 30 W/m2
Low-level wind
Precipitation
Chou and Neelin 2003, J. Clim.
Ventilation by relatively low moist static energy
air from oceanic/nonconvective regions:
helps set poleward extent of monsoons
Chou et al 2001 QJRMS; Chou and Neelin 2003 GRL; 2005 JClim
The role of ventilation in mid-holocene N. Africa
• Precip. change rel. to
control
• Expt: 6 ka bp orbital
parameters & grassland
albedo specified through
all N Africa
• Control: present day
orbital and albedo forcing
• What stops precip from
advancing northward?
Ventilation.
Su & Neelin 2005, JGR
The role of ventilation in mid-holocene N. Africa
N. African zonal avg:
• moisture (dashed)
• critical moisture for
convection (increases with
Temperature)
• 1. Control;
• 2. 6ka orbital PMIP expt;
• 3. 6ka and grassland
albedo over N. Africa
• Despite low albedo,
ventilation by inflow keeps
moisture from rising to
convective threshold in
north
Su & Neelin 2005, JGR
The role of ventilation in mid-holocene N. Africa
• Expts with
ventilation (inflow
moisture advection +
moisture diffusion)
reduced yield greater
poleward movement
of precip
Su & Neelin 2005, JGR
The role of ventilation in mid-holocene N. Africa
• Precip. change rel. to
present
• 1. 6 ka bp orbital
parameters & interactive
vegetation
• 2. Same but reduced
ventilation
• Ventilation leading
control on poleward
extent of 200 mm/y; veg.
feedback enhances prec.
Hales, Neelin & Zeng 2006, JClim
Can one observe a critical moisture for convection?
• Tropical Rainfall Measuring
Mission Microwave Imager
Western Pacific
(TMI) data
• Wentz & Spencer (1998)
Eastern Pacific
algorithm
• Average precip P(w) in each
0.3 mm w bin (typically 104
to 107 counts per bin in 5 yrs)
• 0.25 degree resolution
• No explicit time averaging
Peters & Neelin, 2006, Nature Physics
How well do the curves collapse when rescaled?
• Rescale water vapor and
P by critical value &
amplitude from power law
fit above critical
Western Pacific
Eastern Pacific
Peters & Neelin, 2006
Dependence on Tropospheric temperature
• Averages
conditioned on
vert. avg. temp.
^
T, as well as w
(T 200-1000mb
from ERA40
reanalysis)
• Power law fits
above critical:
wc changes,
same 
• [note more data
points at 270, 271]
Convective margin prototype
Steady-state, 1D temperature and moisture equations
(in moist static energy form) for a semi-infinite land
region lying to the west of an ocean region
M s constant
M q  M qpq
(1)

Land
Ocean
FT >0
E 0
MsvP+ FT
 P  uqxq
(2) M q  v

P   1
c (qqc (T))
P
>0
Lintner and Neelin, 2007, GRL
Perturbations to xc
Stochastic wind smooths margins,
climate pertn to T or inflow q shifts
Between the old and new margins,
precipitation shuts down, so the
largest droughts occur here.
Nonconvecting region solution
For inflow moisture q0 at coast (x=0) for constant inflow uq,
q rises to convective threshold qc(T), giving
convective margin position (distance from coast) xc:
x c  ln[ qc (T)/q0 ]
1
with inflow distance scale

-1 = -uqMs(MqpFT)-1.
Summary=Outline
• Illustration of model precipitation sensitivity
• Global warming/El Niño
• Sahel bafflement
• Some principles:
• Widespread warming/ local precipitation balances
• Moist static energy budget
• A few mechanisms:
upped-ante, rich-get-richer, ventilation
• Role of ventilation: Mid-Holocene example
• Prototype for convective margins
• Convective threshold change versus inflow air
Summary: mechanisms
• tropospheric warming increases moisture gradient between
convective and non-convective regions
• the "upped-ante
mechanism":
 negative precipitation
anomaly regions along
margins of convection
zones with wind inflow
from dry zones
• the “rich-get-richer mechanism" (a.k.a. M' mechanism):
Positive/negative precipitation changes in regions of with
high/low climatological precipitation
• [+ocean heat transport anomaly in equatorial Pacific]
Critical water vapor for onset of precip in TMI data:
Dependence on tropospheric temperature
• Find critical water
vapor wc (vert. int.)
for each vert. avg.
^
temp. T (here in
western Pacific)
• Compare to vert.
int. saturation
vapor value binned
^
by same T
• Not a constant
fraction of column
saturation
(Following Peters & Neelin, Nature Physics, 2006)