Large-scale transient variations of
tropical deep convection forced with
zonally symmetric SSTs
Zhiming Kuang
Dept. Earth and Planetary Sciences and
School of Engineering and Applied
Sciences
Harvard University
Zonally symmetric SST
cases can be interesting
Log(Power)
SPCAM experiment modeled after earlier
experiments by Marat Khairoutdinov
A simple model of convectively coupled waves
Zonally
symmetric
double-ITCZ
Kuang, 2008; Andersen and Kuang, 2008
A moisture-stratiform instability
Question
• Is the intraseasonal signal a moisture
mode that differs fundamentally from
convectively coupled waves?
Moisture mode: Growth comes from negative
effective gross moist stability. The disturbance will
propagate through column MSE sources and
sinks (e.g. Yu and Neelin, 1994; Sobel et al. 2001; Fuchs and
Raymond, 2002, 2005, 2007; Sugiyama, 2009ab)
Decay time of T, q anomalies of an isolated
moist convecting atmosphere column
Kuang, JAS, In press
The most slowly decaying eigenmode
Column MSE
excess is
expressed in
terms of a shift in
the reference
profile of the
Betts-Miller
scheme
Decay time is ~15days, the time for surface flux to remove the
column MSE anomaly (radiation is fixed in this case)
• This slowly decaying mode could provide the
memory that reddens the spectrum in
frequency.
• The decay time will be modified by diabatic
sources, export of column MSE, which could
lead to growth. The disturbance will propagate
through column MSE sources and sinks (e.g. Yu
and Neelin, 1994; Sobel et al. 2001; Fuchs and Raymond, 2002,
2005, 2007; Sugiyama, 2009ab)
• The above moisture modes are either stationary
or propagate by horizontal moisture advection
Is horizontal moisture advection key to the
propagation of the intraseasonal signal seen
in the simulation?
• Use zonally rotated u,v fields to compute
horizontal moisture advection
longitude
u1,v1,q1
normal
u2,v2,q2
u3,v3,q3
u4,v4,q4
Rotate wind for q advection
u1+i,v1+i,q1
u2+i,v2+i,q2
u3+i,v3+i,q3
i is chosen randomly each day
u4+i,v4+i,q4
• This disrupts the coherence between u,v
and q thus removes phase dependent
horizontal moisture advection on time
scales longer than a day
• But preserves the mean horizontal q
advection and allows some coherent subdaily horizontal q advection
Check climatology is not greatly changed
Normalized spectra
Control
0.4
0.3
0.2
0.1
Phase dependent horizontal
moisture advection is removed
• Horizontal moisture advection is key to
the propagation of the intraseasonal
signal but not that of convectively
coupled waves
Control
Homogenized sfc hflx
latitude
Control
Zonally homogenize
surface heat fluxes
Zonally homogenize
radiative heating
Control
Column MSE budgets
Work by Joe Andersen
Why does radiative feedback weaken convectively
coupled Kelvin waves?
Radiative heating anomaly
Work by Joe Andersen
Temperature anomaly
An illustration with column CRM experiments
No anomalous radiative
heating
Anomalous radiative
heating applied more in
the lower troposphere
Vertical distribution of the anomalous radiative
heating can affect the gross moist stability
Conclusions
• The intraseasonal signals appear to be
moisture modes as they depend on
diabatic sources and their propagation
depends on horizontal moisture advection
• Effects of radiative feedback on both
convectively coupled waves and the
intraseasonal variabilities can depend on
the vertical structure of the anomalous
radiative heating
Width of the ITCZ
Medium
Width of the ITCZ
Narrower
Wider
Observations
80km global raveWRF rainfall
climatology
Kuang, Walker, Andersen, Boos, Nie, in preparation
Rainfall Spectra (γ=1)
Rainfall Spectra (γ=20)
Normalized rainfall Spectra (γ=20)
OLR Spectra (γ=20)
Normalized OLR Spectra (γ=20)
Realistic WRF run without sub-seasonal
surface flux variations
Kuang, Walker, Andersen, Boos, Nie, in preparation
Realistic WRF run without sub-seasonal
surface flux variations: OLR spectra
Kuang, Walker, Andersen, Boos, Nie, in preparation
Aquaplanet warm pool
Aquaplanet warm pool rainfall spectra
Normalized
A zonally symmetric case