large-scale transient variations of tropical deep convection forced with zonally symmetric ssts

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

SPCAM experiment modeled after earlier experiments by Marat Khairoutdinov

Log(Power)

Zonally symmetric double-ITCZ

A simple model of convectively coupled waves

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

Column MSE excess is expressed in terms of a shift in the reference profile of the Betts-Miller scheme

The most slowly decaying eigenmode

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

u1,v1,q1 u2,v2,q2 u3,v3,q3 u4,v4,q4

longitudenormal

u1+i,v1+i,q1

Rotate wind for q advection

u4+i,v4+i,q4u3+i,v3+i,q3u2+i,v2+i,q2

i is chosen randomly each day

• 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 sub-daily horizontal q advection

Check climatology is not greatly changed

Phase dependent horizontal moisture advection is removed

Normalized spectra

Control

0.1

0.4

0.3

0.2

• Horizontal moisture advection is key to the propagation of the intraseasonal signal but not that of convectively coupled waves

Zonally homogenize surface heat fluxes

Control

latitude

ControlHomogenized sfc hflx

Zonally homogenize radiative heating

Control

Column MSE budgets

Work by Joe Andersen

Why does radiative feedback weaken convectively coupled Kelvin waves?

Radiative heating anomaly Temperature anomaly

Work by Joe Andersen

An illustration with column CRM experiments

Anomalous radiative heating applied more in the lower troposphere

No anomalous radiative heating

Vertical distribution of the anomalous radiativeheating 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

Width of the ITCZMedium

WiderNarrower

80km global raveWRF rainfall climatology

Kuang, Walker, Andersen, Boos, Nie, in preparation

Observations

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