Sensitivity Study: Modeling the Response of SORCE Solar Variability in WACCM
Aimee W. Merkel1, Jerald W. Harder1, Thomas N. Woods1, Peter Pilewskie1, Odele Coddington1,
1
2
Juan M. Fontenla , Daniel R. Marsh
1
Introduction
The Spectral Irradiance Monitor (SIM) on-board the Solar Radiation and Climate
Experiment (SORCE) satellite provides the first multi-year (since 2004) continuous
measurements of solar spectral irradiance (SSI) variability over the visible and
near-infrared spectral regions (200–2400 nm), accounting for about 97% of the total
solar irradiance (TSI). The SIM observations show a much larger UV change in the
declining phase of the solar cycle than previously predicted. The total solar output,
however, remains the same but compensated in an decrease in radiation at visible and
infrared wavelengths. The consequences of variability in the visible and infrared
portions of the spectrum are less understood and most general circulation and climate
models assume that the spectral variability in the visible and IR follow similar trends to
TSI. The climatic impact of the SORCE SIM observed variability in SSI is just
beginning to be investigated [Haigh et al., 2010, Cahalan et al., 2010]. Our study
applies the measured SIM SSI variability as the solar input for the Whole Atmosphere
Community Climate model (WACCM). We ran two 25-year control case simulations of
low and high solar activity based on standard SSI variability. We ran an additional
25-year control case simulation with the SIM SSI variability in the visible and
near-infrared incorporated in WACCM to test the effects on a global climate model.
Our results show that there is significant atmospheric response to the increase solar
variability. Current estimates of SSI variability based on solar rotation modulation
suggest a smaller long-term trend than the measured values. This should be
considered in future IPCC assessments.
Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO
2
National Center for Atmospheric Research, Boulder, Colorado
WACCM Results
Annual Mean
Solar Min
(Mixing Ratio)
Delta Max-Min
LEAN
Unshaded regions are
significant to 95%.
Delta Max-Min
SORCE
Unshaded regions are
significant to 95%.
WACCM Specifications
Equitorial Average
(25°S - 25°N)
Annual Mean
Delta Max-Min
1 mb
Summary
- Based on the Community Atmosphere Model version 3 (CAM3)
Constituent
O3
LEAN: +0.5%
SORCE: -2.0%
Approach: Case study: Simulate Quiet Sun and Active Sun with both Lean and SORCE.
Active Sun = Year 2004 (between 10/1/2004 to 10/24/2004, Active but quiescent)
Quiet Sun = Year 2007 ((between 8/2007 to 10/2007)
TSI difference is conserved between cases, TSI 2004-2007= 0.41W/m2
Case 2:
Case 3:
Solar Quiet Simulation using Lean spectra
Solar Input: Average Lean spectra for 2007
1
O( D)
LEAN: +0.5%
SORCE: +4.0%
O
LEAN: +0.5%
SORCE: +2.0%
HOX
LEAN: +1.0%
SORCE: +3.0%
LEAN: +0.25K
SORCE: +1.5K
T
Figure 2: Difference between 2004 and 2007 solar spectral
irradiance. The red curve is the difference for the Lean
spectra used as input for Case 2 and Case 1. The blue curve
shows the difference between 2004 composite (Case 3) and
Lean 2007 (Case 1). Significantly more variability in the
SORCE composite.
Discussion
The WACCM results show that there is significant difference
between the response of the atmosphere to a standard SSI model
solar input and the higher variable SORCE solar input. The ozone
vertical distribution shows less ozone higher than 40km and more
ozone lower than 40km. Since there is more UV radiation, the
temperature increases by 1.5K around 1mb. To maintain thermal
equilibrium there is additional cooling so QRS=QRL. Haigh et al.
[2010] presents a similar study. As explained by Haigh, in the lower
mesosphere the major loss mechanism for Ox (=O3+O(3P)+O(1D))
is by reaction with HOx (=OH+HO2) which is mainly produced
through the creation of OH by the reaction of H2O with O(1D). An
enhancement of O(1D) is due to enhanced photodissociation of O3
by the additional UV radiation. The WACCM results support this
explanation where O(1D) is increased by only 0.5% (Max relative
to Min) at 1mb in the Lean experiment but 4% in the same region
with the SIM spectral data. The resulting increases in HOx are over
3% throughout the stratosphere in the SIM experiment but less
than 1% with the Lean model. In the upper stratosphere the major
loss mechanism for O3 is by recombination with O and higher
concentrations of the latter (~2% for SIM, ~0.5% for Lean) which
also tends to reduce O3 concentrations. The sharp decrease in
ozone above 40km with the SIM spectra is consistent with it being
in photochemical steady state with HOx and O as the dominant
sinks. The ozone decreases produce a self-healing effect whereby
more UV radiation is transmitted to lower levels resulting in greater
O2 photolysis and thus more O3 lower than 40km.
Solar observations from the SORCE mission (Solstice and
SIM observations) show enhanced variability (factor of 4) in
the UV and visible part of the spectrum from solar active to
solar quiet conditions.
The TSI is conserved even with the enhanced variability.
Additional UV is compensated by less irradiance at visible and
infrared wavelengths.
QRS
LEAN: +0.05K/day
SORCE: +0.3K/day
QRL
LEAN: -0.05K/day
SORCE: -0.3K/day
Total Shortwave
heating
Figure 1: Average Spectrum for 2004. The black
curve is the Lean spectrum used as input for Case 2. The
colored curves show the 2004 (made up of Solstice, SIM
and the SRPM model) used to scale Lean spectra for Case 3.
See Marsh et al. 2007, Garcia et al. 2007 and Kinnison et al. 2007
Conclusions
Solar Active Simulation Using Lean Spectra
Solar Input: Average Lean Spectra for 2004
Solar Active Simulation representing SORCE variability
Solar Input: SORCE 2004 variability imposed on Lean Spectra
Eliminates differences in calibrations of SORCE vs Lean spectra.
- Horizontal resolution 2° longitude by 2.5° latitude
- Radiation module includes 19 bands, 7 of which are in the
Hartley ozone band (between 200nm and 350nm)
Three control cases: Fixed solar input in each case. Each case contains 25
realizations of FIXED year simulations to build up statistics.
Case 1:
- 66 levels spaced 1.1-1.75km apart (troposphere, stratosphere),
3.5km in MLT.
- Chemistry module (MOZART3) includes 57 species and 211
photochemical reactions.
Solar input
Most recent modeling efforts to study the atmospheric response from the solar
cycle variability use a standard SSI model as input. Lean et al. [2000] provides one of the
best SSI models for this purpose.There are advantages of using the Lean model since it
has excellent agreement with TSI and the parameterization can be extended for future
IPCC assessments. However, the Lean model under-estimates the solar variability in the
UV and visible part of the spectrum between solar active and solar quiet conditions as
observed by the SIM instrument on SORCE. We conduct a sensitivity study using both
SIM and Lean solar variability in the Whole Atmospheric Community Climate Model
(WACCM). See Figures 1 and 2.
- 3-D Global circulation model extending from the surface to the
thermosphere (140km)
Total Longwave
cooling
A case study simulation with Lean and SORCE solar input
in WACCM show significant differences in atmospheric
response. Increased UV effects the amount of O3, O(1D), O
HOX, T, heating and cooling in the stratosphere-lower
mesosphere. This agrees with a similar study by Haigh et al.,
2010.
This sensitivity study applying these solar observations
suggest a very different atmospheric response to solar
variability than what has been currently estimated. Continued
validation of the currently operated irradiance instruments and
a solar spectral irradiance time series that covers a full solar
cycle (or more) are needed to fully assess these findings. An
in-depth analysis of atmospheric observations concurrent to
the SORCE era is an important area of study.
The SORCE observations should be considered in future
atmospheric modeling and IPCC assessments.
Please see authors for references herein.