1. A Comparison of Regional and Global Models during VOCALS
Matthew C. Wyant, Robert Wood, Christopher S. Bretherton, Department of Atmospheric Sciences, Seattle, Washington, USA
and VOCA Modeling Groups
Introduction
Mean 20S profiles
VOCA Experiment Setup
Mean 20S profiles
PreVOCA
Simulations cover the REx field project period: Oct. -15 Nov
Most models are run in short (1-3 day) forecast mode.
Horizontally gridded data was submitted from a coarse larger domain or a fine inner domain or both domains.
Capable models include aerosol species: SO4, sea salt, dust, black carbon, organic carbon
Emissions of aerosol and gas species are specified in a standard protocol for regional models. Many participating models use their own surface emissions formulas.
Sulfate Aerosol Mass at 950 hPa
Pressure [hPa]
PreVOCA compared 15 regional, weather forecast, and climate models (in forecast mode) for October 2006 in the VOCALS region. Many models had large errors in distribution of low cloud cover, though ECMWF and UKMO performed well. Most models produced a marine BL too shallow near the coast at 20S. However, most models qualitatively captured diurnal and day-to-day variability of the cloud and BL despite mean biases. In general, global models outperformed regional models.
The VOCA experiment is designed to compare models in the VOCALS region for the REx period of intensive airplane and ship observations. VOCA has more of a focus on aerosols, aerosol and chemical transport, and aerosol-cloud interactions than PreVOCA. Some of the main research questions for VOCA are:
Observed Droplet Number
VOCA Goals
Bretherton et al. (2010)
Many models show a substantial drop off in droplet number away from the coast as is observed.
Models’ vertical distribution of droplet number is surprisingly diverse.
Do models simulate the variation of droplet concentration Nd along 20S?
Is anthropogenic sulfate the main contributor to geographic Nd variation?
What controls Nd in remote ocean regions?
What is the simulated indirect effect due to anthropogenic aerosols perturbing clouds and net TOA radiative flux in the VOCALS domain?
Fine Domain
CoarseDomain
1x10-10 kg/kg
Many models’ DMS concentrations are much higher than observed (note that observations don’t cover the full diurnal cycle).
Mean Low Cloud Fraction
Most models over-predict sulfate mass.
Most models under-predict CCN concentrations (0.1% supersaturation) both in the boundary layer and above the boundary layer.
Mean Liquid Water Path
Observed DMS
MODIS Total Cloud Fraction
AMSR LWP [kg/m2]
20S CCN (950 hPa)
Observed Droplet Number
Low cloud fraction varies strongly between models.
Some models under-predict coastal low cloud cover.
Many models over-predict clouds in the tropics away from the South American coast.
Liquid water path also strongly varies between models.
Few models show good agreement with observations.
Mean model biases in CCN dominate the comparison.
Observed excursions from coast of higher droplet number concentrations are matched by higher than average modeled CCN concentrations for many models.
Solid colors are MODIS, symbols are aircraft measurements. See Bretherton et al. (2010)
VOCA Participating Models
Participating Institution
Model Name
Model Type
Horizontal Resolution [km]
inner/outer
Vertical levels below 700hPa
Aerosol
Advection
Aerosol-cloud
Interaction
VOCA Participants
COLA
RSM
Regional 25 9
J. Manganello
IPRC/SOEST
IRAM 1.2 30 12
Yes
A. Lauer
NRL
COAMPS 5
15/45 7
X. Hong
PNNL
WRF-Chem 3.2.1 51
Q. Yang
Scripps
SCOAR
20/100
D. Putrasahan
UCLA
WRF 3.1.1 31
A.Hall
U. Iowa
WRF-Chem UIOWA 3.3 47
P. Saide
U. Washington
COSMO
25/100 21
A. Muhlbauer 15
R. George
U. Wisconsin
Milwaukee
WRF-CLUBB
(WRF 3.1.1) 26
L. Grant
ECMWF
36R1
Global Forecast 18
J.-J.Morcrette
UKMO
MetUM 7.5 50 20
J. Mulcahy
GFDL
AM3
Global Climate
200
Y. Lin
LMD
LMDZ 5v2 59 11
F. Codron
NCAR
CAM4 / CAM5
5 / 9
C. Hannay
AGCM v7.1 3
H. Xiao
Cloud Liquid Water at 20S
Longitude
Longitude
g/kg
Pressure [hPa]
Acknowledgements
This work is only possible through the collaborative efforts of all VOCA modeling participants. The emissions data set for VOCA was provided by Scott Spak of the University of Iowa. This work was supported by NSF Grant ATM
Conclusions
Pressure [hPa]
At 20S, boundary layer depth away from the coast also differs strongly between models.
Pressure [hPa]
Large model biases in cloud fraction, liquid water path, and boundary layer depth are present in most models, similar to the PreVOCA study.
Models show large biases in aerosol and chemical concentrations.
Models can reproduce timing of excursions of continental air into the offshore boundary layer.
We will be analyzing a geo-engeering component of the VOCA experiment with cloud droplet number artificially adjusted to 375 cm-3.
References
Allen, G. and co-authors, 2011: South East Pacific atmospheric composition and variability sampled along 20S during VOCALS-Rex, Atmos. Chem. Phys., 11,
Bretherton, C. S., R. Wood, R. C. George, D. Leon, G. Allen, and X. Zheng, 2010: Atmos. Chem. Phys., 10,
Pressure [hPa]
Observed Cloud Top and Base
Pressure [hPa]
750
800
Pressure [hPa]
Pressure [hPa]
900
1000
Longitude
Longitude
Longitude
Longitude
Bretherton et al. (2010)