Transcript AGU_2012_Cronin

Rectification of the Diurnal Cycle and
the Impact of Islands on the Tropical Climate
Poster #
A51C-0070
Timothy W. Cronin*, Kerry A. Emanuel
Program in Atmospheres, Oceans, and Climate, Massachusetts Institute of Technology
77 Massachusetts Avenue, Cambridge 02139; (*[email protected])
Precipitation ( mm/day):
Results: Precipitation
Island Run, h=5 cm, r=96 km, =1.0

10
r = 48 km
r = 96 km
8
300
250
250
y, km
300
200
8
Precipitation ( mm/day):
6
Island Run, h=5 cm, r=48 km
200
6
mm/day
10
4
150
350
150
100
300
100
4
8
2
50
50
Island mean = 6.17 mm/day
250
Domain mean = 2.94 mm/day
0
Precipitation
( mm/day):
0
50
100
150
200
250
300
350 200
Island Run, h=5x,cm,
kmr=24 km, =1.0
y, km
Island mean = 5.48 mm/day
Domain mean = 3.08 mm/day
0
Precipitation
( mm/day):
00
50
100
150
200
250
300
350
Dry Island Run, h=5
cm, r=48 km, =0.0
x, km
10
Rainfall data: Mulligan (2006) database: http://www.ambiotek.com/1kmrainfall
land mask: https://www.ghrsst.org/data/ghrsst-data-tools/navo-ghrsst-pp-land-sea-mask/
• Walker Circulation strength correlates better with MC precipitation than
warm pool precipitation (Dayem et al., 2007)
• 3-5 million years ago: permanent El Nino, and substantially less island
area in the West Pacific?
• Could the interaction of islands and the diurnal cycle strongly affect the
Walker Circulation?
Experimental Design
300
100
250
50
200
0
r = 48 km; dry
350
8
0
100
150
200
250
300
6
350
x, km
4
100
50
Island mean = 6.77 mm/day
Domain mean = 2.9 mm/day
50
100
150
200
250
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00 0
350
0
4
2
0
4
2
50
100
2
Island mean = 11.4 mm/day
Domain mean = 2.87 mm/day
x, km
50
100
150
200
250
300
350
x, km
15
r = 48km
Height (km)
r = 48km; dry
Island = 5 cm slab
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384 x 384 km, 3 km resolution; 64 vertical levels
Non-rotating; no background wind
Top-of-atmosphere insolation: March 20, 45 N
Diurnal average insolation: 310.3 W m-2
Varied Island Size and Surface Wetness
Results are averaged over simulation days 125-250
0.4
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0
3
6
9
12 15 18
Local Solar Time (hours)
21
24
Future Work
• Further sensitivity experiments on island size, wetness, and treatment
of the surface heat capacity
• Comparison of radiative and dynamical mechanisms: role of
advection of moist static energy in the subcloud layer
• Simulations of an equatorial beta channel with embedded island or
islands
• Exploration of effects of diurnal cycle and land/sea breeze dynamics
on soil moisture-precipitation feedback
r = 24km
r = 96km
5
Simulate Radiative-Convective Equilibrium with a mixed
lower boundary using the SAM Cloud-Permitting Model
See Khairoutdinov and Randall (2003) for details on SAM
0.6
0
Results: Temperature
Ocean = 1 m slab
0.8
Domain (mean=0.222)
Ocean (mean=0.22)
Island (mean=0.27)
0.2
• Simulated islands are rainier than the surrounding ocean
• Island size and surface wetness affect the rainfall enhancement
• Dry-island results suggest a strongly negative soil moistureprecipitation feedback
10
Cloud Fraction Diurnal Composite:
Island Run, h=5 cm, r=48 km
10
8
Island mean = 6.17 mm/day
6
Domain
mean = 2.94 mm/day
200
150
0
0
250
150
50
6
warming: in these simulations, clouds have a warming effect
due to their timing, height, and fractional cover
• Diurnal timing of clouds (figure below) is extremely important
for net cloud radiative effect
• Cloud radiative forcing affects the magnitude of island rainfall
enhancement, but dynamics (e.g., land-sea breezes) are more
important
1
300
y, km
y, km
The islands of the Maritime Continent (MC) are
substantially rainier than nearby ocean areas
150
r = 24 km
350
2
• Cloud radiative effects cause most of the island-induced
sunset
350
10
noon
350
y, km
• Tropical islands and continents are rainier than nearby ocean areas
(figure below; Sobel et al., 2011)
• Convection is more vigorous over islands and continents (e.g., Williams et
al., 2004)
• Why? Is the enhancement of island rainfall and greater convective
intensity over land related to the diurnal cycle?
Discussion
sunrise
Precipitation ( mm/day):
Island Run, h=5 cm, r=48 km
Cloud Fraction()
Motivation
0
-1
0
1
2
3
Temperature Deviation from All-Ocean Simulation (K)
• Islands warm the troposphere relative to an all-ocean simulation
• Vertical structure of warming is roughly moist –adiabatic with
increased stability near the surface (nocturnal BL)
• Dry island has larger impact than wet island
Acknowledgments
Thanks to Marat Khairoutdinov for supplying me with the model code, and to Allison Wing
for teaching me how to use it. This work has been supported by NSF Grant #1136480,
“The Effect of Near-Equatorial Islands on Climate,” which is a collaboration with Peter
Molnar.
References
• Dayem, K.E., and David C. Noone, and Peter Molnar, 2007. Tropical western Pacific warm pool and maritime continent
precipitation rates and their contrasting relationships with the Walker Circulation. J. Geophys. Res., 112, D06101.
• Khairoutdinov, M.F., and D.A. Randall, 2003. Cloud Resolving Modeling of the ARM Summer 1997 IOP: Model Formulation,
Results, Uncertainties, and Sensitivities. J. Atmos. Sci., 60, pp. 607-625.
• Mulligan, M., 2006. Global Gridded 1km TRMM Rainfall Climatology and Derivatives. Version 1.0. Database:
http://www.ambiotek.com/1kmrainfall
• Sobel, A.H., C.D. Burleyson, and S.E. Yuter, 2011. Rain on Small Tropical Islands. J. Geophys. Res., 116, D08102.
• Williams, E., T. Chan, and D. Boccipio, 2004. Islands as Miniature Continents: Another Look at the Land-Ocean Lightning
Contrast. J. Geophys. Res., 109, D16206.