New uses of remote sensing to understand boundary layer clouds Rob Wood Jan 22, 2004 Contributions...
Transcript of New uses of remote sensing to understand boundary layer clouds Rob Wood Jan 22, 2004 Contributions...
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New uses of remote sensing to understand boundary layer clouds
Rob Wood Jan 22, 2004
Contributions from Kim Comstock, Chris Bretherton,
Peter Caldwell, Martin Köhler, Rene Garreaud, and Ricardo Muñoz
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Outline
• What is remote sensing?What is remote sensing?
•Recent work at UW:Recent work at UW:-Pockets of open cellsPockets of open cells
- Estimating MBL properties from Estimating MBL properties from combined satellite/reanalysiscombined satellite/reanalysis
• Other new technologyOther new technology
•What’s next? What’s next?
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What is remote sensing?
• Definition: The science, technology and art of obtaining information about objects or phenomena from a distance (i.e., without being in physical contact with them).
• Examples: Radar, lidar, all satellite observations
Ms. Evelyn Pruitt of the United States Office of Naval Research coined the term “remote sensing” in 1958 to include aerial photography, satellite-based imaging, and other forms of remote data collection.
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Recent work at UW
•The mystery of open cell pockets
• Inferring MBL structure by combining knowledge from satellites and reanalysis
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Scanning radars used to observe drizzle from shallow
MBL cloud• Scanning C-band (5 cm) radar
employed during TEPPS (NE Pacific) and EPIC 2001 (SE Pacific).
• Distinct cellular nature of drizzle imaged for the first time
• Allows investigation of links between cloud and drizzle structure
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C-band radar movie from EPIC
40 km
Wind
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C-band example
October 210305 LT
Structure and
evolution of
drizzle cells
60
km
60 km
30 dBZ
20
10
0
-10
0235 0250 0305 0320 0335
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Lifetime of drizzle cells/events
• Typical cell lifetimes 1-2 hours
Mean cloud base rain rates of 0.2-1.0 mm hr-1
Cloud liquid water depletion rates do not
exceed 0.2 mm hr-1
zi(u.qT) ~ 1 mm hr-1
Mesoscale gradients in qT are ~1 g kg-1 over 5-
10 km Typical mesoscale u variations must be 1-2
m s-1
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C-band reflectivity and radial velocity
Mesoscale wind fluctuations related
to drizzle cells Radar reflectivity Radial velocity
fluctuations
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The “POCS” mystery
Pockets Of Open Cells (POCS) are frequently
observed in otherwise unbroken
Sc.
Their cause is unknown
POCS
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The first satellite remote sensor - Tiros 1
TV Camera in space
Mesoscale cellular convection
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MODIS 250m visible imagery
100 km
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POCS associated with clean clouds
11 - 3.75 m brightness temperature difference
Low Tb indicative of low re
0 Tb 5
Figure courtesy Bjorn Stevens, UCLA
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POCS regions drizzle more
Figure by Kim Comstock/Rob Wood
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More vigorous drizzle in POCS
MODIS brightness temperatu
re difference,
GOES thermal
IR, scanning C-band radar
Figure by Sandra Yuter/Rob Wood
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Figure by Bjorn Stevens
DYCOMS II aircraft mm radar
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Climatology of open
and closed cellular regions
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POCS and drizzle – summary
• POCS are often associated with small cloud effective radius and enhanced drizzle (no counter examples found to date)
• Differences in LWP pdf shape are not expected to strongly modulate mean drizzle rate (LWP more skewed in POCS, but with lower cloud fraction)
• Drizzle much more heterogeneous in POCS which may cause large horizontal temperature gradients through evaporative cooling – this in turn leads to density currents (“mini cold pools”) that enhance mesoscale fluxes of moisture and energy
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MBL depth, entrainment and decoupling
• Integrative approach to derive MBL and cloud properties in regions of low cloud
• Combines observations from MODIS and TMI with reanalysis from NCEP and climatology from COADS
• Results in estimates of MBL depth and decoupling (and climatology of entrainment)
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MBL depth, entrainment, and decoupling
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Methodology
• Independent observables: LWP, Ttop, SST
• Unknowns: zi, q (= )
• Use COADS climatological surface RH and air-sea temperature difference
• Use NCEP reanalysis free-tropospheric temperature and moisture
• Iterative solution employed to resulting non-linear equation for zi
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Mean MBL depth (Sep/Oct 2000)
NE Pacific SE Pacific
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Mean decoupling parameter q
Decoupling scales well with MBL depth
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q vs zi-zLCL
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Deriving mean entrainment rates
• Use equation: we=uzi+ws
• Estimate ws from NCEP reanalysis
• Estimate uzi from NCEP winds and two
month mean zi
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Mean entrainment ratesEntrainme
nt rate [mm/s]
◄ NE Pacific
SE Pacific ►
Subsidence rate [mm/s]
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Summary of MBL depth work
• Scene-by-scene estimation of MBL depth and decoupling
• Climatology of entrainment rates over the subtropical cloud regions derived using MBL depth and subsidence from reanalysis
• Decoupling strong function of MBL depth
• Next step: deriving links between turbulence, inversion strength and entrainment by coupling to simple model forced with realistic boundary conditions
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New technology: Multi-angle imaging (MISR)
On Terra (launched late 1999)
9 cameras in fore and aft
direction (-70 to +70)
Unprecedented 3D
examination of cloud
structure
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Example of MISR’s potential
• Movie
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What’s next for MBL cloud remote sensing?SPACEBORNE
• millimeter RADAR in space: CLOUDSAT [launch 2004]; EARTHCARE [ESA, launch 2008]
first spaceborne drizzle measurements• Cloud/aerosol LIDAR: CALIPSO [launch 2004];
+EARTHCARE MBL aerosol characteristic in clear
regions; first direct measurements of MBL depth
at high spatial resolution from space
GROUND BASED• Scanning MM radars – 3D cloud structure
• Scanning LIDAR on aircraft – cloud top mapping and entrainment processes
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Diurnal cycle –The view from space
SE Pacific has similar mean
LWP, but much
stronger diurnal cycle,
than NE Pacific….…Why?
A=LWP amplitude
/LWP mean
From Wood et al. (2002)
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EPIC 2001 [85W, 20S]Diurnal cycle of subsidence ws, entrainment we, and zi/t
NIGHT DAY NIGHT DAY
ws
we
dzi/dt
we=0.24 cm s-1
ws=0.26 cm s-1
zi/t=0.44 cm
s-1
zi/t + u•zi = we - ws
0.05 cm s-1
Conclusion: Subsidence and entrainment contribute equally to
diurnal cycle of MBL depth
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Quikscat mean and diurnal divergence
Mean divergence Diurnal difference (6L-18L)
• Mean divergence observed over most of SE Pacific Coastal SE Peru
• Diurnal difference (6L-18L) anomaly off Peruvian/Chilean coast (cf with other coasts)
• Anomaly consistent with reduced subsidence (upsidence) in coastal regions at 18L
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Cross section
through SE Pacific
stratocumulus sheet
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Diurnal subsidence
wave - ECMWF• Daytime dry heating leads to ascent over S American continent
• Diurnal wave of large-scale ascent propagates westwards over the SE
Pacific at 30-50 m s-1
• Amplitude 0.3-0.5 cm s-1
• Reaches over 1000 km from the coast, reaching 90W around 15 hr after
leaving coast
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Subsidence wave in MM5 runs (Garreaud
& Muñoz 2003, Universidad de Chile)
• Vertical large scale wind at 800 hPa (from 15-day regional
MM5 simulation, October 2001) Subsidence prevails over much
of the SE Pacific during morning and afternoon (10-18
UTC) A narrow band of strong
ascending motion originates along the continental coast
after local noon (18 UTC) and propagates oceanward over the following 12 hours, reaching as
far west as the IMET buoy (85W, 20S) by local midnight.
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Vertical-local time contours (MM5)
• Vertical wind as a function of height and local time of day – contours every 0.5 cm/s, with negative values as dashed lines
Vertical extent of propagating wave limited to < 5-6 km Ascent peaks later further out into the SE Pacific
Heig
ht
[m]
17S-73W 22S-71W 21S-76W
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Diurnal vs. synoptic variability
(MM5)
Diurnal amplitude equal to or
exceeds synoptic
variability (here
demonstrated using 800
hPa potential temperature variability)
over much of the SE Pacific,
making the diurnal cycle of subsidence a particularly
important mode of
variability
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Seasonal cycle of subsidence wave
(MM5)
Seasonal cycle of subsidence wave
(MM5)
• Wave amplitude greatest during austral summer when surface heating over S
America is strongest.
Effect present all year round,
consistent with dry heating rather than
having a deep convective origin
MM5 simulations
broadly consistent with
ECMWF reanalysis data
22-18S, 78-74W
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Effect of subsidence diurnal cycle upon cloud properties and radiation
• Use mixed layer model (MLM) to attempt to simulate diurnal cycle during EPIC 2001 using:
(a) diurnally varying forcings including subsidence rate
(b) diurnally varying forcings but constant (mean) subsidence
• Compare results to quantify effect of the “subsidence wave” upon clouds, MBL properties, and radiative budgets
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MLM results
• Entrainment closure from Nicholls and Turton – results
agree favourably with observationally-estimated values Cloud thickness and LWP from
both MLM runs higher than observed – stronger diurnal
cycle in varying subsidence run. Marked difference in MLM TOA shortwave flux during daytime
(up to 10 W m-2, with mean difference of 2.3 W m-2)
Longwave fluxes only slightly different (due to slightly
different cloud top temperature) Results probably underestimate
climatological effect of diurnally-varying subsidence
because MLM cannot simulate daytime decoupling
SW
LW
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Conclusions• Reanalysis data and MM5 model runs show a diurnally-modulated
5-6 km deep gravity wave propagating over the SE Pacific Ocean at 30-50 m s-1. The wave is generated by dry heating over the Andean S America and is present year-round. Data are consistent with Quikscat anomaly.
• MM5 simulations show the wave to be characterized by a long, but narrow (few hundred kilometers wide) region of upward motion (“upsidence”) passing through a region largely dominated by subsidence.
• The wave causes remarkable diurnal modulation in the subsidence rate atop the MBL even at distances of over 1000 km from the coast.
• At 85W, 20S, the wave is almost in phase with the diurnal cycle of entrainment rate, leading to an accentuated diurnal cycle of MBL depth, which mixed layer model results show will lead to a stronger diurnal cycle of cloud thickness and LWP.
• The wave may be partly responsible for the enhanced diurnal cycle of cloud LWP in the SE Pacific (seen in satellite studies).
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Acknowledgements
We thank Chris Fairall, Taneil Uttal, and other NOAA staff for the collection of the EPIC 2001 observational data on the RV Ronald H Brown. The work was funded by NSF grant ATM-0082384 and NASA grant NAG5S-10624.
ReferencesBretherton, C. S., Uttal, T., Fairall, C. W., Yuter, S. E., Weller, R. A.,
Baumgardner, D., Comstock, K., Wood, R., 2003: The EPIC 2001 Stratocumulus Study, Bull. Am. Meteorol. Soc., submitted 1/03.
Garreaud, R. D., and Muñoz, R., 2003: The dirnal cycle in circulation and cloudiness over the subtropical Southeast Pacific, submitted to J. Clim., 7/03.
Wood, R., Bretherton, C. S., and Hartmann, D. L., 2002: Diurnal cycle of liquid water path over the subtropical and tropical oceans. Geophys. Res. Lett. 10.1029/2002GL015371, 2002
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Ground based radar
• Developed during WWII for aircraft detection
• Operators surprised by unusual signals that turned out to be caused by rain
• Post-WWII: A remote sensing industry is born
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Lidar