Transcript Radar observations of drizzling stratocumulus with an eye

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
Outline
• What is remote sensing?
•Recent work at UW:
-Pockets of open cells
- Estimating MBL properties from combined
satellite/reanalysis
• Other new technology
•What’s next?
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.
Recent work at UW
• The mystery of open cell pockets
• Inferring MBL structure by combining
knowledge from satellites and
reanalysis
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
C-band radar movie from EPIC
40 km
Wind
20
10
60 km
Structure
and
evolution
of drizzle
cells
30 dBZ
0
-10
C-band
example
October 21
0305 LT
0235
60 km
0250
0305
0320
0335
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
C-band reflectivity and radial velocity
Radar reflectivity
Mesoscale wind
fluctuations related
to drizzle cells
Radial velocity
fluctuations
The “POCS” mystery
Pockets Of
Open Cells
(POCS) are
frequently
observed in
otherwise
unbroken Sc.
POCS
Their cause is
unknown
The first satellite remote sensor - Tiros 1
TV Camera
in space
Mesoscale cellular
convection
MODIS 250m visible imagery
100 km
POCS associated with clean clouds
Figure courtesy
Bjorn Stevens, UCLA
Low Tb
indicative
of low re
0
Tb
5
11 - 3.75 m
brightness
temperature
difference
POCS regions drizzle more
Figure by Kim Comstock/Rob Wood
More vigorous drizzle in POCS
MODIS
brightness
temperature
difference,
GOES
thermal IR,
scanning
C-band
radar
Figure by Sandra
Yuter/Rob Wood
DYCOMS II aircraft mm radar
Figure by Bjorn Stevens
Climatology
of open and
closed cellular
regions
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
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)
MBL depth, entrainment, and decoupling
Methodology
• Independent observables: LWP, Ttop, SST
• Unknowns: zi, q (= )
• Use COADS climatological surface RH and airsea temperature difference
• Use NCEP reanalysis free-tropospheric
temperature and moisture
• Iterative solution employed to resulting nonlinear equation for zi
Mean MBL depth (Sep/Oct 2000)
NE Pacific
SE Pacific
Mean decoupling parameter q
Decoupling scales well with MBL depth
q vs zi-zLCL
Deriving mean entrainment rates
• Use equation: we=uzi+ws
• Estimate ws from NCEP reanalysis
• Estimate uzi from NCEP winds and two
month mean zi
Mean entrainment rates
Entrainment
rate [mm/s]
◄ NE Pacific
SE Pacific ►
Subsidence
rate [mm/s]
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
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
Example of MISR’s potential
• Movie
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
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)
EPIC 2001 [85W, 20S]
 0.05 cm s-1
zi/t + u•zi = we - ws
Diurnal cycle of subsidence ws,
entrainment we, and zi/t
NIGHT
DAY
NIGHT
DAY
we
ws
swe=0.24 cm s-1
sws=0.26 cm s-1
szi/t=0.44 cm s-1
dzi/dt
Conclusion: Subsidence and entrainment contribute
equally to diurnal cycle of MBL depth
Quikscat mean and diurnal divergence
• 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
Mean divergence
Diurnal difference (6L-18L)
Cross section
through SE
Pacific
stratocumulus
sheet
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
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.
Vertical-local time contours (MM5)
22S-71W
21S-76W
Height [m]
17S-73W
• 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
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
Diurnal vs. synoptic variability
(MM5)
Seasonal cycle of
subsidence wave (MM5)
22-18S, 78-74W
• 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
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
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 diurnallyvarying subsidence because MLM
cannot simulate daytime decoupling
SW
LW
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).
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 ATM0082384 and NASA grant NAG5S-10624.
References
Bretherton, 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
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
Lidar