New uses of remote sensing to understand boundary layer clouds Rob Wood Jan 22, 2004 Contributions from Kim Comstock, Chris Bretherton, Peter Caldwell,

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?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?

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

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

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

Mesoscale wind fluctuations related

to drizzle cells Radar reflectivity Radial velocity

fluctuations

The “POCS” mystery

Pockets Of Open Cells (POCS) are frequently

observed in otherwise unbroken

Sc.

Their cause is unknown

POCS

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

11 - 3.75 m brightness temperature difference

Low Tb indicative of low re

0 Tb 5

Figure courtesy Bjorn Stevens, UCLA

POCS regions drizzle more

Figure by Kim Comstock/Rob Wood

More vigorous drizzle in POCS

MODIS brightness temperatu

re difference,

GOES thermal

IR, scanning C-band radar

Figure by Sandra Yuter/Rob Wood

Figure by Bjorn Stevens

DYCOMS II aircraft mm radar

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 air-sea temperature difference

• Use NCEP reanalysis free-tropospheric temperature and moisture

• Iterative solution employed to resulting non-linear 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=uzi+ws

• Estimate ws from NCEP reanalysis

• Estimate uzi from NCEP winds and two

month mean zi

Mean entrainment ratesEntrainme

nt 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]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

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

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)

• 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

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

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

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 diurnally-varying 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 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

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