Robin HoganRobin HoganJulien DelanoeJulien DelanoeUniversity of ReadingUniversity of Reading
Remote sensing of ice Remote sensing of ice clouds from spaceclouds from space
OverviewOverview• New variational radar-lidar-radiometer retrieval for ice clouds
– Use of a-priori knowledge of the vertical distribution of ice cloud properties to spread information vertically
• Statistics from a month of CloudSat-CALIPSO-MODIS data– Global coverage from polar-orbiting satellites– Preliminary comparison with the Met Office model
• Spectral analysis to reveal the spatial structure of cirrus clouds from 1 km to 1000 km– What’s the difference between tropical & mid-latitude cirrus?– What determines the variation of power spectra with height?– Can it be represented in a fractal cirrus model?
Variational retrieval method Variational retrieval method • Advantages of combining radar, lidar and radiometers
– Radar ZD6, lidar ’D2 so the combination provides particle size– Include radiances to ensure that the retrieved profiles can be used
for radiative transfer studies
• How the variational approach works– Define the state vector x as a profile of two parameters of the size
distribution (e.g. extinction coefficient and “normalized number concentration parameter” N0*)
– Iteratively find the x that best forward models the observations
• Key advantages– Can include any number/type of observations– Can blend smoothly between regions where both radar and lidar
detect the cloud to where only one is sensitive– But need a good a priori for how cloud properties change in the
vertical
Delanoe and Hogan (JGR 2008)
CloudSat-CALIPSO-MODIS CloudSat-CALIPSO-MODIS exampleexample
• Lidar observations
• Radar observations
1000 km
CloudSat-CALIPSO-MODIS CloudSat-CALIPSO-MODIS exampleexample
• Lidar observations
• Lidar forward model
• Radar observations
• Radar forward model
• Extinction coefficient
• Ice water content
• Effective radius
Forward modelMODIS 10.8-m observations
Radar-lidar retrievalRadar-lidar retrieval
Radiances matched by increasing extinction near cloud top
……add infrared radiancesadd infrared radiances
Forward modelMODIS 10.8-m observations
How to spread information in How to spread information in heightheight
Delanoe and Hogan (JGR 2008)
• Results from a large in-situ database:– Climatologically, N0*/0.6 varies with
temperature independent of IWC– We can use this as an a-priori
• Is this due to aggregation?– Number of large particles reduces with
depth, but mass flux roughly constant?
– Implies a vertical error correlation in this quantity, implemented via a B matrix
• But most clouds are not all seen by both radar and lidar– Radar can miss the tenuous tops, lidar extinguished before the
base– Need to spread information vertically from radar-lidar region to
radar-only and lidar-only regions of the cloud
One orbit in July 2006
A-Train
Model
Comparison with Met Office Comparison with Met Office modelmodel
log10(IWC[kg m-3])
Antarctica
CentralPacific
ArcticOcean
CentralAtlantic
SouthAtlantic
Russia
July 2006 global comparisonJuly 2006 global comparison
• Too little spread in model
• ECMWF coming soon!
Tem
pera
ture
(˚C
)
Model A-Train
Mean effective radiusMean effective radius• July 2006 mean value of
re=3IWP/2i
• Just the top 500 m of cloud
• MODIS/Aqua
Effective radius versus Effective radius versus temperaturetemperature
All clouds
An effective radius parameterization?
Ice water pathIce water path Optical Optical depthdepth• Mean of all skies
• Mean of clouds
• MODIS/Aqua
Structure of Southern Ocean Structure of Southern Ocean cirruscirrus
Observations- Note limitations
of each instrument
Retrievals
-5/3: Cloud-top turbulence &
upscale cascade
Fall-streaks & wind-shear remove smaller scales lower in cloud: steeper power spectra
Hogan and Kew (QJ 2005)
Outer scale 50-100 km
– Slice through Hogan & Kew 3D fractal cirrus model
– Southern Ocean cirrus is just like Chilbolton cirrus!
90 km
Tropical Tropical Indian Ocean Indian Ocean
cirruscirrus
Stratiform region in upper half of cloud?
Turbulent fall-streaks in lower half of cloud?
BurmaIndian Ocean
120 km
Stratiform upper region dominated by larger scales
Turbulent lower
region
600 km
– Sum of two fractal cirrus simulations
– Fall-streak paradigm unsuitable for cloud top
Simulated
Observed
• We can validate the 3D structure using the MODIS infrared window channel image
…not very similar!
Summary and future workSummary and future workNew dataset provides a unique perspective on global ice clouds• Planned retrieval enhancements
– Retrieve liquid clouds and precipitation at the same time– Incorporate microwave and visible radiances– Adapt for EarthCARE satellite (launch 2013)
• Model evaluation– Global forecast models (Met Office and ECMWF): IWC and re
– High-resolution simulations of tropical convection in CASCADE
• Cloud structure and microphysics– What is the explanation for the different regions in tropical cirrus
(e.g. Brewer-Dobson-driven ascent in the TTL)?– What determines the outer scale of variability?– Can we represent tropical cirrus in the Hogan & Kew fractal
model?– Can we resolve the “small crystal” controversy?