Robin Hogan

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Remote sensing of ice clouds from space

• Robin Hogan
• Julien Delanoe
• University of Reading

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Overview

• 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
• Whats 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?

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Variational retrieval method

• Advantages of combining radar, lidar and
  radiometers
• Radar Z?D6, lidar b?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 a 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)
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CloudSat-CALIPSO-MODIS example

• Lidar observations
• Radar observations

1000 km
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CloudSat-CALIPSO-MODIS example

• Lidar observations
• Lidar forward model
• Radar observations
• Radar forward model

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Radar-lidar retrieval

• Extinction coefficient
• Ice water content
• Effective radius

Forward model MODIS 10.8-mm observations
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add infrared radiances

• Radiances matched by increasing extinction near
  cloud top

Forward model MODIS 10.8-mm observations
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How to spread information in height

• 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

• Results from a large in-situ database
• Climatologically, N0/a0.6 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

Delanoe and Hogan (JGR 2008)
9

• One orbit in July 2006

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Comparison with Met Office model
log10(IWCkg m-3)

• A-Train
• Model

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July 2006 global comparison

• Too little spread in model
• ECMWF coming soon!

Temperature (C)
Model A-Train
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• Northern (summer) hemisphere
• IWC concentrated at warmer temperatures
• Southern (winter) hemisphere
• IWC concentrated at colder temperatures

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First comparison with ECMWF
log10(IWCkg m-3)
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Mean effective radius

• July 2006 mean value of re3IWP/2tri
• Just the top 500 m of cloud
• MODIS/Aqua

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Effective radius versus temperature

• All clouds

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Ice water path Optical depth

• Mean of all skies
• Mean of clouds

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Structure of Southern Ocean cirrus

• Observations
• Note limitations of each instrument
• Retrievals

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90 km
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Tropical Indian Ocean cirrus
Burma
Indian Ocean

• Stratiform region in upper half of cloud?
• Turbulent fall-streaks in lower half of cloud?

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(No Transcript)
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120 km
600 km
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3D structure

• We can validate the 3D structure using the MODIS
  infrared window channel image

Simulated Observed
not very similar!
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Summary and future work

• New 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?