Retrieval of Macrophysical Cloud Parameters

Aim to retrieve most obvious macrophysical cloud properties:

• Cloud Top Height CTH (relative to instrument pointing)

• Cloud Top Temperature CTT

• Cloud Extinction Coefficient kext

Cloud Forward Model (CFM): Radiance in MIPAS FOV

Assume that:

1. a cloud in the MIPAS FOV is horizontally homogeneous – that is, has a constant cloud top height across the FOV and can be characterized by a single extinction coefficient.

2. the temperature structure within the cloud can be determined by the wet adiabatic lapse rate estimated downwards from the cloud top temperature.

3. two consecutive spectra view the same cloud

The radiance is considered in the clearest microwindow of the MIPAS A band: 960 cm-1 – 961 cm-1

O3 & CO2 O3

O3 O3 O3 & NH3

O3 & CO2

O3

Gas Correction and Validation with Simulations

Real MIPAS measurements Rm will include significant gaseous radiation contributions Rg, while the CFM calculates only the radiation contribution by the cloud itself Rc. It is thus necessary to deduce what portion of the measured signal is due to the cloud.

Assume that the cloud has a continuum signal and that the gaseous contribution has emission/absorption lines.

The forward model of the radiance encountered in the MIPAS FOV can be expressed as: , where

is the pencil beam radiance for temperature ,

pathlength ,

whereby Re = Earth’s radius, thc = tangent height where cloud is first detected, thFOV = tangent height of FOV modelled, T = temperature, z = relative position within the FOV, Φ = FOV convolution and Γwet = wet adiabatic lapse rate.

Total radiance measured within two MIPAS FOVs (the FOV containing the cloud top and the FOV immediately below) calculated by the CFM for varying cloud top heights and extinction coefficients.

Comparing the CFM output with RFM simulations of cloud corrected for gaseous emission in the manner described above, it is clear that the CFM does a good job at estimating the radiance measured by the MIPAS FOV.

Want blue to overlay red

Retrieval and A Priori DependenceCloud modelling is a highly non-linear process, even when considered on the vastly simplified scale.

Given a pair of radiances from two adjacent sweeps in a scan pattern, there can be two possible clouds present: a high thin cloud, or a low thick cloud.Depending upon which a priori is supplied to an OER, equally valid different solutions will result. NEED TO ADD MORE INFORMATION, to better characterize the a priori extinction.

Easiest way to get information is to use quantity we already have:THE COLOUR INDEX CIwhich should already be highly correlated with Cloud Effective Fraction EF, which is the ‘effective blocking power’ of the cloud in the FOV.

CI and EF are clearly correlated, but how?? Needs optimization down the road …

Using three different extinction coefficients (0.001, 0.01,and 0.1 km-1), four standard RFM atmospheres (day, polar summer, polar winter and tropical) and nine different cloud top heights at a tangent height of 9 km, for a total of 108 different cloudy atmospheric conditions …

Optimal Estimations retrieval of form

with state vector and using:

Real Measurements:

2 radiance measurements from MIPAS spectrum – the first sweep flagged as cloudy and the one immediately below

DIRECT

Pseudo-Measurements:

Tret = temperature corresponding to first flagged cloudy sweep (CIRA climat.)

EF = Cloud effective fraction, as estimated from CI

RELATE

The retrieval has been applied to and validated with RFM simulations for cloudy FOVs with known CTH, CTT and kext.

Application to MIPAS Spectra

•“Hot spot” of high cloud over Indonesian toga core, mountainous regions such as the Southern Andes and Rockies, Amazon Basin and the Congo;•Increasing cloud top height towards the tropics;•Retrieved CTT is nearly fully correlated with CTH;•Retrieved log(kext) is more or less constant over the globe.

Double peaked distribution in kext – thick cloud and cirrus??

cirrus? thick

If there IS a cloud in the FOV at a certain latitude, this shows the probability that it will occur at a given altitude/temperature/extinction …

Future Work

• Add extra microwindows to check for consistent results

• Check retrieval using more sophisticated simulations

• Investigate accuracy to which Tret has to be supplied