turner and ebell, 2013 doe/eu retrieval workshop, köln retrieval algorithm frameworks dave turner...

Turner and Ebell, 2013 DOE/EU Retrieval Workshop, Köln

Retrieval Algorithm Frameworks

Dave TurnerNOAA National Severe Storms Laboratory

Kerstin EbellUniversity of Cologne

DOE / EU Ground-based Cloud and Precipitation Retrieval Workshop


Motivation

• Remote sensors seldom measure the quantity that is really desired

• So we must “retrieve” the quantity we desire from the observations that are made

• Often an ill-defined problem (i.e., there is usually not enough information in the observations)

• Classical analogy from Stephens 1991: “Remote sensing is like characterizing an animal from the tracks it makes in the sand”


Tracks in the Sand

• What type of animal?• Large or small?• Young or old?• Male or female?• What color?


Tracks in the Sand

• What type of animal?


From Observations to Geophysical Variables

Geophysical Variable(What we want to know)

Radiance or Backscatter

(What we observe)

Forward RT Model

Retrieval

X⌃Ym=Y+ε

F

X YX+δX


The Retrieval Challenge• Desire “observations” of geophysical variables to improve

our understanding of the Earth system• Remote sensing observations provide information about

the Earth system, but are not direct observations of the geophysical variables we desire

• Must “retrieve” the geophysical variables from the observations

• Typically is an ill-defined problem• Noise hinders the retrieval; so does resolution• Metadata (data about the data) can help constrain the

problem• Additional observations also help• Important to consider the uncertainties in the retrieved

quantities• Calibration, calibration, calibration


Basic Retrieval Classes

• “Regression” methods– Linear, quadradic approaches, Neural networks, etc– “Tuned” to mean conditions; no guarantee that

retrieved profiles are consistent with observation– Computationally fast and always produces an

“answer”– Could be developed from

• Simulated observations• Collocated observations

• “Iterative” methods– Iterative, uses forward model and a first guess– Retrieved profiles are consistent with observation– Significantly slower than regression methods– Often case-specific error characterization is provided


Example: Liquid Water Path

• Many MWRs observe around 23 and 31 GHz• These observations are sensitive to the LWP and the

amount of precipitable water vapor (PWV) in the column

• Because of the small size of cloud droplets with respect to the wavelength, the cloud droplets are in the Rayleigh scattering regime and thus the MWR observations are insensitive to cloud droplet size

• Observed signal is proportional to the third moment (i.e., <r3>) of the size distribution spectrum


Microwave Spectrum


Real Tb Observations


Real Tb Observations: PWV


Real Tb Observations: LWP


“Orthogonal”

PWV

LWP


Retrieving LWP from the MWR (1)

• Purely a statistical method

• Can use historical dataset to determine coefficients ax

– Requires direct observations of LWP (e.g., from aircraft)

OR– Forward radiative transfer model

• Coefficients are site and season dependent• Fast and easy

€

LWP = a0 + a11Tb ,23 + a12Tb ,232 + a21Tb ,31 + a22Tb ,31

2


Retrieving LWP from the MWR (2)

€

τ =lnTmr −Tbg

Tmr −Tsky

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟−τ dry

€

LWP = L1τ 23 + L2τ 31

Compute “opacity” τ at each frequency

• Also a purely statistical method• Again, use historical data or simulated data to

determine retrieval coefficients Tmr, τdry, Lx

• Advantages over other method:– Linear rather than quadratic– Less scatter than other method (i.e., better statistical fit)

• Coefficients (Tmr, τdry, L1, L2) are site/season dependent


Clear Sky LWP Retrieval

Opacity Regression Retrieval

Liq

uid

Wat

er P

ath

[g

/m2 ]

Hour [UTC]


Improved Regression Retrieval

• More information can often improve retrievals• J.C. Liljegren used surface meteorology to “predict”

the retrieval coefficients Tmr, τdry, L1, L2

• Removed the site and seasonal dependence• Improved accuracy of retrieved LWP€

L = a0 + a1Psfc + a2Psfcesfc + a3esfc2


Improved Clear Sky LWP Retrieval


Liq

uid

Wat

er P

ath

[g

/m2 ]

Hour [UTC]



Iterative Retrieval

• Retrievals are used to ‘invert’ the radiative transfer• Regression approaches frequently will not agree

with the observation in a ‘closure study’• Iterative retrieval uses the actual forward model in

an iterative manner1. Start with first guess of atmospheric property of interest2. Compute radiance (obs) using forward model3. Compute computed “obs” with real observation, and modify

the first guess accordingly4. Repeat steps 2-4 until computed “obs” matches the real

observations (within uncertainties)

• Results will “close” with observations if retrieval converged


Considerations• The forward model may have limited sensitivity to the

desired variable• Forward model may be highly non-linear, which affects

how the solution is found• Multiple solutions may exist for a given observation

(i.e., problem is ill-defined)• Uncertainties in the observations should be propagated

to the retrieved solution• Retrievals often use other data and/or assumptions

that may affect the retrieved solution; uncertainties in these parameters should also be propagated to the solution– Includes model parameters, which are often ignored

• Often only partial prior info on the solution is known


Maximum a Posteriori (MAP)

• One of several iterative retrieval methods• Uses Bayes theorem• Incorporates a priori knowledge into the maximum

likelihood solution

€

P A B( ) =P B A( )P A( )

P B( )

Posterior =Likelihood x Prior

Normalizing Constant

A: the variable we desireB: the observation we have


Estimating the Temperature Outside

Climatology



Obs with itsUncertainty

Climatology



Solution withits uncertainty


Optimal Estimation - 1• Technique is an old one, with long history• Excellent book by Rodgers (2000)• Many good examples exist in literature• Assumes problem is linear and uncertainties are Gaussian

• However, the accuracies of the uncertainty in X is directly related to ability to properly define the covariance matrix of the observations Sε, which is a non-trivial exercise

• Key advantage is that uncertainties in the retrieved state vector X are automatically generated by method !

State vector

A priori

A priori’s Covariance “Obs” Covariance

Jacobian ObservationForward model

€

X n+1 = Xa + Sa−1 + KT Sε

−1K( )−1

KT Sε−1 Y − F X n

( ) + K X n − Xa( )[ ]


Optimal Estimation - 2

• Linear– Forward model of the form y = K x– A priori is Gaussian

• Nearly linear– Problem is non-linear, but linearization about some prior state is adquate

to find a solution

• Moderately non-linear– Problem is non-linear, but linearization is adequate for error analysis but

not for finding a solution Many problems are like this

• Grossly non-linear– Problem is non-linear even within the range of the errors


Optimal Estimation - 3• Moderately non-linear problems

– No general expression for locating optimal solutions as for linear and slightly non-linear problems

– Solutions must be found numerically and iteratively– Follow maximum a posteriori (MAP) approach and minimize the

cost function that is the sum of the “distance” between the observation and current calculation (weighted by observational covariance” and the distance between prior and current state weighted by prior covariance

– Numerical method is the Newtonian method to find successfully better approximations to the roots of the function g

€

J = y −F x( )[ ]TSe

−1 y −F x( )[ ] + x − xa[ ]TSa

−1 x − xa[ ]

€

g x( ) = ∂J∂x


Newton Method

€

x i+1 = x i − ∇xg x i( )[ ]−1

g x i( )

From Wikipedia


Optimal Estimation - 4

• Does not provide an explicit solution• Does provide a class of solutions and assigns a probability

density to each• We chose one state from the ensemble that is described

by the posterior covariance matrix

• Diagonal elements of provide mean squared error of• Off-diagonal elements provide information on the correlation

between elements of

€

ˆ S = Sa−1 + KT Sε

−1K( )−1

€

ˆ S

€

ˆ X

€

ˆ S

€

ˆ X

€

ˆ X


Making It More Concrete: MWR Retrieval

€

X =PWV

LWP

⎡

⎣ ⎢

⎤

⎦ ⎥

€

Y =Tb ,23

Tb ,31

⎡

⎣ ⎢

⎤

⎦ ⎥

€

K i , j =∂Fi

∂X j

X is the state vector Y is the observation vector

K is the Jacobian (2x2 matrix)F is the forward

radiative transfer model

€

Sε =σ Tb 23

2 0

0 σ Tb 31

2

⎡

⎣ ⎢ ⎢

⎤

⎦ ⎥ ⎥

S is the covariance of the observations

€


−1K( )−1


( ) + K X n − Xa( )[ ]


Even Better LWP Retrieval

Iterative Retrieval

Radiometric Uncertainty: 15 g/m2

Liq

uid

Wat

er P

ath

[g

/m2 ]

Hour [UTC]




LWP Relative Uncertainty

• Radiometric uncertainty in MWR results in large relative uncertainty in LWP when the LWP is small

• Combine different observations to improve retrieval

Rel

ativ

e U

nce

rtai

nty

[%

]

Liquid Water Path [g/m2]

From the posterior

€

ˆ S


Improving LWP Retrievals when the LWP is small


Combined Infrared + Microwave Retrieval

€

Y =YMW

YIR

⎡

⎣ ⎢

⎤

⎦ ⎥=

Tb 23

Tb 31

I1

I 2

...

I n

⎡

⎣

⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢

⎤

⎦

⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥

€

F =FMW

FIR

⎡

⎣ ⎢

⎤

⎦ ⎥

€


−1K( )−1


( ) + K X n − Xa( )[ ]

€

Sε =Sε MW 0

0 Sε IR

⎡

⎣ ⎢

⎤

⎦ ⎥

Forward models FMW and FIR need to be consistent!


LWP Relative Uncertainty

• Combining the infrared and microwave significantly reduces the relative uncertainty in LWP for small LWP clouds

Rel

ativ

e U

nce

rtai

nty

[%

]

Liquid Water Path [g/m2]

From the posterior

€

ˆ S


The Classification Problem

• Many retrieval algorithms are only applicable for certain types of clouds (e.g., liquid only stratiform, ice-cloud only)– Running incorrect retrieval method often grossly violates the

assumptions in the retrieval, leading to huge errors

• Need automated methods to classify the cloud conditions at a given time– Allows the correct retrievals to be performed

• Classification algorithms provide discrete (vs. continuous) output

• There is (and will always be) uncertainty in the sky classification; how to capture this uncertainty and propagate it into the retrieval uncertainty?

• Simulaneously retrieve classification and cloud prop’ties?


What is Sε ?• Uncertainty in the observations and forward model• Treated as the sum of two covariance matrices

• Instrument covariance matrix Sy can be difficult to determine– How to quantify this matrix? – How does it depend on conditions?

• Forward model parameter uncertainties in Sb

– Virtually every forward model has some tunable parameters that have some uncertainty – “unknown knowns”

– Many forward models make other assumptions that we may not realize which have uncertainties – “unknown unknowns”

• Often KbSbKbT is orders of magnitude larger than Sy

€

Sε = Sy + KbSbKbT


An Example for SyQuantifying the Noise in the AERI Radiance Observations

Applying NF reduces random error 4x but introduces some correlated error

Unfiltered

PCA Filtered


An Example for SbQuantifying the Impact of other Trace Gases on AERI Retrievals

Spectral region used for H2O Profiling


Summary

• Retrieving geophysical variables from observations is a non-trivial process– Error sources include random noise in the obs, bias in obs or

forward model, retrieval technique applied, small sensitivity, etc.

• Adding information typically improves the retrieval– Reduces noise using ‘redundant’ channels– Improves accuracy when more sensitive channels are added– Try to add channels that are “orthogonal”– Allows additional variables to be retrieved

• Forward model uncertainty and parameters important• Defining prior and observational covariances non-trivial


Let’s Remove the Human Component of the Error!


Good Outcome for WorkshopMy Opinion Anyway

• Quantifying Sε for the different instruments typically used for

cloud / precipitation retrievals– Quantifying Sy

– Identifying the important (tunable) forward model parameters

– Quantifying Sb

• Quantifying Sa for the different geophysical variables

– 1-sigma uncertainty in the atmospheric variable we desire

– Between variables a and b

– Between different height levels i and j

• In the matrices S?, both the diagonal and off-diagonal elements are important!

turner and ebell, 2013 doe/eu retrieval workshop, köln retrieval algorithm frameworks dave turner...

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