data assimilation for the space environment ludger scherliess center for atmospheric and space...

Data Assimilation for the Space Environment

Ludger Scherliess

Center for Atmospheric and Space SciencesUtah State University

Logan, Utah 84322-4405

GEM 2003 MeetingSnowmass, CO

June 2003

Data Assimilation

Create a coherent and objective ‘picture’ of the space environment by combing the information inherent in the physical model and in the data.

This ‘picture’ (analysis) should satisfy the physical laws and fit the data as best as possible (within their errors).

The analysis can than serve as the best initial condition for a forecast.

Historical Background

Data Assimilation in the Atmosphere: Initial Attemps started in the 1950th (NWP) Currently adjoint/variational methods are used Used now also in re-analysis runs to get consistency for the past 40 years.

Data Assimilation in the Oceans: Began with large scales (mean properties) Regional effords (e.g., Gulf stream) [10-15 yrs ago] El Nino, emphasis on equatorial pacific (coupled with atmosphere) GODAE, produce operational upper ocean now- and forecast.

Data Assimilation in Space Sciences

Assimilative Mapping of Ionospheric Electrodynamics (AMIE, Richmond and Kamide, 1988)

Initial Testing of Kalman Filter for Ionospheric Electron Density Reconstructions (Howe et al., 1998)

Kalman Filter to construct the global 3-D Electron Density Field (GAIM)

Until Recently only relatively Sparse Data were available

Global Assimilation of Ionospheric Measurements (GAIM)

Multi-University Research Initiative (MURI):

Utah State University (PI R. W. Schunk) University of Colorado - Boulder University of Texas at Dallas University of Washington

Basic Approach: Use a Physics-Based Ionosphere-PlasmasphereModel as the Basis for Assimilating a Diverse Set of Real-Time (or Near Real-Time) Measurements.

Motivation

Data Assimilation has been proven very successful in

Meteorology and Oceanography

Need for More Accurate Now-Casting and Forecasting of

Plasma Distribution in the Ionosphere and Plasmasphere

(and Magnetosphere?)

Availability of Large Quantities of Data

Diverse Data Sources (Apples and Oranges)

Mature Theoretical/Numerical Space Environment Models

Models contain our ‘knowledge’ of the physics Data contain information about the ‘true’ state

Objectives

Goal: Optimally combine Data and the Model to create coherent Picture of the Space Environment

Solution satisfies the physical laws and ‘agrees’ with the data (within their error bounds)

Both, however, are imperfect:

Models: Forcing, Parametrization, etc.

Data: Observational Errors

The Data Assimilation Cycle

‘Best-Guess’ Background

Short-TermForecast Analysis

DataCollection

QualityControl

Forecast

Physical Model creates a forecast which is adjusted by the Data to create an ‘analysis’, which serves as the start for the next model forecast. In the analysis

the Data Errors and Model Errors are used as weights.

Data Assimilation Techniques

€

3-d Var

4-d Var

Kalman Filter xf = Mx + Pf = MPMT + Q yo = Hx + K = PfHT (HPfHT + R)-1

xa = xf + K(yo - Hxf) Pa = (I-KH)Pf

€

J δx( ) =1/2δxT P−1δx +1/2 H δx + x b( ) − y o

[ ]T

R−1 H δx + x b( ) − y o

[ ]

€

J δx( ) =1/2δx0T P−1δx0 +1/2

i= 0

n

∑ H i M i,o(x0)( ) − y io

[ ]T

R−1 H i M i,0(x0)( ) − y io

[ ]

€

Fundamental Concept of 3D-Var

Start with a forecast of the physical model.

Produce an analysis by minimizing the difference between the analysis and a weighted combination of

the forecast (the background or best guess field) and the observations.

At each time-step take a ‘snapshot’ of the state

This ‘snapshot’ will serve as the initial condition for the next forecast.

We want to minimize a “Cost Function” J which consists of:

J = JB + JO +JC

€

The Cost Function

J = JB + JO + JC

JB: Weighted fit to the background field

JO: Weighed fit to the observations

JC: Constraint which can be used to impose physical properties (e.g., analysis should satisfy Maxwell’s equations, continuity equation, …)

Strong Constraint: Exactly satisfied Weak Constraint: Approximately satisfied

A typical form for the JB term is:

JB = (xA - xB)T B-1 (xA - xB)

Where:

xA: Analysis Variable (e.g., Electron Density,Temperature,…)

xB: Background Field, obtained from the Model Forecast

B : Background Error Covariance Matrix:

How good is your Forecast:Good Forecast high weightPoor Forecast low weight

€

The Cost Function, cont.

A typical form for the JO term is:

JO = [y - H(xA)]T R-1 [y - H(xA)]

Where:

y : Represents all Observations

H : Forward Operator which maps the Grid Point Values to Observations (can be linear or nonlinear)

R : Observation Error Covariance Matrix:

Make use of the probable Errors of the DataGood Data high weightPoor Data low weight

(also includes the representativeness of the data)

€


Constantly compares the data with the model

€


Analysis is found my minimizing J using “standard minimization techniques” (find the gradient of J).

The analysis is found by: Analyzing all points at once Using all available data

The Physical Properties/Model were used to: Obtain the best possible background field To constrain the Analysis

Cost function and the constraints are not explicitly time dependent

€

Fundamental Concepts of 4D-Var

4D-Var introduces the temporal dimension to data assimilation

Find a close fit to the data that is consistent with the dynamical model over an extended period of time.

Find the the closest trajectory

€

J δx( ) =1/2δx0T P−1δx0 +1/2

i= 0

n

∑ H i M i,o(x0)( ) − y io

[ ]T

R−1 H i M i,0(x0)( ) − y io

[ ]

Physical model

4D-Var is a powerful technique to find: Initial conditions External Forcing

The Kalman FilterThe Kalman Filter

x - Model State Vector M - State Transition Matrix - Transition Model Error P - Model Error Covariance Q - Transition Model Error Covariance y - Data Vector H - Measurement Matrix - Observation Error R - Observation Error Covariance K - Kalman Gain

In the Kalman filter the weight for the background (error covariance matrix) evolves with the same physical model as the state!

Kalman Filter EquationsKalman Filter Equations

Model-State Forecast



Model-State Forecast Error



Measurement Equation



Kalman Gain



Model-State Analysis



Model-State Analysis Error


The Dynamical Model entered the Filter:

Evolution of the State Vector (make a Forecast)

Evolution of the Error Covariance Matrix

Error Covariance Matrix becomes time-dependent!

Model Error is explicitly included in the Assimilation

Data Assimilation Tasks

Develop Physical Model

Develop Assimilation Algorithm

Data Acquisition Software

Data Quality Control

Executive System

Validation Software

Kalman Filter Example

Electron Density Along Field Line

Kalman Filter with synthetic data

Truth generated with IFM with Modified equatorial drift

1 Ne Measurement every 15 min at magnetic equator Observation

Truth

Climat

Kalman

Next, consider the more complicated situation:

Global Reconstruction

Many observations

Different kinds of instruments measuring different quantities

Observations are in different places

Global Assimilation of Ionospheric MeasurementsUtah State University, (435)797-2962, [email protected];

Universities of Colorado (Boulder), Texas (Dallas), and Washington“Bringing the pieces together”

We currently assimilate data from 16 globally distributed DISS Stations and more than 100 GPS Ground Stations. The GPS Stations are linked to the Fleet of GPS Satellites. In addition, we assimilate Electron Density Data from two DMSP Satellites and simulated data from the C/NOFS satellite in the Filter.

Data DistributionData Distribution

Kalman Filter ReconstructionKalman Filter Reconstruction

TECTEC

Global Assimilation of Ionospheric MeasurementsUtah State University, (435)797-2962, [email protected];

Universities of Colorado (Boulder), Texas (Dallas), and Washington“Bringing the pieces together”

• In this “test,” the following are variables– TID Equatorward Speed– TID Width in Latitude– TID Amplitude History

• In this “test,” the assumptions are– TID is a perturbation on the background ionosphere.– TID perturbed densities are very noisy– TID moves along a meridian.– Observations lie along the meridian.

• Propagation with simple advective model

A Model TID

1-D TID/TAD

1-D Propagation Propagation along meridian Density Perturbations Network of ~10 observatories

The Observations

10 Stations Density Perturbations 100% Noise

Kalman Filter Reconstruction

# of Stations = 10 100% Noise True Velocity = 2

Reconstruction of TID Density Perturbations

T=1




T=2




T=3




T=4




T=5




T=6




T=10




T=15




T=20


# of Stations = 10 100% Noise True Velocity = 2 Guessed Velocity 50% Off




Reconstruction of TID Density Perturbations Determination of TID Velocity

T=150

Conclusions

Data Assimilation Techniques have proven to be very useful in

Meteorological and Oceanographic Specifications and Forecasts.

Millions of Ionospheric Measurements Per Day Within 10 Years.

Physics-Based Data Assimilation Models Will Provide Real-Time Snapshots of the Global Ionosphere.

Many Outstanding Scientific Problems Will be Resolved.

Results Will be Available for Numerous Ionospheric Applications.

data assimilation for the space environment ludger scherliess center for atmospheric and space...

Documents

data errors

physics data

sparse data

data assimilation techniques

motivation data assimilation

model forecast

picture analysis

model errors