Tangent height offsets estimated by correlation analysis of ground-based data with O3 limb profiles

J.A.E. van GijselY.J. Meijer

Overview

• Introduction– Data description

• Methodology– Visual analysis versus statistical analysis– Example of statistical analysis

• Results– ESA OL 3.0 and IFE 1.63

• Limb ozone profiles• A-priori profiles

Introduction

• Validation of SCIAMACHY limb ozone profiles with collocated ground-based measurements

• Detection and analysis of altitude offset

• Need for objective method to deal with biases

Datasets

• ESA OL 3.0 O3 limb profiles

• IFE 1.63 O3 limb profiles– Both based on ESA level 1 products

• Collocated ground-based observations:– Lidar– Sonde– Microwave (used for IFE 1.63 only)

Datasets II

• ESA OL 3.0: 454 collocated profiles– 332 lidar ozone profiles– 112 sonde ozone profiles

• IFE 1.63: 2346 collocated profiles– 153 unique lidar ozone profiles– 313 unique sonde ozone profiles– 151 unique microwave ozone profiles

Methodology

• Splining of data points to obtain a common altitude grid

• Iterative shifting of the SCIA retrieved limb profiles for the correlation analysis (-5 to +5 km with steps of 200 m)

• Calculation of correlation coefficient between SCIA retrieval and collocated observation over altitude range 20 - 35 km

• Optimal altitude shift can be found at maximum correlation after all iterations

Uncertainties in methodology

• Chosen altitude range can influence results– Reliability of (collocated) instrument varies with

altitude:• ESA OL 3.0 lower ‘trust’ limit set to 20 km• ESA OL 3.0 has a reference height of 40 km where

relative error becomes very high• Sonde data becomes less reliable over 30 – 35 km

• Interpolation over large intervals– For instance microwave data

• Differences in time/space between collocated measurements

Methodology II

Results: ESA OL 3.0 A-priori

• Mean optimal shift is close to 0. • Spread increases towards the poles (as

expected).

ESA OL 3.0 Limb

• Optimal altitude shift = 1.04 km downwards• Standard deviation has decreased with

respect to a-priori

Mean optimal altitude shift using microwave data strongly deviates from lidar & sonde.

Microwave data have a low resolution and the registration of altitude is not very accurate, therefore they will not be further considered

IFE 1.63 A-priori

Lidar

Sonde

Microwave

160 m

-283 m

947 m

IFE 1.63 A-priori II

All West Central-West

Central-East

East

-178 m -79 m -161 m -212 m -273 m

• Dependency on state position due to differences in latitude

IFE lidar+sonde limb

IFE 1.63 Limb

• Mean optimal shift (based on lidar+sonde) = 1.17 km (1.04 km for ESA OL 3.0)

• Difference between East and West for a-priori was ±200 m

All West

C.-West

C.-East

East

Lidar -966

-810

-941 -986 -113

2

Sonde -128

1

-113

3

-1174 -1338 -152

2

Microwave

-437

-265

-352 -478 -727

Distances in table in meters

4 profiles within state

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

Wes

tCen

tral-W

est

Centra

l-Eas

t

East

Opt

imal

alti

tude

shi

ft [m

] A-priori

Lidar

Sonde

Microwave

Linear (Sonde)

Linear (A-priori)

Linear (Lidar)

Linear (Microwave)

Summary

• The ESA OL 3.0 and the IFE 1.63 O3 limb profiles have been validated using ground-based O3 data– Visual analysis

• Not objective

– Statistical analysis• Offsets: ESA OL 3.0: 1.04 Km

IFE 1.63: 1.17 Km

• Ground-based data should be inter-compared to ensure quality

Questions?

• Thank you for your attention !

IFE Microwave limb