Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 1/19

The wet tropospheric correction for coastal altimetry

T. Strub

S. Brown

F. Mercier

G. Wick

R. Scharroo

T. Haack

E. Obligis

B. Khayatian

C. Desportes

L. Eymard

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 2/19

OUTLINE

1) What do we know about coastal water vapor and the distance and time scales over which it changes ?

2) What models are available for making the wet delay correction, what in situ or remotely sensed data do they assimilate, and how should they be interpolated ?

3) How are water vapor radiometers on board altimeters affected by land in the footprint ?

4) How should these data be processed ?

+ 5) Radiometer design for future altimetry missions ?

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 3/19

Surface temperature differences between land and sea create land and sea breeze, and therefore short time scales wind and humidity structuresRelief can modify locally the humidity, the air becomes drier when passing over a mountainAtmospheric rivers: large scale phenomena but imply high spatial variability near coasts

1) What do we know about coastal water vapor and the distance and time scales over which it changes ?

SSM/I Integrated water vapor (cm)

1000 km

G. Wickdh ~ 6cm dh~42cm

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 4/19

Actual global meteorological models (ECMWF, NCEP) are most of the time not accurate enough because:

Their spatial and temporal resolutions are not adapted (6 hours, 0.5° for ECMWF) to coastal humidity variability (see previous slide)

They assimilate a lot of in-situ measurements (radiosondes) but also satellite radiometer brightness temperatures that suffer the same contamination problems in coastal zones !

2) What models are available for making the wet delay correction, what in situ or remotely sensed data do they assimilate, and how should they be interpolated ?

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 5/19

Aladin model : operational mesoscale forecast model of Météo-France (0.1° resolution) over the Mediterranean sea

2) What models are available for making the wet delay correction, what in situ or remotely sensed data do they assimilate, and how should they be interpolated ?

-2cm 2cm

Difference with ECMWFMistral caseLittle structures (<100km)not represented in the ECMWF model

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 6/19

Navy's high-resolution mesoscale model COAMPSRunning operationally at 9km resolution or better for 7 regions around the globe Assimilates radiosondes, surface stations and buoy data (3DVAR).  At a later date TOVS and SSM/I water vapor products10 years of archived forecasts over the U.S. West Coast for validation and research studiesRobustly validated and documentedUsed to get information on atmospheric variability and modeling capability

gradients of 5-6 cm over ~100km.

2) What models are available for making the wet delay correction, what in situ or remotely sensed data do they assimilate, and how should they be interpolated ?

Performed well at offshore sites (Channel Islands and NPS ship) but did not handle the intricacies of local circulations at coastal sites (Pt. Magu, LA, San Diego), or sea/land breeze phenomenaSuggests substantial mesoscale variation in the integrated water vapor field linked to block coastal flow

17.3

9.6

1.9

25 cmCOAMPS (9km) Integrated Water Vapor – 14 June 2006

T. Strub & T. Haack

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 7/19

A

B

Cape Blanco

Cape Mendocino

Mixing ratio (g/kg)

6-hrly profiles from grid 3 (x=5 km)

COAMPS1

Hei

ght (

km)

2.0

1.5

1.0

0.5

0.0

0.0 3.0 6.0 9.0 12.0 15.0

Hei

ght (

km)

2.0

1.5

1.0

0.5

0.0

Point A

Point B

12 UTC 18 UTC00 UTC

2) What models are available for making the wet delay correction, what in situ or remotely sensed data do they assimilate, and how should they be interpolated ?

A and B points separated by ~100km, ~30-50 km from the coast.

Shows significant differences between A and B atmospheresShows diurnal variation of water vapor profile for the 2 points

Strong spatial and temporal humidity variations

T. Haack

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 8/19

The same way as the other radiometers (SSM/I, TMI, AMSU…)

Land emissivity nearly twice sea emissivity + more variable in space and time

For a surface temperature of 300K, a 10% land contamination in the sea pixel will increase the TB of more than 10K several centimenters !

Classical retrieval algorithms developed assuming sea surface emissivity modelling are no more valid

3) How are water vapor radiometers on board altimeters affected by land in the footprint ?

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 9/19

TB contamination by land depends on:radiometer antenna pattern (gaussian shape, defined by the resolution 3dB)land surface emissivity and temperature proportion of land in the footprint

TBland=250K

TBsea=150K

3) How are water vapor radiometers on board altimeters affected by land in the footprint ?

Simulation on a simple case

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 10/19

TOPEX track 187

1

2 3

TM

R 2

1 G

Hz

TB

s (K

)3) How are water vapor radiometers on board altimeters affected by land in the footprint ?

TB contaminationSome example of real data

1- Land/sea transition

2- Overpassing of Ibiza Island

3- Track tangent to the coast

We simulate quite well the contamination so correction is feasible…

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 11/19

Coastal Impact on SSM/I IWV

SSM/I-derived integrated water vapor normalized by latitude and value 1000 km normal to shore

Composite profile suggests land influence for distance lower than 150 km from coast

Depends on radiometer antenna pattern

Composite from 2274 days between 1997-2006 without

atmospheric river events

3) How are water vapor radiometers on board altimeters affected by land in the footprint ?

G. Wick

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 12/19

3) How are water vapor radiometers on board altimeters affected by land in the footprint ?

Near sidelobes

Mainbeam

Far sidelobes

Land contamination can be divided into three categories• Far sidelobe contamination ( > 75 km from coast)

Correctable to acceptable levels (~ 1mm)• Near sidelobe contamination (25 – 75 km from coast)

More difficult, but correction is possible (~2-4 mm)• Main beam contamination ( 0 – 25 km from coast)

Very difficult to correct (>2cm)

PD Error vs Distance to Coast

Mainbeam Near sidelobes Far sidelobes

S. Brown

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 13/19

4) How should radiometer data be processed ?Post processing methods

F. Mercier

-0.2

-0.18

-0.16

-0.14

-0.12

-0.1

-0.08

-0.06

-0.04

32 33 34 35 36 37 38 39 40 41 42 43 44

Tropo Composite

Tropo modèle

Tropo radiomètre

Transition

Transition

Ocean

Ocean

Ocean

Ocean

Petit Trou

Grand Trou

Sardaigne

France

Lybie

-0.2

-0.18

-0.16

-0.14

-0.12

-0.1

-0.08

-0.06

-0.04

32 33 34 35 36 37 38 39 40 41 42 43 44

Tropo Composite

Tropo modèle

Tropo radiomètre

Transition

Transition

Ocean

Ocean

Ocean

Ocean

Petit Trou

Grand Trou

Sardaigne

France

Lybie

Principle:

1°/ to identify several configuration types

2°/ to define corresponding correction algorithms

3°/ to compute a composite correction profile

•Open Ocean (> 50 km) radiometer

•Transition model shifted to the value of the last valid radiometer correction

•Small hole interpolation between the last valid radiom. Corr. Of each side of the hole

•Large Hole Model (shift + slope)

•Coastal path -- > model

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 14/19

Lessons learned II – Beyond flags: new editing strategy Lessons learned II – Beyond flags: new editing strategy

Screening profiles rather than single values Reconstructing /extrapolating profiles where possible

Much more data on average than using standard editing (more in “Data editing” session …)

4) How should radiometer data be processed ?Post processing methods

S. Vignudelli

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 15/19

Analytical methods of correction based on the proportion of land in the footprint p

= land/sea mask smoothed by the antenna pattern

4) How should radiometer data be processed ?

Processing methods

1- Land/sea transition

2- Overpassing of Ibiza Island

3- Track tangent to the coast

Previous decontamination of the TBs Or retrieval algorithms function of p

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 16/19

4) How should radiometer data be processed ?

Processing methodsOptimal combination of radiometer measurements and

meteorological model estimations

Use of a 1d variationnal method Background information (atmospheric profiles) provided by a meteorological model Land emissivity values from atlases computed by Karbou et alProportion of land in the pixelHumidity profiles are adjusted to minimize the difference between simulated and measured brightness temperatures

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 17/19

Use of higher frequencies

•much better spatial resolution (~5km instead if 30km)

•Less sensitive to land contamination

Radiometer design for future altimetry missions ?

AMSU-A23.8 GHz

AMSU-B176.31 GHz

0 100km 0 100km

180K

210K

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 18/19

Addition of 92,130,166 GHz to traditional 18-37 GHz design High frequency channels used to estimate water vapor

when traditional low-frequency channels become contaminated by land

Simulated PD retrieval performances better than 1cm at distances of 3km from land

Radiometer design for future altimetry missions ?

Simulated TBs approaching coast

Extrapolated Path Delay

Linear extrap.

Quadratic extrap.

- True PD

- HF Extrap.

- LF PD

S. Brown

Coastal Altimetry Meeting, Silver Spring, 5-7 February 2008 Page n° 19/19

SUMMARY

Coastal water vapor is highly variable in space and time Small scale structures are hardly predictable with present models despite their constant improvementsMicrowave radiometers onboard altimetry missions are strongly contaminated by land, like the other ones…Different interesting post-processing and processing methods are proposed to improve the coastal products…on going studies

Solutions: use of GPS measurements to provide or validate new productscombination of models and radiometer informationuse of high frequencies radiometer to get a much better spatial resolution