Coastal Waters Research Synergy Framework

Coastal Altimetry Nadim Dayoub, Paolo Cipollini, Helen Snaith

This project has received

funding from the European

Union’s Horizon 2020 Research

and Innovation Programme under

grant agreement no 687289

One of the research applications that will be demonstrated concerns coastal altimetry

So we need to introduce you to satellite altimetry first, then motivate its extension to the coastal zone, and explain some of the processing that support the application

In detail, today’s objectives are:

examine the need for satellite altimetry

introduce the technique

introduce coastal altimetry

make sure you’re familiar with the main processing steps that are needed to produce good data

show how the data can be used for and sea level research, in a hands-on demonstration

2Aim of this lecture

Let’s start from a very simple question:

3Introduction

Coastal Waters Research Synergy Framework

Is the sea really flat?

No, there are ‘bumps’ and ‘troughs’ due

to ocean circulation

And the mean sea level is not constant

but it varies with time.

Picture: funasagran.co.uk

the signature of oceanic currents

the signature of extreme events,

such as storm surges

the long-term variations in sea level

due to melting of glaciers and ice

sheets and to thermal expansion

5What sea level tells us

Oceanic Currents

NEMO Ocean model output

Andrew Coward, NOC

Southampton

At large scales (100+ km) the sea has bumps and

troughs of 10s of cm and there is geostrophy –

balance between pressure gradient and Coriolis

force – so large scale currents are on the sea level

slope as shown below

www.seos-project.eu

Surge due to Hurricane Katrina,29 August 2005

Extreme Events - surges 7

8Long-term Sea Level change

Credit: J. Balog/Extreme ice Survey

Melting of Columbia Glacier, Alaska.

2009 2015

Sea Level Information is essential

… to coastal managers, marine operators,

oceanographers, climate scientists:• rates of sea level rise

• amplitude/phase of periodic variations• statistics of extreme events

Fact: Sea Level is

changing10

Tide gauges at the coast and on islands

good for local rates

but sparse, affected by land movement

(gives relative sea level)

Satellite altimetry

truly global!

measures absolute sea level

data along the track of the satellite can be

interpolated to give 2-D maps

11How do we measure sea

level?

Altimetry: Basic Principles

The altimeter is a radar at

vertical incidence

The signal returning to the

satellite is from quasi-specular

reflection

Measure distance between

satellite and sea (range)

Determine position of satellite

(precise orbit)

Hence determine height of

sea surface:

Height = Orbit - Range

To look for currents

oceanographers require

height relative to geoid

(for mean sea level variations

we don’t need the geoid)

Sea Surface

Geoid

Reference ellipsoid

Satellite

orbit

Ocean dynamic surface topography (DT)

= Orbit - Range - Geoid

Orbit

height

Range

12

It’s done by measuring the time T that the radar pulse takes for the two-way trip

range = c . T / 2

c speed of light in vacuum

BUT we want altimetry to be very accurate ~ 1 cm from ~1000 km

so we need to CORRECT for all possible ‘delays’ to the pulse

(it’s not in vacuum! there’s the atmosphere in between)

13Measuring Range

Range

Instrumental corrections

distance antenna/Center of Gravity; internal path

delay

Ionospheric correction

compensates for delay due to free electrons in the

ionosphere

made with models, or derived by dual-frequency

altimeter ranges

Dry tropospheric correction

compensates for delay due to air molecules

made with atmospheric pressure models

14Range corrections - 1

Wet Tropospheric correction

compensates for delay due to water vapour. Tricky! (WT varies fast in space and time!)

can be done with a WT model (ECMWF for instance)

better: from a microwave radiometer (MWR) co-located on the altimeter satellite. But falls apart in the coastal zone when land enters MWR footprint

best: combination of MWR, models, WT observations at GNSS coastal stations (“GPD+” correction by University of Porto)

Sea State Bias

depends on sea state - compensates for wave skewness and stronger returns from troughs than crests

empirical models are used

15Range corrections - 2

The measured height also contains the signal due to TIDES and to ATMOSPHERIC FORCING on the

surface of the ocean (pressure and wind)

for some applications these signals are left in – for

instance if interested in the total water level in a

storm surge

for other applications there are removed, using

tidal and atmospheric models – for instance to

study long-term sea level variations.

16Geophysical corrections

Sea Surface Height Anomaly (SSHA):

SSHA= ALT – ( Robs + corrections ) – MSS

SSHA= ALT – Robs – Δiono–Δdry–Δwet–Δssb– hocean – hload – hearth – hpole– hatm– MSS

Dynamic Topography (DT): replace MSS with the geoid in the

above equation.

Total Water Level Envelope (TWLE): keep the ocean, pole

tides and atmospheric forcing effects

TWLE= ALT – Robs – Δiono–Δdry–Δwet–Δssb – hload – hearth – MSS

17SSHA, DT & TWLE

1-D (along-track) measurement18

Example: Sea Surface Height along the ground track of a satellite altimeter

19

Coastal Altimetry

Standard altimetry does not quite go all the way to the coast!

Traditionally, data in the coastal zone are flagged as bad and left unused

(coastal zone: as a rule of thumb 0-50 km from coastline, but in practice, any place where standard altimetry gets into trouble as radar waveforms are non-standard and/or corrections become inaccurate)

20+ years of data in the coastal stripcan be recovered!

corruptedwaveform

In recent years a vibrant community of researchers has started to

believe that most of those coastal data can be recovered and that coastal altimetry can be a legitimate component of

coastal observing systems!

20

NOC’s coastal altimetry

processor (ALES) Adaptive Leading-Edge

Subwaveform algorithm (ALES)

(Passaro et al., 2014)

Yields more precise SSH and

SWH in the coastal zone:

Validated for SSH for Envisat,

Jason-1/-2, AltiKa (Passaro et al

2014, 2015a, 2015b, 2016,

Passaro and Quartly 2015)

Validated for SWH for Envisat,

Jason-1/2 (Passaro et al 2015)

21

Geographical region

lon/lat box

Time domain

start/end time of the requested data

Distance limits

Distance in km from the coastline

Range and Geophysical Corrections

mainly for experience users (Default options available)

We get up-to-date corrections from the RADS archive

(rads.tudelft.nl)

Current Selections Status 22

23Choice of the Geo Box

Current Outcome

A structure of arrays (netCDF format)

Time, lat, lon

Orbit, Range, MSS, Geoid

SSHA, TWLE, SWH, Sigma0

Error Fields

All used corrections

24

25Coastal Processor Work Flow

Time span Lon/lat box Distance limits

Catalog

Search

J2 SGDR files

RADS files

ALES

Processor

netCDF

output

Publish

outputs for

user to

download

We will show how to derive along-track profiles of SSHA, TWLE and SWH for a couple of Jason-2

tracks crossing the Portuguese coast

For the reprocessing we will use updated

corrections and the ALES retracking algorithm that

normally yields reliable estimates up to 2-3 km from

the coast

These data support applications on coastal

dynamics, sea level rise, monitoring of the wave

field, etc…

26Specific application

Nadim Dayoub, NOC Southampton (nayoub@noc.ac.uk)

Paolo Cipollini, NOC Southampton

(cipo@noc.ac.uk)

27Contact us