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By Vinca Rosmorduc

CLS, France
 

Altimeter significant wave height tracks
Example of measurements during the Quirin storm
Click for larger image and explanation

Several major storms cross the North Atlantic Ocean every year. These can reach hurricane-force winds, and wave heights follow accordingly - indeed, the highest waves are recorded there. However, studying wind and waves during these storms is a challenge, because there are few in situ observations in the middle of the ocean. Even when there are instruments in the storm region, they barely make it through such extreme weather events. Moreover, wave models are known to underestimate the severity of extreme events. However, remote sensing techniques can measure everywhere and, for radar techniques, in any weather conditions.

Extreme wave height map.
Maximum significant wave height from 20 years of Altimetry
Click for larger image and explanation

This lesson will give you an opportunity to discover the type of information that satellite can provide about storms. It is based on a North Atlantic storm, Quirin, which occurred in February 2011. The highest Significant Wave Height ever measured by a satellite to this day was recorded then. You will be able to display maps of significant wave height (SWH) measured by an altimeter, and compare this with wind speeds measured by a scatterometer.

Introduction

For seafarers, as well as anyone planning coastal installations or living along the coastline, very high waves are a potential danger that cannot be ignored. Altimetry can be used to measure wave heights in the open sea, data which can be integrated rapidly in wave-height forecasting models used by Meteorological bureaus. These wave height forecasts are also broadcast to sailors, either to ensure their safety or during search and rescue. Wave height measurements accumulated over several years along altimetry tracks can also be used to determine trends and estimate maximum wave heights. Moreover, most altimetry satellites also carry a radiometer (see background section 2), meant to correct altimetric height from the perturbations due to the water content of the atmosphere. They can also be used directly, as rain and clouds measurement, and thus as an additional source of information over a storm.

Scatterometry is another active radar technique, which basically measures ocean surface roughness. Since this roughness depends on winds over open water, scatterometers provide both wind speed and direction. Satellites carrying altimeters or scatterometers are not geostationary. Being constrained to their orbit repeat pattern, they are not always able to measure during a storm (because they might not be in the right place at the right time). For example, the Xynthia storm, which caused huge damage in France in 2010 was observed just at its fringe, not at its peak, by one of the 3 altimeters measuring at that time. Despite this limitation many impressive examples of storms are available from the altimetry record.

Lesson Overview

Aim and objectives

At the end of the lesson you should be able to:

Lesson content

The lesson is divided into 4 sections:

  1. Along-track Significant Wave height from altimetry
  2. Radiometer: the water in the atmosphere over oceans
  3. Wind speed from scatterometry
  4. An in-depth view of altimetry: the radar echoes and the wave heights

Data and tools for this lesson

Satellite data sets

RA2_GDR_2PRF-P20110213_231850_000030063099_00275_46840_1267.N1.bz2: Envisat RA-2 and MWR GDR data for 2011-Feb-13 , orbit #275, cycle 099 (ESA http://eopi.esa.int).

JA2_GPN_2PTP096_139_20110214_101739_20110214_111352.nc: Jason-2 GDR data for 2011-Feb-14 , half-orbit #139, cycle 096 (CNES/NASA http://www.aviso.oceanobs.com).

JA2_GPS_2PTP096_139_20110214_101739_20110214_111352.nc: Jason-2 Sensor GDR data for 2011-Feb-14 , half-orbit #139, cycle 096 (CNES/NASA http://www.aviso.oceanobs.com).

ascat_20110214_215400_metopa_22436_eps_o_125_1019_ovw.l2.nc.bz2: Ascat onboard Metop A scatterometer data for 2011-Feb-14 (NRT at OSI-SAF http://www.osi-saf.org; PO.DAAC for older data http://podaac.jpl.nasa.gov).

While the data provided in the above files are limited to the Quirin storm, similar data spanning the globe and archived for many years are available free of charge (some directly on-line, some on request).

Bilko tools used in the lesson

modjet3.pal: colour scale to use for sea level anomalies (SLA).

sla_1997-1999bz2.set: stack of the 36 monthly SLA files ; useful to plot a longitude-time diagram.

ssta_1997-1999bz2.set: stack of the 36 monthly SST anomaly files ; useful to plot a longitude-time diagram.

CRW_SST_ANOMALY.pal: colour scale to use for SST anomalies.

Chlorophyll.pal: colour palette to use with chlorophyll data.

SST_Pathfinder.pal: colour palette to use with SST data.

Downloading the lesson

The lesson downloads contain everything you need to complete the lesson. This includes the data and tools listed above, and three PDF documents:

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