Haiyan Typhoon: the brightest natural source of L-band radiation ever measured over the oceans

Category : CATDS, Ocean

N. Reul

Ifremer

Typhoon Haiyan (known in the Philippines as Typhoon Yolanda) slammed into the Philippines less than 2 weeks ago with sustained winds of 310 kilometers per hour, making it one of the strongest tropical storms to date and the second-deadliest Philippine typhoon on record. Haiyan originated from an area of low pressure in the Federated States of Micronesia on November 2. Tracking generally westward, environmental conditions favoured tropical cyclogenesis and the system developed into a tropical depression the following day. After becoming a tropical storm and attaining the name Haiyan at 0000 UTC on November 4, the system began a period of rapid intensification that brought it to typhoon intensity by 1800 UTC on November 5. By November 6, the Joint Typhoon Warning Center (JTWC) assessed the system as a Category 5-equivalent super typhoon on the Saffir-Simpson hurricane wind scale; the storm passed over the Palau shortly after attaining this strength.

SMOS_Hyan_4to9_bis

Figure 1: SMOS retrieved surface wind speed [km/h] along the eye track of super typhoon Haiyan from 4 to 9 Nov 2013.

SMOS intercepted the typhoon several times along its track. We selected only those passes where the signal was well detected and not too contaminated by RFI or land masses. As illustrated by Figure 1, this let one pass on the 4 as Haiyan was still a Tropical Storm, two on the 6th Nov (with the morning pass capturing only a small portion of the typhoon), one on the 7 prior landing towards Philippines and one interception on the 9, just before it passed over Vietnam. Passes on the evening of the 6 and during the 7th morning were close in time to the maximum intensity reached by that super storm (reached on the evening of the 7th).

As illustrated by Figure 2 top panel, the estimated excess brightness signal (First stokes parameter/2) due to surface roughness and foam-formation processes under the cyclone on the 7th morning overpass (i.e., after correcting for atmosphere, extra-terrestrial sources, salinity and temperature contributions) reached a record value of 41 K. To put such value in perspective of other natural oceanic signals, we plotted together the Tb jump measured during the passage of Hurricane Category 4-5 Igor in 2010, which was only 22 K! In contrast, global changes of surface salinity (32-38 pss) and temperature (0°C-30°C) only modify the Tb by ~5 K. So we believe such signal is very likely a natural extreme of sea surface emission at L-band over the oceans.

compHaiyanIgor

Hyan_20131107_915_sectionNS_130W

Figure 2: Top: North-south section trough the Haiyan Typhoon showing the change of residual brightness temperature (Th+Tv)/2 reconstructed from SMOS data at longitude of 130.05°E on the 7 Nov 2013 at 09:15Z Typhoon (black). The Blue curve is showing an equivalent section through the Igor Category 4 hurricane in 2010. The red line is illustrating the range of brightness temperature variation expected on earth due to sea surface salinity and temperature changes. Bottom: surface wind speed deduced from the excess brightness temperature.

Application of the wind-speed bi-linear retrieval algorithm that we derived in 2012 based on the established relationship between surface wind speed estimates during IGOR and the excess brightness temperature, we obtained the wind speed module shown in Figure 2, bottom. One can easily see that around the cyclone eye, wind speeds largely exceed the 64 knots threshold for typhoons within a more than 50 km radius. The spatial resolution of SMOS however does not allow to resolve the detailed wind speed structure around the eye. The maximum wind estimated from SMOS reaches 142 knots !

SustainedWind

Figure 3: Maximum sustained 1 minute wind speed estimated during Haiyan Typhoon. From SMOS data (black filled dots) compared to Advanced Dvorak Technic (ADT=blue diamond), CIMSS (yellow filled dots), SATCON (red) and Best Track from NHC (cyan). Note the empty circle correspond to the SMOS measurements for the 11/06 morning for which only a small portion of the cyclone signal was intercepted. Maximum 10 minutes wind speed deduced from SMOS algorithm were multiplied by 1/0.93, adopting the conversion factor proposed in (Harper et al., 2010) between one minute winds and 10 min winds.

Given the spatial resolution of SMOS, the wind speed measured is more equivalent to a 10 minute sustained wind than to a 1 minute one, traditionally used by forecasters in the US. Using a 0.93 conversion factor from 1 mn to 10 mn winds (Harper et al., 2010), 1 minutes sustained winds can be estimated from SMOS. The evolution of the maximum sustained wind speed deduced from SMOS is compared to other estimates in Figure 3. SMOS estimate compares well with standard methods. The Advanced Dvorak Technique (ADT) uses longwave-infrared, temperature measurements from geostationary satellites to estimate tropical cyclone (TC) intensity. This step-by-step technique relies upon the user to determine a primary cloud pattern and measure various TC cloud top parameters in order to derive an initial intensity estimate. It continues to be the standard method for estimating TC intensity where aircraft reconnaissance is not available (all tropical regions outside the North Atlantic and Caribbean Sea), however it has several important limitations and flaws. The primary issue centers upon the inherent subjectivity of the storm center selection and scene type determination proceedures. Secondly, learning the Dvorak Technique and its regional nuances and adjustments can take a significant time to master. SMOS winds in TC will be produced operationally next year so as to be ingested by numerical weather forecast model within 6 hours from acquisition. In regions not too polluted by RFI, this new impressive example shows again that SMOS new information shall well complement existing satellite observation to better forecast TC intensification and evolution.


Using SMOS to analyze the variability of the South Pacific Sea Surface Salinity maximum

Category : L3, Ocean

By Jacqueline BOUTIN

Understanding the variability of high-salinity surface waters, as shown in Fig. 1 for the south-eastern tropical Pacific, is important to improve our interpretation of climate and hydrological cycle changes at different time scales. SMOS CATDS-CEC LOCEAN SSS products have been used , in complement to Voluntary Observing Ships (VOS) thermo-salinograph data obtained from the French SSS Observation Service, to validate and understand the seasonal variability of the South Pacific Sea Surface Salinity maximum simulated by an ocean general circulation model with no direct SSS relaxation.

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Fig. 1. Mean 1990-2011 modelled mixed-layer salinity. The blue lines represent the Matisse Ship routes of 2010 and 2011.

All products reveal a consistent seasonal cycle of the displacement of the 36-isohaline barycenter (Fig. 2; about +/-400 km in longitude) in response to changes in the South Pacific Convergence Zone location and Easterly winds intensity respectively associated with changes in precipitation and evaporation.

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Fig.2. Location of isohaline 36 (simulated) and of its barycentre (dots: model; stars: SMOS) for various months (colors).

The SSS from 8 VOS transects compare remarkably well with collocated SMOS SSS averaged over 100km, 18 days (std difference=0.2), as exemplified in Fig. 3 along a shipping track running from New Zealand to Panama ; the comparison with simulated SSS is slightly degraded due to a few degrees latitudinal shift of the simulated SSS maximum (std difference=0.26).

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Fig. 3. Example of comparison between SMOS (dots), VOS (straight line), and simulated (dashed line) SSS as a function of latitude.

Model results and in situ measurements further indicate a low frequency westward shift of the 36-isohaline barycenter (about 1400 km since 1992) that could not be linked to ENSO and may reflect the signature of decadal changes and/or global warming.

Details can be found in: Hasson, A., T. Delcroix, and J. Boutin (2013), Formation and variability of the South Pacific Sea Surface Salinity maximum in recent decades, J. Geophys. Res. Oceans, 118, doi:10.1002/jgrc.20367.

Summary of the SMOS Aquarius Workshop

Category : Cal/Val, Data, L1, L2, L3

SMOS-AQUARIUS SCIENCE WORKSHOP

BREST April 15-17 2013

Summary

The SMOS-AQUARIUS science workshop took place on 15-17 April at IFREMER (France). The workshop was jointly organised between ESA, NASA and IFREMER, with support from CNES and the EC’s COST action SMOS-MODE. Even though it was the first event of this kind, it presents a natural extension of a long-standing cooperation between the SMOS and AQUARIUS satellite teams, which started already in the respective development phases. On a working level, members of both teams are regularly attending relevant science and advisory boards on either side. More than 120 scientists from research institutes worldwide attended the workshop, with more than 80 contributions being submitted.

1. Objectives of the meeting

The main objectives of this workshop were to:

• Provide the SMOS and Aquarius science communities with

  • an overview on the present quality of the provided data, and

  • an outlook on future algorithm developments.

• Explore common L-band sensor and algorithm issues for sea surface salinity and soil moisture retrievals derived by SMOS and Aquarius data.

• Present broader science achievements of both missions, including modelling results.

• Explore synergistic use of and validation approaches for SMOS, Aquarius and other sensors’ data over land and ocean.

• Provide a forum for discussion on specific topics related to improving sea surface salinity and soil moisture retrievals through dedicated working groups.

2. The workshop programme

The above objectives were addressed in five sessions, with associated poster sessions. For the detailed programme and all presentations and posters see –>  www.smosaquarius2013.org.

Session 1 – Opening session: This session included programmatic overviews from ESA, NASA and CNES (Mecklenburg, Lindstrom, Lambin), the overall mission status for SMOS and Aquarius (Kerr and Lagerloef) as well as from the ocean user perspective (Reul) and an introduction to the scope of the two working groups on stratification and inter-calibration (Boutin and LeVine), that were established at the workshop.

Session 2 -Instruments’ performance and inter-calibration, algorithm development: the first set of presentations in this session focussed on the quality and expected processor improvements for SMOS level 1 data (Martin-Neira), inter-calibration and inter-comparison between SMOS and AQUARIUS (Cabot, Bindlish, Macelloni and Skou), and detailed aspects of the processor/retrieval evolutions for level 1 brightness temperatures for both missions (Misra, Colliander, Kainulainen, Kao, Turiel). The second set focussed on retrieval algorithm issues for sea surface salinity data products at level 2 for both missions, including dielectric constant modelling, Galactic noise and sea surface roughness corrections etc (Dinnat, Yueh, Meissner, Spurgeon, Tenerelli and Gourrion) and impacts of ascending/descending biases in Level 1 data on the quality of the retrieved sea surface salinity (Banks). This session also provided the input for the discussion in the working group on inter-calibration.

Session 3 -Product validation & stratification: the first part of the session focussed on problems encountered when comparing/validating satellite derived sea surface salinity with in-situ measurements from ARGO and drifters (Drucker, Ward, Reverdin, Boutin, Chao, Asher), these presentations provided the input for the discussion in the working group on stratification. The second part included presentations focussing on the validation of both SMOS and Aquarius derived sea surface salinity in different regions (Lagerloef, Abe, Button, Bulusu) and an error budget estimation due to small-scale horizontal and vertical variability in salinity fields (Vinogradova).

Session 4 – SMOS and Aquarius science application and synergies: Presentations in this session explored the value of both missions’ data for monitoring (multi-annual) oceanographic features/anomalies (Durand, Lee, Reul) and climate change aspects (Hasson, Reul), the link to the global and local features of the water cycle (Gordon, Sato) and synergistic use of both missions’ data (Xie, Liu).

Session 5 -Beyond salinity: soil moisture, storms and cryosphere: contributions in this session were all presented as posters and comprised storm monitoring (Reul, Calla), cryospheric applications of SMOS and Aquarius data for sea ice monitoring and freeze/thaw detection (Maass, Huntemann, de Matthaeis, Miernecki, Tian-Kunze, Xu), land applications (Miernecki) and iceberg tracking (Slominska).

3. Working groups

Further to the science programme two working groups were established, one on surface stratification and the other on inter-calibration/inter-comparison issues. The summary of the respective discussions and a brief indication how to organise future work can be found below.

3.1 Satellite & In Situ Salinity (SISS) Working Group (WG):

Understanding Stratification and Sub-Footprint Processes Co-Chairs: J. Boutin (LOCEAN/CNRS), Yi Chao (RSSI), C.Banks (NOC)

Contact: To join this working group please contact Jaqueline Boutin at jacqueline.boutin@locean-ipsl.upmc.fr or Yi Chao at ychao001@gmail.com;

A common working group mailing list and webpage/webforum, including document sharing and data links is planned.

Scope: The major goal of this WG is to improve our understanding of the link between L- band satellite remotely sensed sea surface salinity (SSS), as provided by SMOS and Aquarius at approximately 1 cm below the sea surface, and in-situ measured salinity, routinely measured at a few meters’ depth by ships and ARGO floats and recently accessible up to a few cm depth by new profilers, surface drifters etc. and to develop methodologies for relating satellite derived salinity to other estimates of sea surface salinity.

This working group will, amongst others, address the following main questions:

o What is the salinity vertical distribution (stratification) in the first 10m below the sea surface and what is the relation with atmospheric forcing including wind and rain? Under which conditions are the new satellite SSS able to provide new reliable information on SSS within the first cm complementary to existing deeper in-situ information for studying air-sea exchange processes? In case of a rain event, what is the order of magnitude for rain-induced vertical gradients and how to best correct for other perturbing effects impacting L-band radiometric measurements (squall, downdraft/updraft, falling droplets induced roughness, atmospheric rain-induced contribution to the mission)?

o What is the magnitude of SSS variability within a satellite footprint (SMOS (40 km) and Aquarius (50-150 km)); how could this variability be taken into account for in- situ/satellite SSS comparisons?

o How can satellite derived SSS be assimilated/used as input to ocean circulation models? Are current ocean circulation models configured in a way to deal with the surface layer processes inherent in satellite derived SSS?

o Which kind of experimental/numerical experience could support investigations related to the questions above (e.g. field campaigns)?

First activities will concentrate on better defining the physical meaning of the satellite and in-situ sampled salinity, and how L-band satellite salinity and ocean salinity measured at a few centimetres/metres depth should be referred to.

The working group findings and results should be summarized in a report and/or a white paper to be published on the working group webpage. Further publishing could be done in scholarly journals (e.g. BAMS).

3.2 Working group on salinity inter-comparison/cross-calibration

Co-Chairs: David LeVine (NASA), Gary Lagerloef (ESR), Yann Kerr (CESBIO),

JordiFont (ICM/CSIC), M.Portabella (UTM-CSIC)

Contact: To join this working group please contact David LeVine at David.M.LeVine@nasa.gov, who will form a working group mailing list and begin an email « conversation“.

Scope: The focus of this working group is on identifying an approach for inter- comparison/cross-calibration of SMOS and AQUARIUS data. This will include an agreement on the techniques and reference sites being used for this purpose. Before entering the actual inter-comparison, some fundamental issues in the understanding of L-Band remote sensing need to be addressed, specific to both instruments. In the long run this working group aims toward a merged and validated data product using commonly agreed standards. It will need to be confirmed on what level of data the merged product should occur, i.e. level 1 brightness temperatures (i.e. Fundamental Climate Data Records) or level 2 sea surface salinity (i.e. Thematic Climate Data Records).

This working group will, amongst others, address the following main questions:

o Even though both instruments measure in the same spectral region, both data sets are based on

o different technology (SMOS microwave interferometric radiometer versus AQUARIUS conventional pushbroom radiometer and scatterometer);

o a different approach to calibration (SMOS uses cold sky and warm load and AQUARIUS uses ocean and cold sky);

o differences introduced through the instrument hardware itself (orbital and long-term drifts, uncertainties in antenna pattern, pointing, etc);

o differences in the retrieval algorithms for the Level 2 sea surface salinity data product ( e.g. in the choice of auxiliary data and the models used).

Hence it will be necessary to first address these differences and their impact on the results of an inter-comparison. Initial work will concentrate on solving several key common geophysical modelling issues related to galaxy, roughness, emissivity, ascending-descending biases, RFI. This will lead to inter-comparable data sets, even though it is recognised that residual uncertainties will remain related to the above differences.

o The next step will concentrate on agreeing on the approach for inter-comparison (e.g. using a double difference or a tuned-location approach).

4 Areas of research that we should encourage more

o Assimilation of SSS data into ocean circulation models.

o Real synergistic use of the SMOS and AQUARIUS data to exploit their complementarity.

5 Recommendations

o Long-term data continuity required to establish merged/synergistic data products and provide answers in climate change research; how will continuity be ensured beyond SMOS, AQUARIUS and SMAP? What could be possible implementation routes for future concepts such as SMOS Ops or SMOS Next?

o Funding agencies should work together in considering long-term plan to provide L-Band observations continuity.

o Common problems should be pursued on international level: RFI!

o Establish a nomenclature and agreed standard for inter-comparisons between satellite derived and in-situ salinity observations, which will be a pre-requisite for merged products between SMOS and AQUARIUS, also in sight of SMAP.

o Sea surface salinity has already been recognised as an Essential Climate Variable (ECV) but is presently not yet pursued as part of the ESA Climate Change Initiative (CCI) studies. SSS CCI study should be initiated as part of the second phase of ESA’s CCI studies.

o Recognising the benefit of this event, SMOS-Aquarius workshops could be continued in the future, possibly every 1-2 years.

Creation of the Satellite & In Situ Salinity Working Group: Understanding Stratification and Sub-Footprint Processes

Category : Cal/Val, Data, L2, Ocean, Satellite

Following the SMOS-AQUARIUS workshop, Jacqueline Boutin and Yi Chao have worked on defining the terms of reference for the SMOS AQUARIUS working group they co-chair, the:
‘Satellite & In Situ Salinity Working Group: Understanding Stratification and Sub-Footprint Processes’ (SISS in short).

With the help of Olga Hernandez, a mailing list and dedicated website have been established :
www.locean-ipsl.upmc.fr/SISS). The idea being that anyone can use the mailing list to exchange on SISS topics etc
You will find the charges or terms of reference on the web page!
So why wait: visit!