Assessing the sea state in the Arabian Sea during Tropical Cyclone Luban

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Luban went through the life cycle of a tropical storm over the Arabian Sea between 6 October and 14 October 2018.

Date & Time
06 October 2018 06:00 UTC–14 October 18:00 UTC
Meteosat-8 & 10, Sentinel-3A
Infrared Channel, High Resolution Visible (HRV), Significant Wave Height (SWH)

By Shima al Yazidi (DGMET) and HansPeter Roesli (Switzerland)

The Regional Specialised Meteorological Centre (RSMC) in New Delhi upgraded Luban to a category-1 tropical cyclone on 10 October. The sequence of false-colour images of channel IR10.8 from Meteosat-8 (Figure 1) illustrates the evolution of Luban from her birth off the Indian coast to the landfall over eastern Yemen.

Figure 1: Meteosat-8 infrared animation, 6 Oct 00:00 UTC–14 Oct 18:00 UTC. Download animation (MP4, 15 MB)

For cargo vessels and fishing boats in these parts of the Indian Ocean indications about the sea state were of paramount importance, in particular during the most intense period of Luban. In-situ measurements in the area are sparse and limited to few locations, so altimetry measurements from satellites can fill the gaps and provide more spatial coverage.

The Significant Wave Height (SWH) product from the SRAL altimeter on Sentinel-3 is a great aid. SWH is a parameter that is widely used to describe the ocean sea state. It is defined as four times the square root of the integral of the wave spectrum, and closely corresponds to the average height of the highest one third of waves.

Figures 2 and 3 are SWH samples over the Luban track. On 9 October, ahead of being categorised as a Category 1 storm, Luban had estimated wind speeds of 148 km/h–167 km/h (80 kn–90 kn). The SWH swath passed over her eastern side, as shown in Figure 2. The SWH product estimated a SWH maximum of 5.9 m.

Figure 2
Figure 2: Meteosat-10 High Resolution Visible with Sentinel-3 SRAL SWH overlaid, 9 Oct, 05:00 UTC

During the tropical cyclone phase on 12 October an SWH swath was obtained with wind speeds of 185 km/h–204 km/h (100 kn–110 kn). The swath crossed the northern eye wall (Figure 3). There the maximum SWH reached 4.7 m.

On 9 October the highest measured SWH was 6.6 m at lat 14.1, long 62.7. On 12 October the highest was 6.4 m at lat 11.9, long 56.6. Note: These high values do not show in the image due to smoothing of the colour scale along the track.

Figure 3
Figure 3: Meteosat-10 High Resolution Visible with Sentinel-3 SRAL SWH overlaid, 12 October, 06:00 UTC
Figure 4
Figure 4: Scatterplot of Forecast WAM SWH versus altimetry SWH

Altimetry may be employed to evaluate the performance of wave models. Figure 4 shows a comparison of altimetry SWH measurements on 9 October (Figure 1) and SWH estimates from the wave model (WAM). Except for a couple of outliers, the scatter plot shows a close resemblance between the model data and the altimetric measurements.

Note: The SWH measurements from Sentinel-3 SRAL (Ku-band) have an accuracy of around 5% of SWH, globally. Under heavy precipitation the accuracy may be slightly degraded, but there is no significant validation data to assess the quality in these extreme conditions. Such overlays have been published in the past for Hurricane Katrina. There too, satellite altimeters (ERS-2, Envisat, GFO, TOPEX, and Jason-1 at the time) showed consistent results for SWH throughout their tracks across the hurricane up to at least 10 m wave height. See: Scharroo, R., W. H. F. Smith, and J. L. Lillibridge, Satellite altimetry and the intensification of Hurricane Katrina and Reply to Comment on "Satellite altimetry and the intensification of Hurricane Katrina", Eos Trans. AGU.

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