9:50 AM - 10:05 AM
[AAS03-04] Wind speed structure of Typhoon 201821 as seen by CYGNSS and AMSR2
Keywords:remote sensing, typhoon intensity, cyclone global navigation satellite system, advanced microwave scanning radiometer 2, asymmetry of typhoon winds
It is known that typhoons often have asymmetric wind speed distribution in which the wind is stronger in the right semi-circle than in the left with respect to their movement direction. However, since direct observations of synoptic wind field of a typhoon are practically difficult, satellite remote sensing data are required. In this presentation, two different types of remotely sensed wind speed data, CYGNSS and AMSR2, are compared for a case of Typhoon 201821 (JEBI).
The Advanced Microwave Scanning Radiometer 2 (AMSR2) on GCOM-W1 spacecraft measures microwave radiation. In AMSR2 All-weather Sea surface Wind speed (ASW) product provided by JAXA, 2-dimensional wind speed distribution map can be obtained every half day even under heavy wind conditions. Meanwhile, NASA Cyclone Global Navigation Satellite System (CYGNSS) mission operates eight micro-satellites that measures the strength of GPS signals reflected at the sea surface in order to provide frequent observations of wind speeds of hurricanes. The reflected GPS signals are reversely proportional to the sea surface slope created by waves so that wind speed can be retrieved. One micro-satellite provides four 1-dimensional wind speed distribution along trails of GPS refection points during its fly-over movements. We used CYGNSS Ver.3 Level 2 product from NASA JPL PO.DAAC during a period from 27 August to 4 September, 2018 and converted its 1-dimensional data to two-dimensional typhoon coordinate data by referring the best track data from National Institute of Informatics with the assumption that wind speed structures are kept the same during 6 hours of interpolation.
In the early stage of the typhoon from 27 to 31 August, CYGNSS wind speeds show significant asymmetry with stronger winds on the right-hand (northern) side of its westward movement. On 1 September when the typhoon changed its direction to the northwestward, asymmetry of CYGNSS wind speeds became less significant. On the contrary, snapshot wind speed distributions seen by AMSR2 similarly shows asymmetric structures in August, but the asymmetry is much less significant than it seen in CYGNSS data. Reasons of discrepancy between CYGNSS and AMSR2 will be discussed in this presentation, taking account of wave spectrum variations within the typhoon due to swell-wind interactions.
The Advanced Microwave Scanning Radiometer 2 (AMSR2) on GCOM-W1 spacecraft measures microwave radiation. In AMSR2 All-weather Sea surface Wind speed (ASW) product provided by JAXA, 2-dimensional wind speed distribution map can be obtained every half day even under heavy wind conditions. Meanwhile, NASA Cyclone Global Navigation Satellite System (CYGNSS) mission operates eight micro-satellites that measures the strength of GPS signals reflected at the sea surface in order to provide frequent observations of wind speeds of hurricanes. The reflected GPS signals are reversely proportional to the sea surface slope created by waves so that wind speed can be retrieved. One micro-satellite provides four 1-dimensional wind speed distribution along trails of GPS refection points during its fly-over movements. We used CYGNSS Ver.3 Level 2 product from NASA JPL PO.DAAC during a period from 27 August to 4 September, 2018 and converted its 1-dimensional data to two-dimensional typhoon coordinate data by referring the best track data from National Institute of Informatics with the assumption that wind speed structures are kept the same during 6 hours of interpolation.
In the early stage of the typhoon from 27 to 31 August, CYGNSS wind speeds show significant asymmetry with stronger winds on the right-hand (northern) side of its westward movement. On 1 September when the typhoon changed its direction to the northwestward, asymmetry of CYGNSS wind speeds became less significant. On the contrary, snapshot wind speed distributions seen by AMSR2 similarly shows asymmetric structures in August, but the asymmetry is much less significant than it seen in CYGNSS data. Reasons of discrepancy between CYGNSS and AMSR2 will be discussed in this presentation, taking account of wave spectrum variations within the typhoon due to swell-wind interactions.