Recently, the Northwest Pacific Ocean experienced the most intense Tropical Cyclone (TC) worldwide for 2017: Super-Typhoon Noru. Noru was the third longest lasting TC ever of the Northwest Pacific Ocean, from July 19 until August 9, 2017. It experienced a very rapid intensification on July 30, when within 24 hours it went from Category 1 to Category 5.
For most of the time before landfall, Noru travelled in remote locations over the ocean out of reach from in-situ measurements (dropsondes or hurricane-penetrating aircrafts). Satellites provided the only source of observation for the storm wind speeds at the ocean surface, typically derived from scatterometers. However, at the intense wind speeds such as those recorded for Noru, the scatterometer observations become less reliable because they lose sensitivity to the observed physical properties at the ocean surface.
Here we describe the observations from SMAP, a new NASA radiometer at L-band that retains very good sensitivity at intense winds and is not significantly affected by rain (Meissner et al., 2017). We compare intensity and storm size observed by SMAP with the EUMETSAT scatterometer ASCAT (C-band), and with the best track dataset for Noru provided by the Automated Tropical Cyclone Forecasting System (ATCF) developed by the US Navy (Sampson and Schrader, 2000). The best track datasets are developed using different sources of data: dropsondes and aircrafts (when available), microwave and infrared satellite sensors, radars and Dvorak technique or other objective analyses.
The issues we intend to address here are:
- Is SMAP providing reliable observations of the TC intensity (max wind speeds) and the rapid intensification?
- Is the ASCAT scatterometer reliable at these intense wind speeds?
- Do SMAP and ASCAT realistically estimate the storm size, a parameter that is used by forecasters to issue tropical cyclone public advisories?
The following image displays Noru as seen by ASCAT (left) and SMAP (right) on August 3, 2017, when both the satellite sensors had a full view of the storm, albeit 7 hours apart. Both sensors display a circular structure of the storm, with a very wide eye, about 100 km in diameter, and with the highest wind speeds located in the North-East quadrant. However, the storm intensity observed by ASCAT (31 m/s) is much smaller than for SMAP, which peaks at 41 m/s. The uncertainty on the satellite wind speeds is of the order of 10%. The contour lines mark the areas for wind speeds of 17.5, 25.7, and 33 m/s (34, 50 and 64 knots, respectively), typically used for tropical storm advisories.
We compared the peak intensity observed by the two satellite sensors with the best track data from the US Navy for the full duration of the storm (figure below). The best track data are provided as maximum 1-minute sustained wind speeds. For this comparison they were scaled to 10-minute sustained winds using a scaling factor of 0.93 as recommended by the World Meteorological Organization (Harper et al., 2010). The 10-minute sustained winds provide a more meaningful comparison to satellite observations which refer to spatial scales of about 25 km.
The figure displays the evolution of the storm and the rapid intensification on July 30. The evolution of the maximum winds for SMAP is consistent with the best track data, and never exceed the best track 10-min sustained winds over the duration of the storm. On the other hand, ASCAT observations are not able to detect the rapid intensification, and they seem to lose sensitivity above 35 m/s. Differences between ASCAT and SMAP or the best track data lie well outside the range of uncertainty in the satellite measurements. Note that on August 3 (Day 15 in the figure) the intensity decreased between the time of the ASCAT (1 AM) and SMAP (8 AM) observations. Therefore at the time of the ASCAT observation, winds would have been stronger than those observed at the SMAP time, in contrast to the observed storm images displayed above.
In the next figures we examine the spatial extent of the storm radii at three standard wind regimes (34, 50, and 64 kn). Displayed here are the radii for the Northeast quadrants, which experienced peak intensity; similar results were obtained for the other quadrants (not displayed). The 34 kn radius is the most reliable measure of the storm size and is widely used by forecasters to warn the public about the area affected by storm danger (marine advisories). Both the satellite sensors display a 34 kn (17.5 m/s) radius timeseries consistent with the best track data, within the uncertainty range.
Similar results are obtained for the 50 kn (25.7 m/s) winds.
At the Category 1 level (64 kn or 33 m/s) SMAP still compares well with the best track data, while ASCAT doesn’t even record those wind speeds.
Summary and conclusions
These analyses for the super-typhoon Noru provide additional confirmation that the SMAP radiometer is indeed capable of providing reliable measurements at hurricane-force wind speeds. It captures the evolution of the intensity, the rapid intensification, and the storm size. While in general scatterometers provide the most accurate satellite observations of wind speed and direction, they have limited sensitivity at hurricane-force winds. At those intense wind speeds, L-band radiometers like SMAP retain sensitivity and provide reliable measurements. SMAP has therefore a unique capability of providing critical information to forecasters about intense Tropical Cyclones, especially in remote locations.
Additional information and analyses of SMAP hurricane-force winds can be found on the following RSS blog: Extreme winds during cyclone Winston.
Data Sources: SMAP and ASCAT winds displayed here are developed and processed at Remote Sensing Systems (RSS). They are publicly available at www.remss.com. The best track data was provided by the Automated Tropical Cyclone Forecasting System (ATCF) at the US Navy. The analyses presented here were performed at RSS. We thank Mr. Charles Sampson at the Naval Research Laboratory, Monterey, for providing access and information about the best track data.
Harper, B., and J. Kepert, and J. Ginger, 2010: Guidelines for converting between various wind averaging periods in tropical cyclone conditions. World Metrological Organization. WMO/TD no.1555. [Available online at www.wmo.int/pages/prog/www/tcp/documents/WMO_TD_1555_en.pdf].
Meissner, T., L. Ricciardulli, and F.J. Wentz, 2017: Capability of the SMAP mission to measure ocean surface winds in storms. Bulletin of the American Meteorological Society, 98, 8 (August 2017 issue), in press, doi:10.1175/BAMS-D-16-0052.1 .(early online release )
Sampson, C.R. and A.J. Schrader, 2000: The Automated Tropical Cyclone Forecasting System (Version 3.2). Bullettin of the American Meteorological Society, 81, 1231–1240, doi:10.1175/1520-0477(2000)081<1231:TATCFS>2.3.CO;2