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Notes on NCEP-NAM ocean winds during hurricane Sandy

Riha, S.

1 Abstract

We compare sea surface wind data of hurricane Sandy generated by the North American Mesoscale Forecast System (NAM) to buoy observations and satellite data. Buoy data is retrieved from the National Data Buoy Center (NDBC) and scatterometer data is obtained from the Oceansat-2 project. The region of interest is the northern U.S. East Coast. In this post, we simply display plots of all available data in the region for future reference. An interactive map is provided for convenient browsing of NDBC buoys/stations. Interpretation of the plots is deferred to future storm surge studies using the results presented here.

2 Introduction

This is a collection of figures characterizing NAM sea surface wind fields in the northern U.S. East Coast during the occurrance of Hurricane Sandy. The NAM model is operated by the National Centers for Environmental Prediction (NCEP), and most products seem to be publicly available. We provide a complete list of figures comparing NAM data to observations of sea surface winds retrieved from the Oceansat-2 scatterometer (Jaruwatanadilok et al., 2014) and buoys contained in the National Data Buoy Center (NDBC) database. The motivation is to establish a reference resource for future work. Interpretation of the plots is kept to a minimum in this post. The figures show mostly instantaneous data, statistical analysis is deferred to later work.

NAM wind products have been used by several authors studying hurricane Sandy:

In this post we use the NAM Analyses (NAM-ANL) product at roughly 12 km resolution. At the time of writing, the data records are available from March 3, 2004 to the present, and wind data is output 4 times daily at standard synoptic times (00:00, 06:00, 12:00, 18:00 UTC).

3 Methods

We compare the NAM wind to observations during the interval from October 25, 2012 at 12:00 UT, to October 31 at 6:00 UT, covering an interval of 5 days and 18 hours. Figure 1 shows 6-hourly hindcast track positions of hurricane Sandy from the Tropical Cyclone Extended Best Track Dataset (EBTR) (Demuth et al., 2006) during the interval of interest.

Track of hurricane Sandy.
Figure 1: Hurricane Sandy's track and a snapshot of modelled wind speed distribution. The white line connects 6-hourly EBTR hindcast track positions of hurricane Sandy. Labels indicate positions at 00:00 and 12:00 UT. The coastline is shown in black. Color shading shows an exemplary distribution of NAM wind speed (knots) at 10 m height on October 29, 06:00 UT. The white patch in the bottom right corner lies outside the NAM model domain.

Below we compare the NAM wind to the surface wind field measured by the Oceansat-2 scatterometer (Jaruwatanadilok et al., 2014). Figure 2 shows an example wind field observed on October 29, 2012. The wind vector cells composed by the product shown are 12.5 km wide. For the products with coarser resolution (25 km and 50 km), KNMI (2017) state that the measurable wind speed range is from 0-50 m/s (~0-97 knots), and that wind speed estimates above 25 m/s (~49 knots) are less reliable. After a first quick look through the documentation, we are unaware of any temporal smoothing induced by the scatterometer inverse model. The wind vector cells composed by the scatterometer model are 12.5 km wide, which implies some spatial smoothing of instantaneous velocity data.

Surface wind measured with Oceansat-2 scatterometer.
Figure 2: Oceansat-2 image of hurricane Sandy. Black arrows show horizontal equivalent neutral wind vectors at 10 m height (convenient scale), colored dots show wind speed in knots. The color bar is clipped at 50 knots, larger values are mapped to the clipping value. Both quantities are retrieved from the Oceansat-2 scatterometer (SeaPAC, 2013). Data points impacted by rain were removed. The figure title indicates the median time of individual satellite measurements. The thin black line follows part of the U.S. East Coast.

Below we also use wind speed recorded on buoys registered at the NDBC. Figure 3 shows a map of the subset of buoys in the region which supplied wind data between October 25 and October 31, 2012. NDBC provides averaged wind direction and wind speed measured by the buoy anemometer(s). The data is averaged over an eight-minute period for buoys and a two-minute period for land stations. For a given interval, the averaging methods are payload dependent .

Map of buoys in the region.
Figure 3: A map of NDBC buoys which supplied wind data between October 25 and October 31, 2012. Data records of individual buoys may not cover the entire interval. Red rectangles indicate moored buoys, blue rectangles indicate instruments classified as "other" in the NDBC station metadata sheet . Labels show station IDs printed in small fonts to avoid overlapping. This figure is a vector graphic and allows arbitrary zoom levels without loss of resolution.

Wind data for use in stress parameterizations is usually given at a standard height of 10 m. Hence, wind measured at anemometer height has to be adjusted to 10 m. At the time of writing, we were not able to find adjusted wind speeds for the interval/region of interest after searching through the respective folder at NDBC. However, for each buoy/station and each continuous measurement time series, the anemometer height is listed in the NDBC station metadata sheet . NDBC publishes a guide to calculate the wind using two different methods, the more comprehensive of which was described in Liu et al. (1979). As a first quick solution, we look for a simple method to obtain equivalent neutral wind at 10 m, which can be directly compared to scatterometer data. A detailed discussion is deferred to future work. Here, we follow Bidlot et al. (2002) and use the steady-state neutrally stable logarithmic vertical wind profile

$$U(z)=\frac{u^*}{\kappa}\ln{\left(\frac{z}{z_0}\right)}$$
$$z_0=\alpha\frac{u^{*2}}{g}$$

to solve for \(u^*\) given \(U(z_{an})\), where \(z_{an}\) is the anemometer height, \(z_0\) is a roughness length, \(\alpha\) is the Charnock parameter (Charnock, 1955), \(k\) is the von Kármán constant \((k=0.41)\) and g is the gravitational acceleration. The Charnock parameter is set to \(\alpha=0.018\). The function \(U\) is then evaluated at \(z=10\). We are currently not yet certain if this approach is appropriate for data retrieved during a hurricane. In particular, the choice of \(\alpha\) is typically interpreted to be dependent on the sea state, e. g. the age and slope of the dominant wave length. Fairall et al. (1996) write that previous authors have used values for \(\alpha\) between 0.01 and 0.035. Hersbach (2011) writes that the commonly used range of \(\alpha\) is from 0.01 (for swell) up to 0.04 (for steep young ocean waves). He states that values up to 0.1 are used in extreme cases, and that a typical value is 0.018. Further literature review is necessary.

4 Results

This section contains two exemplary plots with captions. The appendices contain the remaining figures for other times or stations. Figure 4 shows a snapshot of scatterometer and NAM data on October 29 in the region of interest. Appendix A contains a complete list of analogous figures for other times, i.e. all available times for which Oceansat-2 recorded data during the interval from October 25, 2012 at 12:00 UT, to October 31 at 6:00 UT.

A comparison of model predictions and observations.
Figure 4: Black arrows show equivalent neutral wind vectors at 10 m height, recorded by Quickscat-2. Green arrows show NAM surface wind interpolated to scatterometer cell centers. Both vector fields are conveniently scaled by the same scaling factor. A reference vector is located above the color bar. Colored dots show the difference in wind speed (knots) between modelled and observed wind. The color bar is clipped at 20 (-20) knots, larger (lower) values are mapped to the clipping value. The dashed black line in the bottom right corner delineates the NAM domain boundary. Data points impacted by rain were removed. The figure title indicates the median time of individual satellite measurements.

Figure 5 shows time series data recorded by NDBC buoys. Appendix B contains a complete list of analogous figures for other stations, i.e. all stations shown in Figure 3. Note that the figures in Appendix B are bitmap figures and have lower resolution than the examplary Figure 5. This is to make the web version of this document load quicker.

Observed wind speed by buoys and NAM wind.
Figure 5: Solid red dots show averaged wind speed (knots) measured by NDBC buoys. Red strokes symbolize wind vanes and indicate the wind direction (dots point upstream). The title displays buoy ID number (id) and anemometer height (ah). The data is subsampled to a minimum period of 2 hours. Note that at the time of writing (2019/10/28), anemometer height data is inconsistent for at least one station (ID 44067) when comparing the station home page to the metadata sheet. Blue circles show wind speed at 10 m assuming the neutrally stable logarithmic vertical wind profile relation used e.g. by Bidlot et al. (2002). Black circles (strokes) show scatterometer neutral wind speed (direction) of wind vector cells close to the station. Cells are shown if they are located within a square of roughly 25 km width centered at the station. Data extracted from NCEP-NAM is plotted in green.

5 Acknowledgements

Data was provided by NCEP, NDBC, the NASA Jet Propulsion Laboratory QuikSCAT Project, the Indian Space Research Organization and others. G.N. Seroka provided valuable feedback on the original version of this post. Open source software used includes NumPy , Matplotlib and Okular .

6 Appendix A

Observed Oceansat-2 wind speed and NAM wind.
Figure 6: Same as Figure 4, but for the time indicated in the title.
Observed Oceansat-2 wind speed and NAM wind.
Figure 7: Same as Figure 4, but for the time indicated in the title.
Observed Oceansat-2 wind speed and NAM wind.
Figure 8: Same as Figure 4, but for the time indicated in the title.
Observed Oceansat-2 wind speed and NAM wind.
Figure 9: Same as Figure 4, but for the time indicated in the title.
Observed Oceansat-2 wind speed and NAM wind.
Figure 10: Same as Figure 4, but for the time indicated in the title.
Observed Oceansat-2 wind speed and NAM wind.
Figure 11: Same as Figure 4, but for the time indicated in the title.
Observed Oceansat-2 wind speed and NAM wind.
Figure 12: Same as Figure 4, but for the time indicated in the title.
Observed Oceansat-2 wind speed and NAM wind.
Figure 13: Same as Figure 4, but for the time indicated in the title.
Observed Oceansat-2 wind speed and NAM wind.
Figure 14: Same as Figure 4, but for the time indicated in the title.
Observed Oceansat-2 wind speed and NAM wind.
Figure 15: Same as Figure 4, but for the time indicated in the title.
Observed Oceansat-2 wind speed and NAM wind.
Figure 16: Same as Figure 4, but for the time indicated in the title.
Observed Oceansat-2 wind speed and NAM wind.
Figure 17: Same as Figure 4, but for the time indicated in the title.
Observed Oceansat-2 wind speed and NAM wind.
Figure 18: Same as Figure 4, but for the time indicated in the title.

7 Appendix B

Wind data recorded by buoys and NAM wind.
Figure 19: Same as Figure 5, but for station 44067.
Wind data recorded by buoys and NAM wind.
Figure 20: Same as Figure 5, but for station 44024.
Wind data recorded by buoys and NAM wind.
Figure 21: Same as Figure 5, but for station 44022.
Wind data recorded by buoys and NAM wind.
Figure 22: Same as Figure 5, but for station 41002.
Wind data recorded by buoys and NAM wind.
Figure 23: Same as Figure 5, but for station 41048.
Wind data recorded by buoys and NAM wind.
Figure 24: Same as Figure 5, but for station 44057.
Wind data recorded by buoys and NAM wind.
Figure 25: Same as Figure 5, but for station 41038.
Wind data recorded by buoys and NAM wind.
Figure 26: Same as Figure 5, but for station 44009.
Wind data recorded by buoys and NAM wind.
Figure 27: Same as Figure 5, but for station 44007.
Wind data recorded by buoys and NAM wind.
Figure 28: Same as Figure 5, but for station 44065.
Wind data recorded by buoys and NAM wind.
Figure 29: Same as Figure 5, but for station 44034.
Wind data recorded by buoys and NAM wind.
Figure 30: Same as Figure 5, but for station 44037.
Wind data recorded by buoys and NAM wind.
Figure 31: Same as Figure 5, but for station 44040.
Wind data recorded by buoys and NAM wind.
Figure 32: Same as Figure 5, but for station 44042.
Wind data recorded by buoys and NAM wind.
Figure 33: Same as Figure 5, but for station 44025.
Wind data recorded by buoys and NAM wind.
Figure 34: Same as Figure 5, but for station 44062.
Wind data recorded by buoys and NAM wind.
Figure 35: Same as Figure 5, but for station 44058.
Wind data recorded by buoys and NAM wind.
Figure 36: Same as Figure 5, but for station 44008.
Wind data recorded by buoys and NAM wind.
Figure 37: Same as Figure 5, but for station 44013.
Wind data recorded by buoys and NAM wind.
Figure 38: Same as Figure 5, but for station 44064.
Wind data recorded by buoys and NAM wind.
Figure 39: Same as Figure 5, but for station 44029.
Wind data recorded by buoys and NAM wind.
Figure 40: Same as Figure 5, but for station 41001.
Wind data recorded by buoys and NAM wind.
Figure 41: Same as Figure 5, but for station 44011.
Wind data recorded by buoys and NAM wind.
Figure 42: Same as Figure 5, but for station 44005.
Wind data recorded by buoys and NAM wind.
Figure 43: Same as Figure 5, but for station 44030.
Wind data recorded by buoys and NAM wind.
Figure 44: Same as Figure 5, but for station 44020.
Wind data recorded by buoys and NAM wind.
Figure 45: Same as Figure 5, but for station 41013.
Wind data recorded by buoys and NAM wind.
Figure 46: Same as Figure 5, but for station 44027.
Wind data recorded by buoys and NAM wind.
Figure 47: Same as Figure 5, but for station 41029.
Wind data recorded by buoys and NAM wind.
Figure 48: Same as Figure 5, but for station 44043.
Wind data recorded by buoys and NAM wind.
Figure 49: Same as Figure 5, but for station 44063.
Wind data recorded by buoys and NAM wind.
Figure 50: Same as Figure 5, but for station 44061.
Wind data recorded by buoys and NAM wind.
Figure 51: Same as Figure 5, but for station 41036.
Wind data recorded by buoys and NAM wind.
Figure 52: Same as Figure 5, but for station 41024.
Wind data recorded by buoys and NAM wind.
Figure 53: Same as Figure 5, but for station 44032.
Wind data recorded by buoys and NAM wind.
Figure 54: Same as Figure 5, but for station 44039.
Wind data recorded by buoys and NAM wind.
Figure 55: Same as Figure 5, but for station 44059.
Wind data recorded by buoys and NAM wind.
Figure 56: Same as Figure 5, but for station 44041.
Wind data recorded by buoys and NAM wind.
Figure 57: Same as Figure 5, but for station 45012.
Wind data recorded by buoys and NAM wind.
Figure 58: Same as Figure 5, but for station 41037.
Wind data recorded by buoys and NAM wind.
Figure 59: Same as Figure 5, but for station 44033.

References