# Presentation: 344042

## Path of Hurricane Gert,

**Abstract:**  On September 21, 1999, the eye of Hurricane Gert approached within 300 km of Bermuda, causing 110-knot winds and extensive beach erosion on the island. On September 22, 1999, data acquired by the Sea-viewing Wide Field-of-view Sensor
- (SeaWiFS) indicated an apparent plume of sediment originating on the Bermuda Platform generated by this strong wind forcing. SeaWiFS normalized water-leaving radiances were submitted to an analytical algorithm to calculate the suspended matter concentration in this sediment feature. The algorithm is based on minimization of the spectral difference between measured and modeled seawater reflectances. A feature of this algorithm is the capability to vary parameters describing spectral backscatter and absorption when inverting the reflectance model. A computationally fast, linear least-squares technique is used to retrieve bio-optical properties from the reflectance model. The retrieved suspended matter backscattering coefficient is converted to suspended matter concentration using a simple regression: bb(555) [ 1/m] = 0.015 S [mg/l]. The calculated suspended matter concentrations for each pixel were summed to provide an initial mass transport estimate. The physical characteristics of the Bermuda Platform and the sediment plume, and the assumptions used in this calculation, provide a minimum estimate of the mass of carbonate sediment in the plume. This novel application  represents a preliminary step in the use of remote sensing data to quantify this important marine geochemical process.

**Path of Hurricane Gert,**
**September 12-23, 1999**

**True-color SeaWiFS image **
**of Bermuda acquired on **
**September 22, 1999**

**nLw(555) image of Bermuda,**
**September 22, 1999**

**SeaWiFS image of Hurricane Gert, **
**September 22, 1999**

**Event Description:**  Hurricane Gert reached hurricane strength on September 13, 1999.   On September 21, 1999,  Gert’s center approached to within 300 km of Bermuda.   The island experienced 110-knot winds and extensive beach erosion.  On September 22, SeaWiFS  viewed Bermuda from a near-nadir position.   Examination of the true-color image revealed an apparent plume of sediments originating on the Bermuda Platform.  Analysis of the spectral signature of this image and comparison to similar events (Acker *et al*., in press)  confirmed the identification of this feature as a sediment plume.

**Analytical Methods**
- The Level 1A SeaWiFS data file for September 22, 1999 was acquired by the HRPT station located at the
-  Bermuda Biological Station for Research, station HBBS.    This data has an approximate resolution of 1 km
- at nadir.  The data file was obtained from  the Goddard Distributed Active Archive Center (DAAC) and
- processed using the SeaWiFS Data Analysis System (SeaDAS), Version 4.0.
- Normalized water-leaving radiances generated from the SeaWiFS data for the sediment plume detected
- offshore of Bermuda were initially submitted to a prototype coccolithophore calcite algorithm (Gordon *et al.*
- 2001) coded into SeaDAS.  The results indicated that the feature was of insufficient optical brightness to
-  provide a sediment mass estimate.  In order to estimate the mass of sediment in the feature, an analytical
- algorithm was applied.   This algorithm, described in Vasillkov (1997) uses a linear least-squares  technique to
- retrieve inherent optical properties (IOP) from a water-leaving radiance model.    The model uses the oceanic
- radiance model of Gordon (1988), where ocean reflectance is expressed as
- 			***R / Q = l******1******X + l******2******X******2***		                                                            (1)
- where Q is the ratio of the upwelling radiance to the upwelling radiance toward zenith, l1= 0.0949,
- l2 = 0.0794, and
- 			***X = b******b****** /  (a + b******b******)***		                                         (2)
- In this model, the total IOP are a sum of the IOP for pure seawater and for the three major scattering and
- absorbing species:  phytoplankton pigment, suspended particulates, and  dissolved organic matter (DOM).
- Equation 2 is then expressed as a set of linear equations that are solved iteratively for the IOP using nLw data
- for the six SeaWiFS visible bands.   In the linear equations, the values of ***S*** (DOM spectral slope) and ***n***
- (backscatter wavelength ratio exponent) can be varied:
-            ***a******ph0******a*************ph****** (λ) + a******0****** exp [-S (λ-λ******0******)] + b******0****** (λ******0******/ λ)******n****** V (λ) =  - a******w******(λ) – b******w******(λ)V(λ)***        (3)
- where aph is the phytoplankton absorption coefficient;  a**ph* is the chlorophyll-specific absorption coefficient;
- a*ph*0, b0, and a0 are the phytoplankton absorption coefficient, backscattering coefficient, and total absorption
- coefficient at the reference wavelength λ0;  V = 1-1/X (from  Equation 2);  and aw, bw are the IOP for pure
- water.
- Sediment concentration **S** is then calculated using the generated value of bb(555) using the regression equation
- 			***b******b******(555) [m******-1******] = 0.015 ******S****** [mg/l]                                             ***(4)
- which was determined by analysis of spectral reflectance data for North Sea coastal waters  (Althuis *et al.*
- 1996).   Note that the value of the coefficient (0.015) is particle-size dependent and can be varied.
**Results**
- The algorithm described above was coded for use in SeaDAS, and the SeaWiFS data for the sediment
- plume was  analyzed, generating an image of sediment concentration.
- The area of the sediment plume was then covered by a polygonal blotch, and the amount of sediment
- contained within the blotch was summed and multiplied by the volume of seawater contained in the top
- meter of the surface area covered by the blotch.
- This calculation yields a mass estimate of  1.23 x 109 grams (1.23 million kg) of sediment in the plume.
- Due to the  geologic nature of the Bermuda Platform, this sediment is expected to consist largely  of neritic
- calcium carbonate  (CaCO3) sediments from the shallow waters of the Bermuda platform, and thus represents
- an estimate of the mass of CaCO3 exported offshore by this event.

**References:**
- Acker, J.G., Brown, C.W., Hine, A.C., Armstrong, E., and Kuring, N., 2002: Satellite remote sensing observations and aerial photography of storm-induced neritic carbonate transport from shallow carbonate platforms.   *International Journal of Remote Sensing*, in press.
- Acker, J.G., Brown, C.W., and Hine, A.C., 1997: The ‘CORSAGE’ programme:  continuous orbital remote sensing of archipelagic geochemical effects. *International Journal of Remote Sensing*, **Vol. 18 (2),**  pages 305-321.
- Althuis, IJ.A., Vogelzang, J., Wernand, M.R., Shimwell, S.J., Gieskes, W.W.C., Warnock, R.E., Kromkamp, J., Wouts, R., and Zavenboom, W., 1996:  On the colour of case 2 waters:  particulate matter North Sea, Part II:  Instruments, methods, and database.  *Netherlands Remote Sensing Board Report 95-21B*, 73 pages.
- Gordon, H.R., Boynton, G.C., Balch, W.M., Groom, S.B., Harbour, D.S., and Smyth, T.J., 2001: Retrieval of coccolithophore calcite concentration from SeaWiFS imagery*.   Geophysical Research Letters,* **Vol. 28 (8),** 1587-1590.
- Gordon, H.R., Brown, O.B., Evans, R.H., Brown, J.W., Smith, R.C., Baker, K.S., and Clark, D.K., 1988:  A semianalytic radiance model of ocean color.   *J. Geophys. Res.,* **Vol. 93,** 10,909-10,924.
- Hubbard, D.K., 1992:  Hurricane-induced sediment transport in open-shelf tropical systems - an example from St. Croix, U.S. Virgin Islands.  *J. Sed. Pet.,* **Vol. 62,** 946-960.
- Vasillkov, A., 1997:  A retrieval of coastal water constituent concentrations by least-squares inversion of a  radiance model.   *Proceedings of the Fourth Int. Conf. on Remote Sensing for Marine and Coastal  Environments,* Orlando, Florida, 17-19 March 1997, Vol. II, pages 107-116.

**Discussion**
- For several reasons, the mass estimate calculated for this event is a minimum estimate of the carbonate mass
- exported offshore.   The primary reason that this is a minimum estimate is that a sediment plume is not a
- shallow surface feature – it possesses a definite three-dimensional character due to the rapid settling rates of
- CaCO3 sediments.   This observation means that the mass of sediment in the terminal eddy of the feature has
- been underestimated by the algorithm which uses surface water-leaving radiances, because the sediment in the
- terminal eddy has very likely settled several meters below the water surface.    So the lower radiances in the
- terminal eddy (as compared to the higher radiances observed nearshore) are not necessarily due to a lower
- concentration of sediments;  they may be due to the fact that the water-leaving radiances are attenuated by
- scattering in the intervening water column.   We believe that this indicates that a larger mass of sediment has
- been exported than is estimated by the algorithm.
- Another factor is that this method for estimating IOP and deriving sediment concentrations allows the
- variability of the parameters *S* and *n*.   Vassillkov (1997) indicates that knowledge of the correct values of
*S* and *n* allows considerable improvement of in the retrieval accuracy of the sediment backscatter and DOM
- absorption.   Determination of these values would require field observations, which could be accomplished for
- small tidally-generated sediment plumes.
- For comparison, Hubbard (1992) estimated that Hurricane Hugo removed approximately 2 million kg
- of sediment from the Salt River submarine canyon on the north coast of the island of St. Croix.   This value
- indicates that our estimate of carbonate mass in the sediment plume adjacent to Bermuda is likely accurate to
- within an order of magnitude.
- CaCO3 sediments are an important part of the oceanic carbonate cycle, notably due to the fact that they can
- dissolve in  the deep ocean.   Neritic carbonates typical of the Bermuda Platform and other shallow carbonate
- platforms are composed of high-magnesian (high-Mg) calcite and aragonite, which are more soluble than
- coccolithophore and foraminiferal calcite.   For this reason, continuing observation of sediment transport events
- generated on carbonate platforms  is advocated to improve quantification of this process in the global carbonate
- and global carbon cycles (Acker *et al.* 1997;  Acker *et al.*, in press).

**Addendum:  Plume or Bloom?**   During October 2002, strong sustained winds on Bermuda generated a surface optical feature that was observed by SeaWiFS.   SeaWiFS data acquired on October 6, 2002 are shown below.   This feature has very little radiance at 555 nm, and at 443 nm the feature has lower nLw than adjacent waters, indicating absorption by chlorophyll.   Thus, this feature appears to be a phytoplankton bloom generated by wind-driven upwelling.   These observations indicate that it is not difficult to discriminate between sediment plumes and phytoplankton blooms, even though they may appear similar.

- Use of SeaWiFS Data to Quantify Carbonate Mass
- for a Hurricane-Forced Neritic Sediment Transport Event

- http://daac.gsfc.nasa.gov

- E-mail:   acker@daac.gsfc.nasa.gov