# Underwater Optical Path Loss after Passage of a Tropical Storm
## 5 Effect of Super Typhoon Guchol's Passage  
With the beam attenuation coefficient \(c(z)\) collected from the three NAVOCEANO Seagliders depicted in Section 3, the volume back scattering coefficient \(b(z)\) is calculated with a single scattering albedo, \(\omega=b\)/\(c\). The light albedo (\(\omega\)) for light with a wavelength of 514 nm was calculated to be 0.241 for the clear ocean, 0.551 for the coastal ocean, and 0.833 for a turbid harbor. At 470 nm, \(\omega\) would be higher and less variable because of lower absorption than at 514 nm [18]. Thus, the value of \(\omega\) was selected as 0.75, an average between 0.60 (\(>\)0.551) and 0.90 (\(>\)0.833) for our study area (the western North Pacific). Between the two Seagliders, NG226 was under Super Typhoon Guchol's influence (see Figure 5a in [6]) and NG270 was not (see Figure 6 in [6]). We assumed that there was no natural optical source, i.e., \(S=0\). The same albedo was used for all cases. It seems likely that a storm event will change the composition of the scattering and absorbing water constituents and hence the albedo. We will study this issue in the near future.  
In the simulation, the light propagation direction was assumed vertically downward in consistence with the IOP data. The following model parameters were adopted: \(z_{j}\) was the same as the observational depth of the individual Seaglider's profile with \(\Delta z\) nearly 1 m and the source depth at the first observational point, the lateral step size \(\Delta\rho\) = 0.01 m, the receiver aperture = 0.1 m, and \(K\) = 32. The Gauss-Seidel iteration (4) was used to solve the 2D RTE (3) with the observed beam attenuation coefficient \(c(z_{j})\) and the calculated volume back scattering coefficient \(b(z_{j})\) with a single albedo \(\omega=0.75\) to obtain the radiance \(L_{I,\,j,\,k}\) and, in turn, calculate the normalized received power \(P_{n}(z)\).  
The cross sections of the beam attenuation coefficient \(c(z)\) from January 6 to February 28, 2014, without typhoon activity along the track of the NAVOCEANO Seaglider NG270 (Figure 3a), show small values c \(<\) 0.05 m\({}^{-1}\). The corresponding normalized received power \(P_{n}(z)\) reduces slowly from 1 at the surface to 0.1-1 at 40 m (Figure 3b), which indicates the feasibility of using UWOC without strong events like typhoons. Usually, a UWOC link can be as long as 20 m. The minimum received optical power of \(-\)19 dBm is required to achieve a bit-error rate (BER) of \(1.0\times 10^{-3}\) to establish a 9 m underwater optical channel at 1 Gbps [19].  
On the other hand, the cross section of the beam attenuation coefficient c(z) from January 25 to January 30, 2012, after Super Typhoon Guchol's passage along the tracks of the NAVOCEANO Seaglider NG226 (Figure 4a) shows large values \(c>0.75\) m\({}^{-1}\). The corresponding normalized received power \(P_{n}(z)\) reduces rapidly from 1 at the surface to \(10^{-10}\) at z = 40 m along NG226 (Figure 4b). This indicates that Super Typhoon Guchol caused huge path loss and made UWOC impossible even 7 to 13 days after passage.  
Figure 3: (**a**) Cross section of the observed volume attenuation coefficient \(c(z)\) along the tracks of NAVOCEANO Seaglider NG270 from January 6 to February 28, 2014 (red) without typhoon activity, and (**b**) the calculated normalized received power \(P_{n}(z)\). The horizontal axis shows the glider’s location along the magenta-colored track in Figure 2a with Station-1 located at 19\({}^{\circ}\)36\({}^{\prime}\)32\({}^{\prime\prime}\) N, 124\({}^{\circ}\)58\({}^{\prime}\)56\({}^{\prime\prime}\) E at 05:01 UTC, January 9, 2014 and Station-825 at 19\({}^{\circ}\)59\({}^{\prime}\)27\({}^{\prime\prime}\) N, 127\({}^{\circ}\)34\({}^{\prime}\)05\({}^{\prime\prime}\) E at 17:26 UTC, February 28, 2014. Note that there was no typhoon passing by during that period.