Results

Magdalena D. Anguelova and Michael H. Bettenhausen

maggie.anguelova@nrl.navy.mil

Remote Sensing Division, Naval Research Laboratory, Washington, DC, USA

Sea spray layer is non-reflectiveGradual change of spray layer permittivity yields Impedance matching with sea surface Facilitates absorption close to the surface May diminish reflection of GPS signal

Seawater permittivity from MW12 can be used instead of KS77 (see refs) Percent difference < 1% for real parts (reflection at boundary) Percent difference < 0.1% for imaginary parts (losses in medium)

Skin depth < 5 cm of a spray layer with only Q = 20%Enough to provide a signal at L1Thicker spray layers may form in hurricane conditions

Conclusions

Effect of a sea spray layer on L-band signal from ocean surface

Spray layer vertical properties

We developed a radiative transfer model for es with profile of seawater content Q(z) in power law form (Fairall et al., 2009)

Approach

4.0,,*

kku

VmzzQ

gm

High wind and breaking waves in hurricane conditions create a sea spray layer with vertically inhomogeneous seawater content Q

We use Q(z) profile to obtain spray radiative properties above the waves:

Effective dielectric constant of sea spray layer s (z)

With quadratic mixing rule and Seawater permittivity (Ts, S)

Meissner and Wentz (2004; 2012) Sea spray layer attenuation (z)

Sea spray layer skin depth d

We calculate sea spray layer emissivity es with: All radiative contributions

Upwelling, downwelling, transmitted to seawater No scattering at L-band because

( 20 cm) >> spume diameter ( 1 mm)

sWsDsUs eeee

Input parameters: GPS freqs F = (L1, L2, L5, Aq) = (1.5754, 1.2276, 1. 1765, 1.4) GHz

Friction velocity U10N = 10 – 60 m s-1

With Hs = 5-7 m and z/Hs = 3-5, spray layer from 15 m to 35 m

Mean Vg 1 m s-1 for droplets of 100 – 200 m Seawater temperature Ts = 28 C

Salinity S = 34 psu

Veron, 2015

Andreas, 1998

Rastigeyev et al., 2011

Background

Sea spray adds to the interfacial heat fluxes due air-sea differences of temperature and humidity

Sea spray droplets exchange sensible and latent heat with the ambient air

Droplet cooling and evaporation change the temperature and humidity profiles over the sea surface

Spray-mediated heat fluxes increase the enthalpy flux in the marine boundary layer

Increased enthalpy flux intensifies the hurricanes

Large, directly teared spume droplets

Small, bubble-mediated film and jet droplets

Volume flux of sea spray droplets is important for heat exchange

Large spume droplets are most important for spray-mediated heat fluxes

Sea spray volume size distribution peaks at droplet radii of 100-200 m

Droplets with these sizes stay air-borne in turbulent flow long enough to contribute to heat exchange the most

Turbulence (friction velocity u* and Re number) and the terminal velocity (gravitational settling) Vg of the spume droplets determine how thick the spray layer is

Spume droplets are produced at the wave crests

Spray layer above the waves is thus investigated

Spray layer is scaled with the significant wave height Hs

2

1 zQzQzεs

zε.c

Fπzα sIm

2

1

0

dzzα

d

The sea spray layer is lossy medium due to Q and affects the microwave radiation reaching the seawater, thus the reflected signal We obtain the reflectivity r of the sea spray layer from its emissivity e:

ss er 1

Andreas et al., 2015

Following Fairall et al., 2009

ReferencesAndreas, 1998, JPO, 28, 2175-2184Andreas et al., 2015, Q. J. R. Meteorol. Soc., 141, 642–654Fairall et al., 2009, JGR-Oceans, 114, C10001Klein and Swift, 1977, J. Oceanic Eng., OE-2, 104-111 (KS77)Meissner and Wentz, 2004, TGRS, 42, 1836-1849Meissner and Wentz, 2012, TGRS, 50, 3004-3026 (MW12)Rastigeyev et al., 2011, Boundary-Layer Meteorol., 141, 1-20Veron, 2015, Annu. Rev. Fluid Mech., 47, 507–38

Large droplets and high winds create

similar Q(z) profiles

Spray layer reflectivity

The shape of the r(U10N) curve at a given polarization strongly depends on the droplet size and its terminal velocity