The Role of Water Droplets in Air-sea Interaction: Rain and Sea Spray

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of Marine Science and Policy College of Earth, Ocean, &

1 The Role of Water Droplets in Air-sea Interaction: Rain and Sea Spray Fabrice Veron Air-Sea Interaction Laboratory School of Marine Science and Policy College of Earth, Ocean, & Environment University of Delaware USA Air-sea Interaction laboratory

2 Collaborators Work done with: Dr. James Mueller (former student) Dr. Emily Harrison (former student) Marc Buckley (PhD student) Chelsea Hopkins (Undergraduate) Jake Steinberg (Undergraduate)

3 Sea Spray - Spume Northern CA, ~10-15m/s, 2010 Hurricane Isabel (Black et al. 2007) Spume production water droplets are ripped from wave crests by the wind when U 10 exceeds about 7 ms -1. Droplets range from ~40µm to ~1mm in diameter.

4 Spray Fluxes Air-Water Interface Total Air-Water Transfer Two Distinct Pathways Spray (Rain fall) The spray-mediated fluxes depend on three controlling factors: I - Transport and exchange model the duration of suspension within the atmospheric boundary layer, the rate of momentum, heat and mass transfer between the drops and the atmosphere, and the number and size of drops formed at the surface. II - Source function model & measurements

5 Transport & Exchange Model Flowchart (1) (2) (3) (4) Input boundary conditions (10-m and surface) Generate environment for droplet (surface waves, air-sea fluxes, profiles) Initialize droplet (location, temperature, velocity) Determine local environment at droplet Two-way coupling One-way coupling (5) Droplet motion Droplet microphysics Repeat steps 4-5 with updated position and attributes until quasi-equilibrium or reentry

a stochastic Lagrangian turbulence model allowing for:

6 Lagrangian Stochastic Model I - Transport and exchange model Triple decomposition: u = Ū + Ũ + u Velocity u is modeled with a stochastic Lagrangian turbulence model allowing for: Stratification & wave effects -> Inhomogeneity Anisotropy Unsteadiness

7 Transport & Exchange Model Particle residence time 8000 drops 500 breaking events

8 Transport & Exchange Model Particle impact velocity Impact horizontal velocity Impact vertical velocity Large particle slow response time -> no time to accelerate to terminal Small particle fast response time, small inertia -> follow the turbulence

9 Transport & Exchange Model Particle impact temperature 100 µm drops

10 Transport & Exchange Model Particle re-entry temperature At low winds, only the smallest drops are suspended long enough to approach thermal equilibrium At high winds, all but the largest drops approach thermal equilibrium The smallest drops have less thermal inertia and reheat before reentering

11 Transport & Exchange Model Particle re-entry mass

12 Spray fluxes (spectral) Momentum Flux Sensible heat m s m s m s m s m s m s m s m s r(µm) NEED the number and size of drops formed at the surface r(µm) r(µm) Latent heat

13 Spume visualization in the laboratory II - Source function model & measurements

14 Spume Generation Function II - Source function model & measurements Wind-Wave Tank Round Jet Work by Fabrice Veron, Chelsea Hopkins, & Emily Harrison From Marmottant and Villermaux (2004)

15 Spume Generation Function II - Source function model & measurements Spray generated by breaking waves Drops Formed at Surface Ligaments form along breaking wave crests Calculation of volume of water in ligaments Calculation of size distribution after fractionation Drops Transported Vertically From Mueller & Veron (2009)

16 Spume Generation Function Number source function Volume source function Peak diameter of formed drops decreases with wind speed Peak diameter of suspended drops increases slightly for low to moderate wind speeds From Mueller & Veron (2009)

17 Spray-Mediated Stress Integrate m s m s m s m s m s m s m s m s -1 X Generation function r(µm) 10 2 Total τ (Pa) LS + Fairall (1994) Andreas (2008) LS + Veron & Mueller (2009) U 10 (m s -1 )

18 Exchange coefficients Drag Coefficient Enthalpy exchange Coefficient

19 Exchange coefficients Significant discrepancies past 30 m/s Differences between generation spray functions How about the data?

20 Laboratory Measurements of the Spume sizes

21 Spume Concentration Function 6.3 cm x 4.2 cm

22 Spume Concentration Function Observation of very large droplets.

23 Spume Generation mechanism

24 Summary SPRAY Reversal of spray heat fluxes during re-entry leads lower mean fluxes per drop At low wind speeds, the larger drops in the source function compensates for these smaller mean fluxes At high wind speeds, the source function is critical The spume generation mechanism is very complex (beautiful!) rendering physically based parameterizations difficult. Perhaps there is two distinct generation mechanisms: 1 - lenticular canopy/bag breakup 2 sheet rim / filament breakup

25 Rain Another spray generation mechanism is from the impact of rain on the surface Rain also generates subsurface turbulence and disrupts the molecular layers with consequences on air-sea fluxes.

26 Wind-wave-current tank 1.4mm radius drops 8 rain modules 20,000 needles Rain on 50% of the surface area

27 Instrumentation 3 rain rates 8 wind speeds 24 experiments

28 Gas transfer velocity: k(600) 11 ms -1 cutoff for rain effects

29 Kinetic energy flux ratio: 11 ms -1 cutoff for rain effects

30 Nonlinear model for k(600)

31 Extrapolating to the field KEF r Laws-Parsons drop size distribution k (600) w Ho et al. (2006)

32 Global gas transfer velocity 2009 JAN FEB MAR APR MAY JUN JUL AUG SEP 100% Wind OCT NOV DEC 25% Rain

33 Rain-Turbulence generation Fresh- and saltwater 7 rain rates (2 drop sizes), 5 wind speeds 70 experimental conditions 1.3mm radius drops

determine the fluctuating density field Cross-correlation analysis of the

34 Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF) Panels a & b include the fluorescent dye concentration used with LIF to determine the fluctuating density field Cross-correlation analysis of the particles in image 1 and 2 (c & d) is used to determine the 2D fluid velocity

35 PIV velocity and vorticity fields Velocity fields from cameras 1 and 2 have been merged to form a single velocity field

36 LIF fluctuating density fields Results from cameras 1 and 2 have been merged to form a single fluctuating density field, ρ 90 mm/h

37 Fluctuating density: ρ b : average density of flow Measured with the profiled temperature-conductivity sensor R 40 mm h -1 R 90 mm h -1 R 180 mm h -1

38 Turbulent kinetic energy: R 40 mm h -1 R 90 mm h -1 R 180 mm h -1

39 Average turbulent kinetic energy and dissipation: KE t (ε) decrease (increases) significantly at the highest rain rate

40 Buoyancy flux: B t >0: Buoyant production of KE t B t <0: Buoyant destruction of KE t KE t is being destroyed by buoyancy at the highest rain rate

41 Summary Rain significantly influences the gas flux, especially at low wind speeds Turbulence measurements beneath rain falling on a still surface show: Initially, turbulence levels increase with rain rate Then, turbulence levels decrease when buoyancy forces take over at higher rain rates

42 Summary

43 Droplets are Fun! Thank you!

Dynamic Effects of Airborne Water Droplets on Air-Sea Interactions: Sea-Spray and Rain

Dynamic Effects of Airborne Water Droplets on Air-Sea Interactions: Sea-Spray and Rain Fabrice Veron University of Delaware College of Marine Studies 112C Robinson Hall Newark DE, 19716 phone: (302) 831-6658