1. Inferring the Skin Temperature from its Distribution Measured with an Infrared Camera
A.T. JESSUP 1, W.E. ASHER1, M.A. ATMANE2,
K.R. PHADNIS1, C.J. ZAPPA3, M.R. LOEWEN2
1Applied Physics Laboratory, University of Washington, USA 2Civil and Environmental Engineering ,University of Alberta, Canada 3Lamont-Doherty Earth Observatory, Columbia University, USA
Acknowledgements
S.R. Long, NASA, Wallops Flight Facility, Wallops Island, VA
National Science Foundation
2011 NASA SST Science Team Meeting
Jessup, A. T., et al. Geophys. Res. Let., 2009.

2. Thermal Signature of Boundary Layer Disruption
Boundary Layer Thickness
Skin Temperature
Random Eddy Penetration
Not all eddies fully disrupt BL
BL thickness varies
Skin temperature varies
Seek to determine:
Does complete disruption of TBL occur?
If so, how often?
Implications
Surface Renewal Theory
Can DT be inferred from distribution?
Zappa et al [1998] (adapted from Harriott [1962] and Gulliver [1991])

3. Heat loss due to evaporation
Cool Skin Effect at an Air-Water Interface
Heat loss due to evaporation
Causes surface to cool
AIR
WATER SURFACE
d  1 mm
THERMAL
BOUNDARY LAYER
Tbulk
IR Optical Depth
is 10 mm, so IR measures Tskin
Tskin
DT  0.1 to 0.5 C
Temperature
Profile
For those who may not be familiar with the cool skin effect, I will take a moment to explain its significance to infrared remote sensing of a water surface. At any air-water interface where there is a net heat flux from the water to the air, a thermal boundary layer develops that is on the order of a millimeter thick under low wind conditions. Because turbulent motions are suppressed close to the air-water interface, heat transfer across the thermal boundary layer is primarily by conduction, which results in a temperature difference between the bulk water below and the temperature at the surface of a few tenths of a degree. Since the optical depth of the infrared radiation we detect is about 10 microns, the temperature we measure corresponds to the surface or skin temperature at the very top of the thermal boundary layer. If this boundary layer is disrupted from below, the bulk water will be brought to the surface and result in an infrared signature that corresponds to the temperature of the bulk water. For the unstratified conditions illustrated here, disruption of the surface will result in a warm signature. But if the sub-surface temperature profile is not uniform but stratified, then we can anticipate that the temperature signature of a surface disruption may depend on the degree of stratification.
Bulk-skin temperature difference
Tbulk also referred to as Tsubskin

4. Wire Wake Disruptions: FLIP 1992
Measured DT
IR radiometer
Thermistor at 0.1 m
Compared with DTwakes = Tin wake – Toutside wake
Results –
Low wind
DT = 0.55 K
DTwakes = 0.45 K
High wind
DT = 0.2 K
DTwakes = 0.1 K
Within measurement uncertainty
Complications
Accuracy of IR: no external calibration
Tbulk measurement depth
Skin temperature recovery
Zappa et al [1998]

5. Wire Mesh Surface Disrupter: FLIP: COPE 1995 & FAIRS 2000
Schiff [2006]
Branch [2006]

6. DT from Tsurf Distribution
PDF for Tsurf
Surface Renewal with constant Q
Tskin
combined with lognormal PDF for t
Tbulk
[Soloviev and Schlϋssel., 2004]
Then fit to PDF is given by
[Garbe et al., 2004]
Tskin = mean of PDF
Tbulk = intercept of T-axis
[Garbe et al., 2004]

7. Flux Exchange Dynamics Study (FEDS) 4 NASA Air-Sea Interaction Research Facility, Wallops Island, Virginia
Instrumentation Measurement
IR Camera kH via ACFT, p(Tsurf)
IR Radiometer Tskin, calibrated “LabRad”: Accuracy ± 0.05 K
Air: U, T, q profiles Qnet, u*
SeaBird T sensors Tbulk, calibrated: Accuracy K
Fast T sensor Twater, sub-skin profiles
Gas Chromatograph Bulk kG
LabRad IR radiometer
Tskin (calibrated)
IR camera
kH via ACFT
Tbulk via PDF
CO2 laser
FLUXES
Qsensible
Qlatent u*
Tair
qair u
Wind
45 cm
Tprofile
Tbulk
28 mm
72 mm
150 mm
76 cm

8. Experimental Conditions
Data Set: 22 runs of 5-min duration
Winds Speed: to 9.3 m s-1
Friction Velocity: to 0.55 m s-1
Heat Flux (up): 20 to 442 W m-2
Air-water DT: -6 to 3.9 K
Relative Humidity: 70 to 81 %
Bulk-skin DT: to 0.24 K

9. Sub-skin Temperature Profile
Profile, Bulk, and Skin Temperature
Correction to Tb vs Tb-Ts
Tb - Tp(zmin) < 0.04 K
Well mixed
Tb: T from sensor at 28 mm
Tp(z): Profile T
Ts: Skin T from LabRad

10. Surface Temperature PDF
Long tails typical
Ts from LabRad
Assign to mean
Tb from profile
Mean Percentile 99.90
99.80 or above for 19 of 22 runs
Occurrence of Tb in PDF implies complete surface renewal
Can DT be inferred from distribution?

11. Surface Salinity Profiler
R/V Kilo Moana
6-16 Dec
Samoa to Hawaii
IR Camera
LTAIRS –
Lighter-than-Air
Remote Sensing
M-AERI Mk II
Compare IR PDF with Tskin and Tbulk
Investigate Spatial Variability
SSP
Surface Salinity Profiler
ISAR

12. Conclusion Laboratory investigation of surface disruption
Tskin measured using calibrated IR radiometer
Tbulk measured & occurred at %-ile in p(Tsurf)
Demonstrated that:
Complete renewal occurs
Partial renewal is a common occurrence
Tbulk given by Tmax in PDF from IR image
Estimate of Saunders’ l consistent with others

13. Implication of High Percentile of Tb in PDF
Very rapid cooling if all renewals complete OR
Partial renewal also occurs
Compare DT to cooling expected in mean t
Comparable: Most events complete
Significantly less: partial renewal common

14. Surface Renewal Time Scale from Active Controlled Flux Technique
Measurement
k: thermal diffusivity

15. DT Recovery in time t Change in time t: DTt = 0.06 ± 0.03K
Soloviev and Schlüssel [1994]
Q: net heat flux
r: density of water
cp: specific heat
Change in time t: DTt = 0.06 ± 0.03K
Measured bulk-skin difference: DT = 0.16 ± 0.04 K
On average, a water parcel renewing the surface
does not have enough time to cool down to mean Ts

16. Degree of Partial Renewal
Consider t*
Time for complete
renewal based on DT
Compare to t
Ratio t/ t
>>1 for low Q
approaches 1 for large Q
Behaves like surface renewal only when strongly forced

17. Saunders’ Constant l Conduction Eq. Dimensional Analysis
Measured: l= 5.9 ± 1.9
Dimensional Analysis
Saunders predicted l=6

18. Compare to Ward and Donelan [2006]
Measured dc
Found l=2.4 ± 0.5
via
but
So l=6.3