Improved Validation of SatelliteSST for Climate Research
C.J. Donlon, P.J. Minnett, C.L. Gentemann, T.J. Nightingale, I.J.Barton, B. Ward, & M.J. Murray
Submitted to Journal of Climate, 3/9/2001
Operational strategy for validationof satellite-derived SSTs
1. VOS with high-quality skin radiometers2. Buoy/Drifter in-situ
The bulk of validation retrievals will comefrom in-situ measurements located belowthe skin layer, but within the diurnal warmlayer.Better characterization of the SSTskin andSSTdepth relationship and its dependenceon wind speed.
SSTskin and SSTdepth relationship
The cool skin / warm layer effects aregermane to:
• Interpretation of SST used in calculation ofair-sea fluxes, climate studies, NWP, &GCMs.
• Multi-sensor SST blending efforts(increased fidelity, consistency, andresolution)
Upper Ocean Thermal Structure
SSTskin ~500µmconductive and diffusiveheat transfer processesdominate
SSTsub-skin – 1mm,molecular and viscousheat transfer processesdominate
SSTdepth – 1m, turbulentheat transfer processesdominate
Cruise TracksM-AERI: Fourier-Transform infrared spectroradiometer
SISTeR: scanning infrared SST radiometer: skinmeasurement
Sonic anemometer: wind speeds
Diurnal Warm Layer
Thermal stratification can cause large differences betweenSST measured at 1 µm, 1 mm, and 1 m.
Three vertical profilestaken by SkinDeEP,showing the diurnal warmlayer. Small circles areskin measurements fromMAERI, showing coolskin.
6AM 1PM 5PM
Skin Bulk DifferencesDAYTIME
U > 6 m/s : there exists a cool skinof 0.17 K ± 0.07 K RMS. SSTdepth,corrected for this small bias, may betermed SSTskin.
U < 6 m/s : modeling of diurnalwarming and cool skin effects isclearly mandatory.
NIGHTTIME
U > ~2 m/s : there exists a cool skinof 0.14 K + 0.30e(-U/3.7)
U < ~2 m/s : convective andmolecular heat transfer processesdominate maintaining a significanttemperature gradient.
Validation Methodologies
Direct: (preferable)• Contemporaneous satellite and in situ
SSTskin encompassing a range of oceantemperatures and atmosphere profiles.
• To meet sampling requirements, thereshould be ~15 deployed in situ SSTskinradiometers.
• Target low wind areas
Validation Methodologies
In-direct:
• Collocated satellite and in situSSTdepth encompassing a range ofocean temperatures and atmosphereprofiles
• Need contemporaneous wind retrievals
• Target regions with winds > 6 m/s
Percent of year winds < 6 m/s
SSM/I wind speeds 1995-1999 highlightregions where indirect validation shouldbe targeted. Average wind speed is 8.3m/s and 30% of winds are < 6 m/s. 3% ofwinds are less than 2 m/s.
Skin Temperature IR SSTsATSR shows diurnal warmingup to 7 m/s. Mean differencefor U > 6 m/s :
Night: –0.17 K ± 0.46 K RMS
Day: –0.07 K ± 0.47 K RMS
MPF shows diurnal warming upto 2.5 m/s. Mean difference forU > 2.5 m/s :
Night: –0.06 K ± 0.50 K RMS
Day: –0.15 K ± 0.53 K RMS
Diurnal warming of skinSST as a function of windspeed.
Diurnal warming of Microwave SST
At wind speeds less than5 m/s, diurnal warming ofup to 2°C, peaking at 2pm.
Cooling of skin duringnight at very low windspeeds.
Conclusions
We need a better characterization of upperocean thermal structure ____________________
• Rigorous validation of satellite SSTs• Fundamental to development of long-term
multi-sensor SST blended products• Vital parameterization for SST analyses that
incorporate temperatures measured at differentdepths (satellite and in situ data).