Freezing Spray Algorithms and Their Sensitivities by Serge Desjardins Tracey Talbot * National Lab for Marine and Coastal Meteorology (aka MAC Lab) *Atlantic Storm Prediction Centre Halifax, Nova Scotia EC/NOAA Marine Collaboration Workshop February 2014 Halifax, Nova Scotia

MAC Lab R&D Areas MISSION: Improved scientific understanding and prediction of high-impact meteorology in marine and coastal environments. Marine Boundary Layer Couche Limite Marine HIGH RESOLUTION : LAM2.5 km Fog / Brouillard Coastal area / Région Côtière Boundary Layer / Couche Limite Wind Profiler / Profileur de vent Tropical Systems / Systèmes Tropicaux Ocean Surface Environment Environnement Surface de l’Océan Waves / Vagues Storm Surge / Ondes de tempête Coupling / Couplage METAREA MISSION: Amélioration de la compréhension et la prévision de la météorologie à fort impact dans les milieux marins et côtiers. MBL OSE

National Lab for Marine & Coastal Meteorology (aka MAC Lab) Staff ( MT7,MT5,MT6, MT6,MT6,MT6(Gander) ) Serge DesjardinsManager Hal Ritchie Senior Scientist (RPN) Collaborative works with RPN Researchers (OSE) Teresa Canavan Marine Boundary Layer Jamie McLean National Wave Program / Ocean Surface Environment Jim MurthaNational Wave Program (RPN-E) (Assign.-Aug 2013) Mark Pilon Marine Boundary Layer (Assignment – April 2013) Devon Telford (Gander)Ocean Surface Environment (Assignment) Doug MercerOcean Surface Environment (Assignment) G

Contents Let the Equations Talk –Coded Algorithms (based on 2011 Ross Brown’set : frzg_comp.for) Overland Modified (Original) Stallabrass –Their Sensitivities –From the Theory Main differences from the algorithms –Spray flux –Freezing aspect

SAWADA : ƒ(Wind Speed, Temperature) OVERLAND : ƒ(Wind Speed, Temperature, Salinity, Sea Surface Temperature) STALLABRASS : ƒ(Wind Speed, Temperature, Salinity, Sea Surface Temperature, Wave, RH) Wind Speed Temperature Category Light Moderate Severe Extreme “Cloud of Possibilities” The Freezing Spray Algorithms

Compare All Freezing Spray Algorithms Cold (T air =-15ºC)Warm (T air =-5ºC) ** Wind / wave correlation used SST= 0 C

2D Comparisons Overland _ NOAA Modif Stallabrass Sawada -4 Severe Category Moderate Category 42 HR forecast valid at 26 January 2014

Wind Speed (kt) Air Temperature ( ° C) -5NIL TRCLGTMDT SVR XTM -10NIL LGTMDT SVR XTM -15NIL LGTSVR XTM -20NIL LGTSVR XTM -25NIL LGTSVR XTM Sawada (Still quite used by Canadian Forecasters- Atlantic) Weakness : no dependency on SST Probably applicable for SST above ZERO In CODE : 1 knot and Air temperature Intervals

Overland Original: “Warm” vs. “Cold” Wind required to Generate Freezing Spray SST= 0 C

Freezing Spray – Overland Algorithm PPR = Icing Predictor (m o Cs -1 ) V a = Wind Speed (m s -1 ) T f = Freezing point of seawater (usually -1.7 o C or -1.8 o C) T a = Air Temperature ( o C) T w = Sea Temperature ( o C) - freezing spray categories -extreme conditions - Calibrated with ship observations (85) [20-75m] in Alaskan Waters between

Freezing Spray – Overland Algorithm PPR = Icing Predictor (m o Cs -1 ) V a = Wind Speed (m s -1 ) T f = Freezing point of seawater (usually -1.7 o C or -1.8 o C) T a = Air Temperature ( o C) T w = Sea Temperature ( o C) 30% 23% 19% 15% 12% Near Ice Edge - freezing spray categories - extreme conditions

Overland Original : Extreme Conditions extreme conditions Always assume that the ship has been exposed to these conditions for a long period of time In an unlimited fetch environment

Overland Fetch Factor : Where is it useful? W h =W max 1

Overland Fetch vs. Modified Stallabrass SST Sensitivity Warm (T air =-5ºC)Cold (T air =-15ºC) ***Solid lines are Overland Dashed lines are Stallabrass (Modified) ** Wind / wave correlation used

Freezing Spray – Stallabrass Algorithm  Considers air temperature, water temperature, wind speed, and wave height  Assume 20 knots for relative to the wave vessel speed (SCRIBE)  Note in Ross’s code : 10m/s gives important icing rates for low sea states  Predicts an average and maximum icing rate o1200 Events from 138 different vessels (Trawlers [38-57 m]. Data collected by Jim Stallabrass 1968 to 1980 oEach event described by 67 fields oIcing categorized by severity: Trace, Lights, Moderate, Heavy 3 m 9 m

Slightly Deeper understanding of the equations Ri : rate of ice buildup per unit area : Icing Rate Rw : rate of generation/delivery spray per unit area : Spray Flux n : fraction of the total incident water that freezes : Frozen Spray Challenges In Rw : what does cause the spray ? -Parametrisation (Wind, Wave and Ship Vessel movement) -How high does the spray go (vessel size and shape) ? In n : what does not freeze ? -Runoff water - thermodynamic processes associated with ice formation (parametrisation) In Ri : what type of ship vessel (how high is the deck ?) From reading : Norm L. Henry report, A, Newfoundland, October 1995

Where Vr = 10 m/s : relative velocity of wave-vessel interactions Slightly Deeper understanding of the equations LWC : Liquid Water Content Overland Stallabrass (Modified) (ORIGINAL) CALIBRATION

Slightly Deeper understanding of the equations The thermodynamic involves Structure Ice Film of water Ta,RH SEA : SST Air Td Ts Runoff Rate of icing building on an ideal cylinder Model calibration correlating the theoretical rate of icing on the cylinder to observational data From reading : Norm L. Henry report, A, Newfoundland, October 1995 Droplet time of flight

Rate of energy release due to the latent heat of freezing Heat transfer between the surface film (water) and the incident spray : source/sink Convective Heat transfer between the air/water interface : source Evaporative Heat transfer between the air/water interface : source Runoff parameter Slightly Deeper understanding of the equations HUGE ASSUMPTION: The incident spray is immediately cooled (or heated) to the temperature of the surface film Major impact on the first term on the right hand side

Ts : temperature of surface film : F ( freezing point of salt water) : Salinity Td : Spray Droplet temperature : F ( SST, droplet time of flight) : SST, Wind, Tair, RH In the Evaporative process : smaller RH contribute to more evaporative cooling Slightly Deeper understanding of the equations

Overland  STALLABRASS Computation of Td makes Stallabrass less sensitive to SST Evaporative cooling Droplet cooling Runoff Spray cloud residence time ? Runoff

Modified Stallabrass: Salinity Sensitivity Ocean Water: 30 ppt vs. 32 ppt Ocean (32 ppt) vs. Fresh Water (0 ppt) 0 ppt 32 ppt 0 ppt Cold Warm ** Wind / wave correlation used Ts : temperature of surface film : F ( freezing point of salt water) : Salinity

Modified Stallabrass: Relative Humidity Higher RH Lower RH Cold Warm ** Wind / wave correlation used

Wind wave curve Modified Stallabrass : Waves (Winds) Less realistic realistic

CONCLUSIONS OVERLAND Wind Wave (Fetch Factor) Sea Surface Temperature Air Temperature SPRAY : in 0.3 FREEZING - Very Sensitive to Sea Surface Temperature - Air Temperature departure from freezing point of sea water Extreme Categorical predictions (…unrealistic icing rate) Not used in Canada (unrealistic : near coast : fetch factor to “fix”)

CONCLUSIONS Modified Stallabrass (Used in Canada) Wind Wave –can be seen as a function of wind –Swell could contribute as well in light winds Temperature RH : freezing spray condition more likely behind a front in cold and dry air SST : not too sensitive but lower contribute to faster and more freezing Salinity : affect the freezing point of salt water (which does not vary a lot -1.5 to -1.8) SPRAY FREEZING Developed for Canadian East Coast offshore fishing vessels 3 m 9 m In the Algorithm Are we too high ? Superstructure height ?