advertisement

Direct Mixing Measurements using χpods in IWISE Profiling Dissipation Measurements using χpods on Moored Profilers (Moum/Nash) Shipboard LADCP/χpod profiling of Internal Wave Structure (Nash/Moum) Jonathan D. Nash & James N. Moum College of Earth, Ocean and Atmospheric Sciences Oregon State University with help from: Byungho Lim (OSU), Andy Pickering and Matthew Alford (APL-UW) and thanks to Ming-Huei Chang, Maarten Buijsman, Luc Rainville, Alexander Perlin, Ray Kreth, Mike Neeley-Brown, John Mickett, Eric Boget, Amy Waterhouse, Zoe Parsons, Jen MacKinnon, Harper Simmons Objectives General • quantify turbulence dissipation where large amplitude internal waves are generated Particular • capture the energetics of the largest scales that directly extract energy from the barotropic tides • while simultaneously measuring mixing associated with the turbulence that occurs at millimeter and millisecond scales. • through direct observation, to assess the means by which waves form and break, elucidate the structure/evolution of the wave breaking, and quantify the dissipation that induces irreversible mixing Methods • χpods on moorings Stablemoor Methods • χpods on moorings • χpod-like devices on moored profilers Methods χpod -LADCP • χpods on moorings • χpod-like devices on moored • χpod on shipboard CTD for full ocean depth turbulence profiling Methods • χpods on moorings • χpod-like devices on moored profilers MP χpods • χpod on shipboard CTD for full ocean depth turbulence profiling • fabricated and deployed 5-component array of moorings to capture the 2D evolution of the largerscale dynamics Profiling Dissipation Measurements using χpods on Moored Profilers data example fast thermistors on APL MP Kraichnan theoretical spectrum { εχ=N2 χ/(2 Γ Tz2) Products → Testedprofiling – Puget experiment Sound 2009 1st continuous deep-ocean LT Thorpe (overturn) scales Refined for 2010 (MPN) ndrate χ temperature variance dissipation mesoscale dominates half T current (Kuroshio?) Deployed 2011 on N1,2 N2 KT turbulence diffusivity (Osborn-Cox) • elevated turbulence at base of current all data returned ε TKE dissipation rate (indirect) • is this friction on a western boundary Km turbulence viscosity current? New mean speed / dissipation sensor for use on χpods and in general on moorings inexpensive, lightweight, low power, standalone velocity sensor χpod mooring N1 – χpod at 2000 m 1 day time series compensated pitot tube leading to a new GustT combination probe speed sensor at low f dissipation sensor at high f ε=2x10-7 m2s-3 not acoustic, hence requires no scatterers quiet characterization of sensor includes tests in • wind tunnel • tidal channel 2nd continuous deep turbulence profiling time series N2 (1830m water depth) 2011 2 units constructed and deployed – both worked – only 1 MP profiled Chipod-LADCP-CTD fast-T 3-axis accel 3-axis gyro compass USB-data Nash & Moum inferred turbulence (blue) direct turbulence (green) OSU Ocean Mixing χ-pod/LADCP direct measurements of abyssal turbulence from standard shipboard CTD. permits rapid deep profiling direct turbulence differs from that inferred from overturns low noise-floor (but N2-dependent) Above: TKE dissipation rate from LADCP/chipods (green) and Thorpe analyses (blue) at one of the most energetic stations sampled during IWISE. contrasting structures from detailed measurements at 2 ridges 1) broadly-distributed dissipation on the east ridge 2) big breaking lee waves on the west ridge west MP chipods east A1 – mid-column dissipation at the generation site 1440 m water depth mid-column dissipation not dominated by a single breaking wave… Byungho Lim A1 – mid-column dissipation at the generation site observation / model comparison (MITgcm / Buijsman) similar tidal fields, but water-column instabilities are not captured by MITgcm and model dissipation is mostly near the bottom. Byungho Lim T-Chains on the West Ridge eastward westward west Jonathan Nash T1 T2 N2 T3 Buijsman et al T4 30-50 m sensor spacing to detect overturns 500 m overturns 2-sec sampling to capture inertial subrange Vertical synopticity (test sampling schemes of other platforms) 3 months data east 700 m waves T-Chains on the West Ridge spring tides / diurnal inequality T1 T2 N2 T3 Mooring T3 during spring + diurnal inequality T4 T-Chains on the West Ridge neap tides / semidiurnal period T1 T2 N2 T3 Mooring T3 during neap/semidiurnal period T4 Time – mean structure / east ridge Springs (diurnal) T1 T2 N2 T3 T4 Dissipation tied to lee waves Strong spring/neap changes Isopycnals displaced down in the mean? Lee-wave shifts closer to ridge crest during neaps? Neaps (semidiurnal) Dissipation evolution and scaling ε~ u3bt T1 T1 T2 T2 N2 N2 T3 T3 T4 T4 dissipation scales with u3bt (nonlinear!) consistent with Klymak et al (2010)’s “recipe” for ε over a supercritical ridge … u3 because flux into trapped lee waves ~( ubt x u2bt ) … Summary Results • 1st continuous turbulence profiling away from ship-based upper ocean measurements • χpod-CTD measurements have led to beginning of contribution to Global Repeat Hydrogaphy Program • NEW VELOCITY SENSOR - speed + turbulence leading way to new possibilities • observational confirmation of Klymak etal (2010) ε scaling • breaking waves: vertically-integrated ε O(1 W/m2) comparable to flux divergence 5 kW/5 km suggests significant local dissipative losses • vertically-distributed turbulence may be difficult to model but significant to water mass mixing through vertical flux divergence Summary Results continued contributions to NRL field science MORT Mixing Over Rough Topography BWE Breaking Wave Effects in High Winds technological: • loan of OSU-developed instrumentation • technical-level analysis scientific: • participation in science-level analysis • contribution to publications Moored profiler χpod estimates of turbulence dissipation rate, ε LT – large-eddy length scale statistic simply computed from 1D profiles - but an imperfect statistic in an evolving 3D field Lo – large-eddy length scale defining buoyancy limit on turbulence Lo = √(ε/N3) if LT = Lo, then ε = LT2 N3 is LT = Lo ? Moored profiler χpod estimates of turbulence dissipation rate, ε LT – large-eddy length scale statistic simply computed from 1D profiles - but an imperfect statistic in an evolving 3D field Lo – large-eddy length scale defining buoyancy limit on turbulence Lo = √(ε/N3) if LT = Lo, then ε = LT2 N3 is LT = Lo ? same data – different definition of N2 pod How do we know χpods work? 7 χpods on EQUIX mooring yields 7 time series of χ, ε Equatorial Internal Wave Experiment 2008 16-day experiment at 0, 140W Oct/Nov 2008 24h continuous profiling of χ, ε 6-10 profiles/h 5 χpods on TAO mooring yields 5 time series of χ, ε Perlin & Moum, 2012 JAOTech pod How do we know χpods work? χ ε profiler χpods χpods Perlin & Moum, 2012 JAOTech Comparison of ε computed from χpod and from pitot tube A1 – mid-column dissipation at the generation site Observation / model comparison at T3 Observation / Model comparison at T3 Andy Pickering model does a pretty good job with the vertical distribution and daily-averages details are a little different Andy Pickering T1 T1 T2 T2 N2 N2 T3 T3 MITgcm / Buijsmann et al 2013 Dissipation evolution / compare to MITgcm T4 T4 First continuous deep turbulence profiling time series MP-N 2010 mesoscale current (Kuroshio?) dominates 2nd half elevated turbulence at base of current is this friction on a western boundary current? T-Chains on the West Ridge spring diurnals vs. neap semidiurnals diurnal composite / spring semidiurnal composite / neap Summary Results Integrated Dissipation from big breaking waves contributes O(1 W/m2) vertically-integrated ε this suggests ΔFε= 5 kW/m in 5km… ε is significant to FE! Distributed Mixing (detached from bottom) is difficult to model Can models accurately capture mid-column ε and its vertical distribution? ε~ u3bt Buijsman et al Conclusions Can we assign error bounds on model ε? ε~ u3bt log10 ε observed