Assessing the Influence of Surface Wind Waves to the Global Climate by Incorporating WAVEWATCH III in CESM! Phase I: Langmuir Mixing in KPP

Transcription

1 Assessing the Influence of Surface Wind Waves to the Global Climate by Incorporating WAVEWATCH III in CESM Phase I: Langmuir Mixing in KPP Image: NPR.org, Deep Water Horizon Spill Qing Li Baylor Fox-Kemper, Todd Arbetter Brown University Adrean Webb TUMST 19 th Annual CESM Workshop Breckenridge, CO 1

2 Langmuir Mixing in KPP Choosing a better parameterization κ CORE2 Inter-Annual Forcing WAVEWATCH-III Stokes drift; 4 o x3.2 o LM in KPP POP2 U, T, HBL; gx3v7 z Diffusivity stronger Boundary layer deeper Enhancement factor to vertical velocity scale W Aligned wind and waves Misaligned wind and waves us SL VR12g Enhancement Factor E = q 1+.8La 4 t E = p 1+(3.1La t ) 2 +(5.4La t ) 4 La 2 t = u u s () θ α us SL u* u* LC La 2 SL,proj = u cos( ) hu s i SL cos( ) Including Stokes shear Making the boundary layer even deeper VR12h La MS2K VR12a Ri b = d [b r b(d)] hu r i hu(d)i 2 + Ut 2 + u s () 2 McWilliams and Sullivan, 2; Van Roekel et al., 212 2

3 Summer Mixed Layer Depth (JAS for NH & JFM for SH) RMSE (m) Global m 12 No Langmuir Mixing S-2N South of 3S OBS CTRL 19.5 Shallow bias in the Southern Ocean Setup WAVEWATCH-III (Stokes drift; 4 o x3.2 o ) <-> POP2 (U, T, HBL; gx3v7) CORE2 interannual forcing (Large and Yeager,29) 4 IAF cycles; average over last 5 years for climatology Same forcing but different Langmuir Mixing parameterizations OBS: de Boyer Montégut et al. 24 3

4 Summer Mixed Layer Depth (JAS for NH & JFM for SH) RMSE (m) Global m 12 No Langmuir Mixing S-2N South of 3S OBS CTRL Aligned u* and us MS2K VR12a Enhanced mixing, but too much in MS2K OBS: de Boyer Montégut et al. 24 4

5 Summer Mixed Layer Depth (JAS for NH & JFM for SH) RMSE (m) m 12 Global No Langmuir Mixing S-2N South of 3S 19.5 OBS CTRL Aligned u* and us MS2K VR12a Misaligned u* and us With Stokes shear VR12g OBS: de Boyer Montégut et al. 24 VR12h 5

6 Winter Mixed Layer Depth (JFM for NH & JAS for SH) RMSE (m) m 4 Global No Langmuir Mixing S-2N South of 3S OBS CTRL Aligned u* and us MS2K VR12a Misaligned u* and us With Stokes shear VR12g OBS: de Boyer Montégut et al. 24 VR12h 6

7 D ASARO ET AL.: QUANTIFYING SURFACE WAVE TURBULENCE Percentage change in MLD by Langmuir Mixing Second-moment closure model a 6 N 45 N 3 N 15 N DEC JUN 15 S The surface wind waves increase the mixed layer depth at high latitude by 2% ~ 6% 3 S 45 S 6 S 6 E % 18 W 1% c VR12h - CTRL Latitude 3% With Stokes shear 12 W 2% b VR12g - CTRL Misaligned u* and us 12 E % 2 3% Winter D Asaro et al. 214 With wave forcing Percentiles 25% 5% (median) 75% Summer JUN 4% No wave forcing DEC 2 JUN DEC Summer (JAS for NH & JFM for SH) MLD (m) 3% 4% 2 6 W MLD (m) Figure 3. Contribution of Langmuir turbulence to global mixed layer depth. (a) Percentage increase in mixed lay with wave forcing relative to no wave forcing when Langmuir turbulence is parameterized [Harcourt, 213] in mixed layer model (section S7) 18 days after a near-summer solstice initial profile; (b) Zonal median (thick line layer depth and 25th and 75th percentiles (thin lines) 18 days after near-summer solstice initial profile Ð with (bl 8% without (red) wave forcing; (c) As for Figure 3b 365 days after initial profile. [9] Buoyancy of the floats induces errors in the measurement of wrms [Harcourt and D Asaro, 21]. The resulting vertical motion of the float relative to the water (section S4.2.2) changes both the measured vertical velocity and causes the mixed layer to be nonuniformly sampled. The floats buoyancy was calibrated at least daily, with some low-wind days dedicated to calibration profiles. Direct measurements of float-relative velocity in Lake Washington imply buoyancy errors in wrms of less than 5% (section S4.2.3). The finite size of the float also introduces errors by averaging smaller turbulent eddies. We correct for this by fitting a universal spectral form to the vertical velocity spectrum (section S4.2.5) for each drift and removing the component due to finite float size. This correction increases wrms by 1 2% from the measured value and introduces 8 15 m s 1, mixed layer depths of 2 1 m and w.5 7 m. The wide range of wave conditions for wind speed allows us to assess whether waves ar tant and ask whether Langmuir turbulence can ex measured effect. Winter (JFM for NH & JAS for SH) 3. Theory [11] Models of Langmuir turbulence predict wrms the Stokes drift profile us (z), the mixed layer thic and the air-sea buoyancy flux B = g c 1 w Q, where acceleration of gravity, is the thermal expansio cient of seawater, and c 1 w is its specific heat; we igno salinity effects. For monochromatic surface waves us ()ekz, where k is the horizontal wave number of 7

8 pcfc11 Bias Observation Anomaly Southern Hemisphere 28 6 RMSE 2.2 Langmuir Mixing enhance ventilation and reduce low pcfc11 biases 15 OBS No Langmuir Mixing CTRL -6 RMSEs are reduced by 6% ~ 18% MS2K Aligned u* and us VR12a Misaligned u* and us VR12g With Stokes shear VR12h 8S EQ 8S EQ OBS: Key et al. 24 (GLODAP) 8

9 pcfc11 Bias Observation Anomaly Equatorial Region 28 4 RMSE 15. Langmuir Mixing enhance ventilation and reduce low pcfc11 biases 8 OBS No Langmuir Mixing CTRL -4 RMSEs are reduced by 6% ~ 19% MS2K Aligned u* and us VR12a VR12g VR12h Misaligned u* and us With Stokes shear 8 2S 2N 2S 2N OBS: Key et al. 24 (GLODAP) 9

10 Summary and Future Work (by Jan, 215) WAVEWATCH III as a component of CESM and coupled with POP Langmuir mixing Reduces the shallow bias of mixed layer depth in the Southern Ocean (RMSE reduction: summer 2%; winter 1%). Reduces low biases in zonal mean pcfc11 both in the Southern Ocean and Equatorial region (RMSE reduction: ~18%). Equatorial region, depend on the MLD definition. LM enhances ventilation; Mean MLD might not be a good indicator there. More tests: Fully coupling with active atmosphere and sea ice model. Considerations in the Equatorial region. CVMix. An efficient but accurate data wave model. 1

11 Model Setup WAVEWATCH III v3.14 3rd generation wave model Solves the spectral action density balance equation Res: WW3a (4 o x3.2 o ) CESM1.2; Ocean-Wave only Compset: CIAF_WAV Res: gx3v7 CORE2 Interannual forcing; 4 IAF cycles (62 years); average over the last 5 years for climatology CFC active starting near the end of the 3 rd cycle (model year 17, corresponding to data year1931) and through the 4 th cycle CFC11 comparison: annual mean of model year 233 (corresponding to data year 1994) Mixed layer depth definition OBS: MLD in density with a variable threshold criterion (equivalent to a.2 C decrease) CESM: Shallowest depth where the local, interpolated buoyancy gradient matches the maximum buoyancy gradient between the surface and any discrete depth within that water column 11

12 Stokes Drift At the surface u s () = 2 Surface layer mean Z 2 Z 1 (cos, sin, ) 3 g S(, )dd hu s i SL = 1 H SL Z 2 Z 1 (cos, sin, )(1 e 2 2 HSL g )S(, )dd 12

13 Langmuir Mixing in KPP Aligned Wind and Waves Turbulent Langmuir Number Vertical velocity scale Enhancement factor (MS2K) Diffusivity E = La 2 t = u u s () W = ku E q 1+.8La 4 t κ Shape function apple v = WH BL G( ) G( )= (1 ) 2 = d The boundary layer depth is determined from H BL z Ri b = Enhancement factor (VR12a) H BL [b r b(h BL )] hu r i hu(h BL )i 2 + U 2 t.3 E = p 1+(3.1La t ) 2 +(5.4La t ) 4 13

14 Langmuir Mixing in KPP Misaligned Wind and Waves us SL Surface layer averaged, projected Langmuir Number us SL LC La 2 SL,proj = u cos( ) hu s i SL cos( ) Angle between Langmuir cell and wind " # tan 1 sin ( ) u u s ()apple ln ( H BL/z 1 ) + cos ( ) θ α u* u* Enhancement factor (VR12g, VR12h) q E = cos 1+(1.5La SL,proj ) 2 +(5.4La SL,proj ) 4 The boundary layer depth (VR12h) is determined from Ri b = H BL [b r b(h BL )] hu r i hu(h BL )i 2 + U 2 t + u s ()

15 Summer Mixed Layer Depth (JAS for NH & JFM for SH).3 kg/m3 density criterion RMSE (m) 12 Global No Langmuir Mixing S-2N 6.39 South of 3S OBS CTRL Aligned u* and us MS2K VR12a Misaligned u* and us With Stokes shear VR12g OBS: de Boyer Montégut et al. 24 VR12h 15