Wind, Current, Wave, and Stress Coupling in the Boundary Layer and A Plan for Observing This Coupling from Space

Transcription

1 Wind, Current, Wave, and Stress Coupling in the Boundary Layer and A Plan for Observing This Coupling from Space Mark A. Bourassa and Qi Shi COAPS, EOAS & GFDI, Florida State University With input from Dudley Chelton, Dmitry Dukhovskoy, Tom Farrar, M. Mar Flexas, David Long, Thomas Kilpatrick, Nikolai Maxeminko, Dimitris Menemenlis, Steven L. Morey, Alexis Mouche, Dragana Perkovich, Roger Samelson, Bryan Styles, Andrew Thompson, Frank Wentz, Shang-Ping Xie and the Ocean Observation Panel for Climate 1

2 Where is the Warming Going? And how is it getting there? 2

3 Links to Climate Change: Ocean Heat Content How is the energy getting to the deeper ocean? Heat content anomaly (10 21 J) The rate of vertical transport is Important for interpreting time timescale and lag for these processes. Highly dependent on processes, which depend on scales, physics considered and parameterizations IPCC AR5 3

4 Motivation for Interaction over Currents Substantial bias and errors have been found in air-sea fluxes over western boundary currents (WBC) regions from numerical model products (Moore and Renfrew 2002; Roberts et al. 2012; Zhang et al. 2016). Ocean modeling studies show that including currents reduces the Eddy Kinetic Energy production by about 25% Adding currents caused a 27% reduction relative to the control Errors arise in air-sea fluxes come from different sources: Limited observations for air-sea fluxes Missing feedback mechanisms in uncoupled or non-fully coupled numerical models Coarse-resolution ocean models 4 4

5 What Improvements are Needed For Surface Vector Winds? Finer spatial resolution to calculate spatial derivatives (including curl and divergence) Divergence and curl calculated on the scale (e.g., diameter) of one or two grid cells are quite noisy compared to those calculated on the scale of 3 or more grid cells. Spatial derivatives are very important for Coupling with clouds (divergence of winds) Ocean currents near the surface, in the Ekman layer, and in the deep ocean (curl of stress) Frontogenesis (gradient of winds) Ocean upwelling and primary productivity All of which are important for air/sea coupling We need to observe winds roughly every 5km to address these interests About 2.5 times finer than the existing measurements 5

6 Coupling among three modeling components Surface currents Ocean (ROMS) Wave length, height, period and direction Wind stress, Surface heat fluxes Radiation, Precipitation, Sea level pressure Sea surface temperature Surface currents Atmosphere (WRF) Wave (SWAN) 10 meter wind Wave height and wave length Atmospheric, ocean and wave models are synchronized every 10 minutes to exchange data fields through the Model Coupling Toolkit (MCT). 6 6

7 Experimental design These experiments were designed to separate the ocean currents effect on the wind stress from the wave effect. The four experiments differ only in how wind stress is calculated in the bulk parameterization equation. 7 7

8 Changes in October Wind Stress Magnitude Relative to model with stress not dependent on waves and currents CUR CTL Not Reasonable CUR&WAV CTL Seemingly Reasonable WAV CTL Not Reasonable Currents alone tend to reduce stress beyond reasonable values Waves alone tend to increase stress But not as needed Waves together with currents are not a linear sum of the two Good for ocean EKE 8

9 Changes in October Ocean Ekman Pumping CUR CTL WAV CTL CUR&WAV CTL More like Observations In the upper two images the heat budget is dominated by horizontal transport processes. In the bottom case, the heat budget is dominated by vertical motion Curl of stress is greater over SST gradients (more like observations) 9

10 Coupling mechanism Dominant process ±5 W/m 2 ±3 W/m 2? +12% ±5% Wave current interaction All numbers are median value of 30 day averaged difference between CUR+WAV and CTL over the Gulf Stream 10 10

11 Winds and Currents Mission Measurement Concept 700 km 800 km Ka-band rotating pencil beam Doppler scatterometer Winds measured from Ka-band measurements at multiple azimuth angles Heritage: QuikSCAT, RapidScat, OSCAT Surface currents from Doppler measurements at multiple azimuth angles Heritage: SAR Doppler, Along-track interferometry 5km stress (sort of winds) and currents Temporal coverage achieved by a roughly 1600 km swath 11

12 What Current Velocity are we Measuring? Surface scatterers (resonant gravity/gravity-capillary waves satisfying the Bragg condition) motion is due to several effects: group velocity of resonant patch; orbital wave velocity; advection due to surface currents. Additional platform motion and scattering effects can also appear. Bragg group velocity can be estimated using the dispersion relation (for water waves) and knowledge of the wind direction. Orbital wave velocity component averages mostly out after averaging over kilometer scales, with the non-cancelling part contributing to the surface current through Stokes drift. Residual motion (Doppler current minus group velocity) is due to the surface current. 12

13 Retrieved Surface Velocity Components 2017 California Institute of Technology. Government sponsorship acknowledged. 13

14 DopplerScatt physical measurements Engineering Flight June 22nd California Institute of Technology. Government sponsorship acknowledged. Fore looking Surface radial velocity from Doppler, after subtracting platform motion and rough Bragg contribution. Notice tidally induced internal waves at the continental shelf. Fore looking backscatter cross section corresponding to radial velocity. Notice increased brightness due to wind Aft looking strengthening off Point residual radial Sur. velocities Velocity Leadership (m/s) College of Arts and Sciences Council 2017 California Institute of Technology. Government sponsorship acknowledged

15 Finding Partners to Help Make it Happen Satellites are expensive! Partners help manage the costs The Indian Space Research Organization (ISRO) wants to collaborate with NASA, given that NASA formally supports such a mission The Japanese Aerospace Exploration Agency (JAXA) and ISRO are collaborating on the GCOM-W2 mission, and would like to have an advanced radiometer and Doppler Scatterometer on the same platform 15

16 Other Benefits Aircraft observations should lead to breakthroughs in sub-mesoscale forcing, and will like help identify future goals Ocean and atmospheric data assimilation & modeling Ice edge detection & Iceberg tracking Finer resolution Ice motion Finer resolution is hugely important The Doppler measurements of velocity should work well Ice age (highly likely) Upper canopy moisture content Ocean biology and biochemistry Search and rescue Oil spill planning and response 16

17 Wind, Current, Wave, and Stress Coupling in the Boundary Layer and A Plan for Observing This Coupling from Space Mark A. Bourassa and Qi Shi COAPS, EOAS & GFDI, Florida State University With input from Dudley Chelton, Dmitry Dukhovskoy, Tom Farrar, M. Mar Flexas, David Long, Thomas Kilpatrick, Nikolai Maxeminko, Dimitris Menemenlis, Steven L. Morey, Alexis Mouche, Dragana Perkovich, Roger Samelson, Bryan Styles, Andrew Thompson, Frank Wentz, Shang-Ping Xie and the Ocean Observation Panel for Climate 17