Measurements of Mass and Energy Exchange using Aircraft-based Sensors R.L. Desjardins, D. Worth, Mauder, M., Metzger, S., and R. Srinivasan 15th EMS Conference, Sept. 2015, Sofia, Bulgaria

2 Length Aircraft Tower Chamber 10 2 m 10 3 m 1 h 10 4 h 10 3 h 100 h Time 10 m 10 4 m 10 5 m Models Spatial and temporal scales of mass and energy exchange

3 Flux Measurement Platform – The Twin Otter Aircraft Laser Altimeter Side-looking video camera Satellite simulator Greeness indicator Gas analyzers (CO 2, CH 4, H 2 O, O 3 ) Duct pressure, temperature sensors Litton 90 inertial reference system Intake REA system Data recorder Accelerometers rate gyros Global positioning system antenna Console, keyboard & navigation system controls Dew point sensor Rosemount 858 ( , , airspeed, altitude) Video camera Altitude gyro Radio altimeter (CO 2, CH 4, N 2 O, VOC, agrochemicals)

4 Reynolds: Flux:.. x EC technique is a useful tool because it permits to quantify fluxes for an entire ecosystem without disturbing it. It is based on assumptions of stationarity and horizontal homogeneity. The temporal mean may not be spatially representative due to large-scale organized structures. The Eddy Covariance Method Time (with tower) or distance (with aircraft) vertical wind velocity scalar vertical wind velocity scalar

5 Relaxed Eddy Accumulation (REA) Alternate to eddy covariance technique to measure fluxes of trace gases for which fast- response analyzers are not operational (N 2 O) Air samples from updrafts and downdrafts are collected in two separate reservoirs for later analysis In EA, sample flow rate is proportional to w; this requirement is ‘relaxed’ in REA (i.e., full flow into up or down reservoir depending on the direction of the vertical wind) Desjardins et al Alternate to eddy covariance technique to measure fluxes of trace gases for which fast- response analyzers are not operational (N 2 O) Air samples from updrafts and downdrafts are collected in two separate reservoirs for later analysis In EA, sample flow rate is proportional to w; this requirement is ‘relaxed’ in REA (i.e., full flow into up or down reservoir depending on the direction of the vertical wind) Desjardins et al. 2000

CO 2 Fluxes (kg CO 2 ha -1 h -1 ) Measurements of carbon dioxide absorption over a 15 km x 15 km grassland site using the NRC Twin Otter aircraft. The data is superimposed on a satellite image. CO 2 flux measurements during FIFE, 1989

Evapotranspiration measured using the Twin Otter aircraft over the Konza Prairies during FIFE

8 Tower and Aircraft Flux Measurements sensible and latent heat QEQE QEQE QEQE QHQH QHQH QHQH SGP project 1997 USA

Tower (W m -2 ) Aircraft (W m -2 ) Whole run, 12 km Segment, 2.8 km AC / Tower Comparison of Sensible Heat Fluxes (line 8) 1:1

10 Lack of energy budget closure: Implications for sensible heat, CO 2 and H 2 O flux measurements? From the basic energy balance equation, Q n – Q G – ΔQ S = Q E + Q H However, experimentally it has generally been found that on a short time scale (hours) Q n – Q G – ΔQ S > Q E + Q H

11 turbulent energy fluxes: Q H + Q E (W m -2 ) available energy: -Q * s – Q G (W m -2 ) 22 European sites: residual of 20% of the available energy on average (Wilson et al. 2002) Lack of Energy Budget Closure Energy Balance at 6 European forest sites (Aubinet et al., 2000) Are we underestimating the CO 2 flux as well?

12 Mesoscale Circulation T < [T], w < [w]  Q H > 0 QHQH

An example of the long term impact of surface heterogeneity on mass and energy exchange- 56 passes (Desjardins et al. 1997) 13 Desjardins, R.L., MacPherson, J.I., Mahrt, L., Schuepp, P.H., Pattey, E., Neumann, H., Baldocchi, D., Wofsy, S., Fitzjarrald, D., H. McCaughey and D.W. Joiner Scaling up flux measurements for the boreal forest using aircraft-tower combinations. J. Geophys. Res. 102: 29,125-29,134.

wavelength Eddy flux Mesoscale contribution Turbulent flux Handling of nonstationary conditions

15 Candle Lake – Wavelet analysis Why wavelet analysis? 1.Does not require stationarity and homogeneity (in contrast to Fourier analysis) 2.Gives quantitative information, where in space and on what wavelength flux contributions occur 3.Allows to distinguish between small-scale turbulence and mesoscale fluxes ( 2 km) 4.Allows to compute fluxes at a relatively small spatial resolution (  1 km) without neglecting flux contributions from longer wavelengths 5.With wavelet analysis the residual term is substantially reduced

16 Candle Lake – Wavelet analysis Analysis of low-frequency flux contributions as reason for the underestimation of eddy-covariance fluxes: Wavelet analysis of aircraft measurements Canada 115 km

17 Candle Lake – Wavelet cross-scalogram Flight 1 BOREAS 1041 – 1116 CST 25 May 1994 Distance (km) Logarithmic wavenumber (m) Mauder, M., R. L. Desjardins, and I. MacPherson Scale analysis of airborne flux measurements over heterogeneous terrain in a boreal ecosystem. J. Geophys. Res., 112, D 13112, doi: /2006JD Positive flux contribution Near zero flux Negative flux contribution Legend

How important is mesoscale transport in the surface layer? Candle Lake Runs (BOREAS/BERMS) at 30 m measurement height 20 flights analyzed  5 – 20% mesoscale flux contribution (2 km) Mauder, M., R. L. Desjardins, and I. MacPherson Scale analysis of airborne flux measurements over heterogeneous terrain in a boreal ecosystem. J. Geophys. Res., 112, D 13112, doi: /2006JD

19 Mesoscale flux contributions (i.e., wavelength > 2 km) in % of the flux averaged over the entire flight track Datetime (CST)HλECO 2 fluxO 3 flux 25-May %10% 25-May %13%11%8% 25-May %14%9%13% 27-May −2%5%−5%2% 01-Jun %14% 22% 06-Jun %17%12%13% 06-Jun %12%9%7% 21-Jul %12%11%5% 23-Jul %14%9%13% 25-Jul %16%17%25% 25-Jul %17% 18% 25-Jul %30%19%37% 27-Jul %12%11%10% 08-Sep %10%2%8% Mauder, M., R. L. Desjardins, and I. MacPherson Scale analysis of airborne flux measurements over heterogeneous terrain in a boreal ecosystem. J. Geophys. Res., 112, D 13112, doi: /2006JD

20 Measuring nitrous oxide emissions soy cereals pasture/grass alfalfa forest corn town LEGEND

21 Regional N 2 O fluxes during and right after snowmelt at the Eastern Canada study sites in 2001 Canada Mixed farmland e.g. REA can be used to measure the regional (≈ km 2 ) flux of N 2 O from agricultural land. Each data point represents the average of 3 samples, collected during two consecutive 10 km flight legs (total flight distance for one data point is ≈ 20 km). After accounting for 20% of the indirect N 2 O emissions (not considered by models such as DNDC) cumulative N 2 O emission estimates between the DNDC model and measurements were comparable. e.g. REA can be used to measure the regional (≈ km 2 ) flux of N 2 O from agricultural land. Each data point represents the average of 3 samples, collected during two consecutive 10 km flight legs (total flight distance for one data point is ≈ 20 km). After accounting for 20% of the indirect N 2 O emissions (not considered by models such as DNDC) cumulative N 2 O emission estimates between the DNDC model and measurements were comparable. Source: Desjardins, R.L., Pattey, E., Smith, W.N., Worth, D., Grant, B., Srinivasan, R., MacPherson, J.I., and Mauder, M Multiscale estimates of N 2 O emissions from agricultural lands. Special Issue of Agriculture and Forest Meteorology 150 (6)

The NRC Twin Otter 22 Instrumented nose boom in-flight REA sample collection & post-flight REA sample analysis using Picarro G1301 CH 4 Analyzer (G2301) and real-time display

CH 4 emission estimates at a regional scale (2011)

24 Summary Aircraft-based flux measuring facility based on EC and REA techniques A powerful tool to measure mass and energy exchange over a wide range of ecosystems Presented flux measurements for CO 2, CH 4, N 2 O, H 2 O and O 3 Energy budget closure – a poor validation tool. Mesoscale flux contributions have a similar magnitude as the flux underestimation by tower- based systems Aircraft-based flux measuring facility based on EC and REA techniques A powerful tool to measure mass and energy exchange over a wide range of ecosystems Presented flux measurements for CO 2, CH 4, N 2 O, H 2 O and O 3 Energy budget closure – a poor validation tool. Mesoscale flux contributions have a similar magnitude as the flux underestimation by tower- based systems