Helen Cleugh

A re-evaluation of long-term flux measurement techniques - Part I: Averaging and coordinate rotation

Experience of long term flux measurements over tall canopiesduring the last two decades has revealed that the eddy flux of sensible plus latentheat is typically 30% smaller than the available radiant energy flux. This failureto close the energy balance is less common close to the surface over short roughnessbut is still sometimes seen, especially in complex topography. These observationscast doubt on the results obtained from long term flux studies where daily and annualnet ecosystem exchange is usually the small difference between large positive andnegative fluxes over 24 h. In this paper we investigate this problem by examiningsome fundamental assumptions entailed in analysis of surface exchange by the eddyflux method.In particular, we clarify the form and use of the scalar conservation equation thatunderlies this analysis and we examine the links between averaging period androtation of coordinates in the situation where coordinates are aligned with thewind vector. We show that rotating coordinates so that the x axis is alignedwith the mean wind vector has the effect of high pass filtering the scalar covariance,¯wc, such that contributions to the aerodynamic flux from atmosphericmotions with periods longer than the averaging period are lost while those of shorterperiod are distorted.We compare the effect of computing surface exchange by averaging many shortperiods, in each of which the coordinates are rotated so that the mean verticalvelocity is zero (the method currently adopted in most long-term flux studies),with analysis in long-term coordinates and show a systematic underestimationof surface exchange in the former case. This is illustrated with data from threelong-term forest field sites where underestimations of sensible and latent heatfluxes of 10–15% averaged over many days are seen.Crucial factors determining the loss of flux are the averaging period T, themeasurement height and the content of the scalar cospectrum at periods longerthan T. The properties of this cospectrum over tall canopies in both homogeneousand complex terrain are illustrated by measurements at our three sites and we see thatover tall canopies on flat ground in convectiveconditions, or on hilly sites in near neutralflow, the scalar cospectra have much more low frequency contentthan classical surface-layerspectral forms would predict. We believe that the filtering of this low frequencycovariance by the averaging-rotation operations in common use is a large contributoryfactor to the failure to close the energy balance over tall canopies.

Nocturnal carbon efflux: reconciliation of eddy covariance and chamber measurements using an alternative to the u*-threshold filtering technique

Micrometeorological measurements made on single towers often underestimate nighttime respiration of terrestrial ecosystems because they cannot account for vertical and horizontal advection, thereby causing systematic errors in estimates of net ecosystem carbon exchange. We show that there is a maximum in the sum of the turbulent flux and change in storage of CO2 in the early evening, Rmax, that is in close agreement with concurrent and independent estimates of net carbon exchange from soil and plant chambers.We hypothesize that the peak occurs because there is a time delay between the onset of radiative cooling and the development of temperature gradients that are strong enough to initiate thermally-driven horizontal and vertical flows that remove the stored CO2. We propose taking advantage of this time delay to develop relationships between Rmax and soil temperature and moisture. The new parameterization leads to realistic values of nighttime respiration, and therefore to improved estimates of net ecosystem exchange.

Estimating catchment evaporation and runoff using MODIS leaf area index and the Penman‐Monteith equation

[1] This paper shows the feasibility of using steady state water balances of gauged catchments to calibrate a spatially explicit evaporation model and then applying this to estimate mean annual runoff for 120 gauged catchments in the Murray-Darling Basin (MDB) of Australia from 2001 to 2005. We used remotely sensed leaf area indices from the Moderate Resolution Imaging Spectrometer (MODIS) mounted on the polar-orbiting Terra satellite with the Penman-Monteith equation, gridded meteorology, and a two-parameter biophysical model for surface conductance (G s ) to estimate 8-day average evaporation at 1-km resolution. Parameters for the G s model were optimized using steady state water balance estimates (precipitation minus runoff) in the gauged catchments in three precipitation zones of the MDB, and the calibrated evaporation model was then used to estimate evaporation (E RS ) and runoff from gauged and ungauged catchments in the MDB. Mean annual calibrated estimates of E RS compared well with water balance estimates, indicated by a root-mean-square error (RMSE) of 78.6 mm/a and the Nash-Sutcliffe coefficient of efficiency (CE) of 0.68. Reasonable agreement was obtained between the estimated mean annual runoff (R RS ) (rainfall minus E RS ), and the measured runoff (RMSE = 71.0 mm/a and CE = 0.75). Cross validation showed that estimated E RS and R RS were almost as good as the calibrated ones. Furthermore, R RS has an accuracy similar to that of a seven-parameter conceptual rainfall-runoff model in the gauged catchments. The results show that the evaporation model can be easily applied to estimate steady state evaporation and runoff and that E RS can be used with rainfall-runoff models to improve accuracy of estimated runoff in ungauged catchments.

The utility of the eddy covariance techniques as a tool in carbon accounting: tropical savanna as a case study

Global concern over rising atmospheric CO2 concentrations has led to a proliferation of studies of carbon cycling in terrestrial ecosystems. Associated with this has been widespread adoption of the eddy covariance method to provide direct estimates of mass and energy exchange between vegetation surfaces and the atmosphere. With the eddy covariance method, fast-response instruments (10–20Hz) are typically mounted above plant canopies and the fluxes are calculated by correlating turbulent fluctuations in vertical velocity with fluctuations in various scalars such as CO2, water vapour and temperature. These techniques allow the direct and non-destructive measurement of the net exchange of CO2 owing to uptake via photosynthesis and loss owing to respiration, evapotranspiration and sensible heat. Eddy covariance measurements have a high temporal resolution, with fluxes typically calculated at 30-min intervals and can provide daily, monthly or annual estimates of carbon uptake or loss from ecosystems. Such measurements provide a bridge between ‘bottom-up’ (e.g. leaf, soil and whole plant measures of carbon fluxes) and ‘top-down’ approaches (e.g. satellite remote sensing, air sampling networks, inverse numerical methods) to understanding carbon cycling. Eddy covariance data also provide key measurements to calibrate and validate canopy- and regional-scale carbon balance models. Limitations of the method include high establishment costs, site requirements of flat and relatively uniform vegetation and problems estimating fluxes accurately at low wind speeds. Advantages include spatial averaging over 10–100ha and near-continuous measurements. The utility of the method is illustrated in current flux studies at ideal sites in northern Australia. Flux measurements spanning 3years have been made at a mesic savanna site (Howard Springs, Northern Territory) and semi-arid savanna (Virginia Park, northern Queensland). Patterns of CO2 and water vapour exchange at diurnal, seasonal and annual scales are detailed. Carbon dynamics at these sites are significantly different and reflect differences in climate and land management (impacts of frequent fire and grazing). Such studies illustrate the utility of the eddy covariance method and its potential to provide accurate data for carbon accounting purposes. If full carbon accounting is implemented, for ideal sites, the eddy covariance method provides annual estimates of carbon sink strength accurate to within 10%. The impact of land-use change on carbon sink strength could be monitored on a long-term basis and provide a valuable validation tool if carbon trading schemes were implemented.

Neural Error Regression Diagnosis (NERD): A Tool for Model Bias Identification and Prognostic Data Assimilation

Abstract Data assimilation in the field of predictive land surface modeling is generally limited to using observational data to estimate optimal model states or restrict model parameter ranges. To date, very little work has attempted to systematically define and quantify error resulting from a models inherent inability to simulate the natural system. This paper introduces a data assimilation technique that moves toward this goal by accounting for those deficiencies in the model itself that lead to systematic errors in model output. This is done using a supervised artificial neural network to “learn” and simulate systematic trends in the model output error. These simulations in turn are used to correct the models output each time step. The technique is applied in two case studies, using fluxes of latent heat flux at one site and net ecosystem exchange (NEE) of carbon dioxide at another. Root-mean-square error (rmse) in latent heat flux per time step was reduced from 27.5 to 18.6 W m−2 (32%) and monthly f...

Spatial and Temporal Variations in Fluxes of Energy, Water Vapour and Carbon Dioxide During OASIS 1994 and 1995

This paper introduces the micrometeorological field campaigns known asOASIS (Observations At Several Interacting Scales) and then summarizesseveral companion studies that have used the OASIS dataset. Instrumentedtowers, aircraft and atmospheric sondes were used for measurements overthree paired sites (crops and pastures), approximately equi-spaced along an88-km transect in south-eastern New South Wales, Australia, during the australsprings of 1994 and 1995. Measurements included standard meteorologicaldata and the fluxes of solar and net radiation, sensible heat, water vapour andthe greenhouse gases CO2, N2O, CH4. Descriptions of the site, andthe spatial and temporal variations of climate fields and fluxes, are presented.There were strong contrasts in fluxes and surface conductances, evaporationratios and water use efficiencies between the 1994 drought year and the normalrainfall year of 1995. Despite greater incoming solar radiation in 1994 associatedwith less cloud cover, net radiation was lower than in 1995 because of greateroutgoing thermal radiation caused by higher surface temperatures. In 1994 dailysensible heat fluxes were about 50% higher and evaporation rates about half thosefor 1995. Rainfall in the three-month growing season prior to the field campaignswas the key determinant of leaf area index, surface conductances and the fluxes ofsensible and latent heat and CO2. Antecedent rainfall distribution also controlled variation in fluxes and surface properties along the transect within each year. There was a net loss of CO2 to the atmosphere at the drier central sites in 1994, and a net uptake at the wetter north-eastern sites. Both sites recorded uptake of CO2 in 1995, but values were lower at the central site than at the north–east site due to the strong rainfall gradient along the transect in the three months prior to each fieldcampaign. Differences in fluxes between crops and pastures at each site were smallerthan between sites.

MODELLING REGIONAL SCALE SURFACE ENERGY EXCHANGES AND CBL GROWTH IN A HETEROGENEOUS, URBAN-RURAL LANDSCAPE

Over the last decade, simple models of theconvective boundary layer (CBL) have beensuggested as an approach to inferring regionallyaveraged land-air exchanges of heat, water and tracegases, because the properties of the CBL respond toan average of the underlying small-scaleheterogeneity. This paper explores the use of anintegral CBL method to infer regionally averagedfluxes in a landscape that has at least three majorsources of heterogeneity – irrigated andnon-irrigated rural land use and a large urban area(Sacramento region, California).The first part of the paper assesses the validity ofthe simple slab model of the CBL – this isintegrated forwards in time using local-scalemeasured heat and water vapour fluxes, to predictmixed-layer depth, temperature and humidity. Of thefour different CBL growth schemes used, the Tennekesand Driedonks model is found to give the bestperformance. Evaluation of the model performancewith different weightings of heat and water vapourfluxes based on the land use characteristics in theregion suggest that the source area for theboundary-layer sonde measurements is larger thanphysically-based estimates would suggest.Finally, measured time series of potentialtemperature are used to infer regionally averagedsensible heat fluxes using an integral CBL (ICBL)method. These ICBL fluxes are compared with thosemeasured at the local scale over the three land usetypes that comprise the region of interest. They arefound to be closest to the heat fluxes calculated byappropriately weighting the measured heat fluxes inthe source area calculated for the ICBL. We concludethat the integral CBL budget method providesadequate estimates of regionally-averaged surfaceheat fluxes in a landscape that is characterised bysurface types with distinctly different surfaceenergy budgets.

Estimation of Regional Evapotranspiration by Combining Aircraft and Ground-Based Measurements

Two methods are examined for combining measurements from instrumented aircraftand towers to estimate regional scale evapotranspiration. Aircraft data provided spatially averaged values of properties of the surface, the evaporative fraction and maximum stomatal conductance. These quantities are less sensitive to meteorological conditions than the turbulent fluxes of heat and water vapour themselves. The methods allowed aircraft data collected over several days to be averaged and thus to reduce the random error associated with the temporal under-sampling inherent in aircraft measurements. Evaporative fraction is estimated directly from the aircraft data, while maximum stomatal conductance is estimated by coupling the Penman–Monteith equation to a simple model relating surface conductance to the incoming shortwave radiation and specific humidity saturation deficit. The spatial averages of evaporative fraction and maximum stomatal conductance can then be used with routine tower data to estimate the regional scale evapotranspiration. Data from aircraft flights and six ground based sites during the OASIS field campaign in south–east New South Wales in 1995 have been used to check the methods. Both the evaporative fraction and the maximum stomatal conductance derived from the aircraft data give information on the spatial variability of the surface energy budget at scales from 10 to 100 km. Daily averaged latent heat fluxes estimated using these methods for the OASIS study region agree with the available observations in quasi-stationary conditions or in weakly non-stationary conditions when the data from several aircraft flights are averaged to reduce the impact of short term imbalances in the surface energy budget.

A simple method to determine Obukhov lengths for suburban areas

Abstract A simple scheme is presented to calculate Obukhov stability lengths L for suburban areas when measurements of sensible heat flux Qh or temperature gradients are not available. The scheme replaces Qv with χQ*, where Q* is net all-wave radiation, and χ = QH/Q*, which is based on data collected in North American cities. The results suggest that the simple two-part model with either a variable or fixed value of χ for daytime works satisfactorily.