Extending the applicability of the eddy-covariance flux-measurement technique

Surface-atmosphere exchange of momentum, energy and atmospheric constituents affects the atmosphere--from alterations in local microclimates and mesoscale weather to climate modification. These exchange processes can be studied using direct eddy-covariance (EC) measurements of vertical turbulent transport, but the technique has not yet readily been applied in non-prevailing ecosystems. Thus, the aim of this thesis is to extend the applicability of the EC technique in two ways: to non-standard sites and by further developing the technique itself. To reach the aim, EC measurements over a boreal lake and three urban sites in Helsinki were performed. Long-term measurements over a lake revealed that the water below the thermocline was decoupled from the atmosphere and thus not important for atmospheric vertical turbulent fluxes. The energy exchange between the lake and the atmosphere departs from vegetated surfaces especially due to large nocturnal evaporation fuelled by lake-water heat storage. Long-term measurements at a semi-urban site in Helsinki showed that the surface-atmosphere exchange is altered by anthropogenic activity: changes in surface-cover and an additional anthropogenic heat release (13 W m-2) led to an altered surface energy balance, and anthropogenic CO2 emissions led to a large positive annual CO2 balance (1.8 kg C m-2). Intra-site and intra-city variation in surface-cover led to differences in atmospheric stability and CO2 emissions. The EC technique evaluation demonstrated that (i) the 'energy imbalance problem' in EC measurements is not primarily surface-cover dependent, and that (ii) common calculation errors in EC calculations can be almost 30% of the flux. Water vapour flux measurements with a closed-path analyser were affected by sorption: the signal's arrival is delayed and it is attenuated. A new spectral-correction method based on wavelet analysis was developed to automatically correct for this signal attenuation of constituents. The conclusions of this thesis improve the understanding of surface-atmosphere exchange over non-standard ecosystems. The lake measurements will continue to be used for improving weather forecasts, and the results from the urban studies can be used in city-planning. The EC technique is developed by offering guidance in calculations at urban sites and by introducing a new correction algorithm.Pyörrekovarianssitekniikalla maanpinnan ja ilmakehän välisen vuorovaikutuksen jäljillä Maanpinta ja ilmakehä ovat vuorovaikutuksessa keskenään erilaisten energian ja materian pystyvirtausten, eli voiden, kautta. Turbulenttiset pystysuuntaiset vuot kuljettavat muun muassa lämpöä, vesihöyryä ja hiilidioksidia. Turbulenttisten voiden suuruus riippuu pinnan tyypistä sekä ilmakehän ominaisuuksista, ja vuot vaikuttavat sekä paikallissäähän että ilmastoon. Ihmisten aiheuttamat lämpöpäästöt voivat esimerkiksi aiheuttaa korkeampia lämpötiloja kaupungeissa ja hiilidioksidipäästöt kiihdyttävät ilmastonmuutosta. Näitä turbulenttisia pystyvoita voidaan mitata suoralla pyörrekovarianssi-mittaustekniikalla. Se perustuu korkeataajuisiin tuulen, lämpötilan ja kaasujen pitoisuusmittauksiin, jotka tehdään korkeassa mastossa, tutkittavan ekosysteemin yläpuolella. Tämän väitöstyön tavoitteena oli laajentaa pyörrekovarianssi-tekniikan soveltuvuutta uusiin ympäristöihin (järvi ja kaupunki) sekä kehittää uusia vuonlaskentamenetelmiä. Tutkimus perustui useamman vuoden mittauksiin eteläsuomalaisella järvellä sekä kolmella mittausasemalla Helsingissä. Työn uusia ympäristöjä käsittelevässä osiossa osoitettiin, että järven pinnan energiatase eroaa suuresti maanpinnan taseesta, koska järvi haihduttaa vettä myös läpi yön ja sillä on suuri lämmönvarastointikapasiteetti. Nämä tulokset painottavat kuinka tärkeää on huomioida järvien olemassaolo säänennustusmalleissa. Kumpulan kampuksella Helsingissä tehdyt mittaukset osoittivat, että ihmisperäinen lämmönpäästö on suurta (13 W m 2), ja että hiilidioksidipäätöt aiheuttavat positiivisen vuositaseen (1.8 kg C m 2), vaikka kasvillisuus onkin hiilidioksidinielu. Kaupungin sisäisen vaihtelun analyysi osoitti, että Helsingin keskustan suuret lämpöpäästöt pitävät meteorologiset inversiotilanteet loitolla myös talvisin, ja että keskustassa on myös jatkuvasti suuremmat hiilidioksidipäästöt kuin Kumpulassa, koska kasvillisuutta on vähemmän. Näitä tuloksia voidaan hyödyntää kaupunkisuunnittelussa. Laskentamenetelmiä kehiteltäessä huomattiin, että pyörrekovarianssimenetelmälle tyypillinen pinnan energiataseen epätasapaino ei riipu pinnasta, ja että tyypilliset virheelliset laskentamenetelmät voivat aiheuttaa helposti 30 prosentin virheen tuloksissa. Vesihöyryn takertuminen mittauslaitteiston putkistoon etenkin aiheuttaa suurta signaalihäviötä. Tätä varten kehitettiin uusi aikasarja-analyysiin perustuva korjausalgoritmi joka palauttaa hävitetyn signaalin. Näitä tuloksia voidaan käyttää pyörrekovarianssimittauksien analysoimisessa ympäri maailman, riippumatta mitattavasta pinnasta

Urban energy fluxes in Helsinki and their high frequency spectral corrections

Inadvertent climate modification has led to an increase in urban temperatures compared to the surrounding rural area. The main reason for the temperature rise is the altered energy portioning of input net radiation to heat storage and sensible and latent heat fluxes in addition to the anthropogenic heat flux. The heat storage flux and anthropogenic heat flux have not yet been determined for Helsinki and they are not directly measurable. To the contrary, turbulent fluxes of sensible and latent heat in addition to net radiation can be measured, and the anthropogenic heat flux together with the heat storage flux can be solved as a residual. As a result, all inaccuracies in the determination of the energy balance components propagate to the residual term and special attention must be paid to the accurate determination of the components. One cause of error in the turbulent fluxes is the fluctuation attenuation at high frequencies which can be accounted for by high frequency spectral corrections. The aim of this study is twofold: to assess the relevance of high frequency corrections to water vapor fluxes and to assess the temporal variation of the energy fluxes. Turbulent fluxes of sensible and latent heat have been measured at SMEAR III station, Helsinki, since December 2005 using the eddy covariance technique. In addition, net radiation measurements have been ongoing since July 2007. The used calculation methods in this study consist of widely accepted eddy covariance data post processing methods in addition to Fourier and wavelet analysis. The high frequency spectral correction using the traditional transfer function method is highly dependent on relative humidity and has an 11% effect on the latent heat flux. This method is based on an assumption of spectral similarity which is shown not to be valid. A new correction method using wavelet analysis is thus initialized and it seems to account for the high frequency variation deficit. Anyhow, the resulting wavelet correction remains minimal in contrast to the traditional transfer function correction. The energy fluxes exhibit a behavior characteristic for urban environments: the energy input is channeled to sensible heat as latent heat flux is restricted by water availability. The monthly mean residual of the energy balance ranges from 30 Wm-2 in summer to -35 Wm-2 in winter meaning a heat storage to the ground during summer. Furthermore, the anthropogenic heat flux is approximated to be 50 Wm-2 during winter when residential heating is important

Eddy covariance methodologies revisited for urban measurements

Non peer reviewe

Revised eddy covariance flux calculation methodologies - effect on urban energy balance

Eddy covariance (EC) measurements of turbulent fluxes of momentum, sensible heat and latent heat—in addition to net radiation measurements—were conducted for three consecutive years in an urban environment: Helsinki, Finland. The aims were to: (i) quantify the detection limit and random uncertainty of turbulent fluxes, (ii) assess the systematic error caused by EC calculation-procedure choices on the energy balance residual, and (iii) report the energy balance of the world’s northernmost urban flux station. The mean detection limits were about 10% of the observed flux, and the random uncertainty was 9–16%. Of all fluxes, the latent heat flux— as measured with a closed-path gas analyzer—was most prone to systematic calculation errors due to water vapor interactions with tube walls: using a lag window that is too small can cause a 15% lack of data (due to the dependency of lag time on relative humidity) and omitting spectral corrections can cause on average a 26% underestimation of the flux. The systematic errors in EC calculation propagate into the energy balance residual and can be larger than the residual itself: for example, omitting spectral corrections overestimates the residual by 13% or 18% on average, depending on the analyzer.Peer reviewe

Quantifying the uncertainty of eddy covariance fluxes due to the use of different software packages and combinations of processing steps in two contrasting ecosystems

We have carried out an inter-comparison between EddyUH and EddyPro (R), two public software packages for post-field processing of eddy covariance data. Datasets including carbon dioxide, methane and water vapour fluxes measured over 2 months at a wetland in southern Finland and carbon dioxide and water vapour fluxes measured over 3 months at an urban site in Helsinki were processed and analysed. The purpose was to estimate the flux uncertainty due to the use of different software packages and to evaluate the most critical processing steps, determining the largest deviations in the calculated fluxes. Turbulent fluxes calculated with a reference combination of processing steps were in good agreement, the systematic difference between the two software packages being up to 2.0 and 6.7% for half-hour and cumulative sum values, respectively. The raw data preparation and processing steps were consistent between the software packages, and most of the deviations in the estimated fluxes were due to the flux corrections. Among the different calculation procedures analysed, the spectral correction had the biggest impact for closed-path latent heat fluxes, reaching a nocturnal median value of 15% at the wetland site. We found up to a 43% median value of deviation (with respect to the run with all corrections included) if the closed-path carbon dioxide flux is calculated without the dilution correction, while the methane fluxes were up to 10% lower without both dilution and spectroscopic corrections. The Webb-Pearman-Leuning (WPL) and spectroscopic corrections were the most critical steps for open-path systems. However, we found also large spectral correction factors for the open-path methane fluxes, due to the sensor separation effect.Peer reviewe

Urban nitrous-oxide fluxes measured using the eddy-covariance technique in Helsinki, Finland

Peer reviewe

Variability in cold front activities modulating cool-season evaporation from a southern inland water in the USA

Understanding seasonal variations in the evaporation of inland waters (e.g., lakes and reservoirs) is important for water resource management as well as the prediction of the hydrological cycles in response to climate change. We analyzed eddy covariance-based evaporation measurements from the Ross Barnett Reservoir (32◦26N, 90◦02W; which is always ice-free) in central Mississippi during the cool months (i.e., September–March) of 2007 and 2008, and found that the variability in cold front activities (i.e., passages of cold fronts and cold/dry air masses behind cold fronts) played an important role in modulating the exchange of sensible (H) and latent (λE) heat fluxes. Our analysis showed that 2007’s warmer cool season had smaller mean H and λE than 2008’s cooler cool season. This implies that the warmer cool season did not accelerate evaporation and heat exchange between the water surface and the atmosphere. Instead, more frequent cold fronts and longer periods of cold/dry air masses behind the cold fronts in 2008 resulted in overall larger H and λE as compared with 2007, this primarily taking the form of sporadic short-term rapid ‘pulses’ of H and λE losses from the water’s surface. These results suggest that future climate-induced changes in frequency of cold fronts and the meteorological properties of the air masses behind cold fronts (e.g., wind speeds, temperature, and humidity), rather than other factors of climate change, would produce significant variations in the water surface’s energy fluxes and subsequent evaporation rates.Peer reviewe

The effect of local sources on aerosol particle number size distribution, concentrations and fluxes in Helsinki, Finland

Peer reviewe

Comparison of static chambers to measure CH4 emissions from soils

The static chamber method (non-flow-through-non-steady-state chambers) is the most common method to measure fluxes of methane (CH4) from soils. Laboratory comparisons to quantify errors resulting from chamber design, operation and flux calculation methods are rare. We tested fifteen chambers against four flux levels (FL) ranging from 200 to 2300 g CH4m−2 h−1. The measurements were conducted on a calibration tank using three quartz sand types with soil porosities of 53% (dry fine sand, S1), 47% (dry coarse sand, S2), and 33% (wetted fine sand, S3). The chambers tested ranged from 0.06 to 1.8 m in height, and 0.02 to 0.195 m3 in volume, 7 of them were equipped with a fan, and 1 with a vent-tube. We applied linear and exponential flux calculation methods to the chamber data and compared these chamber fluxes to the reference fluxes from the calibration tank. The chambers underestimated the reference fluxes by on average 33% by the linear flux calculation method (Rlin), whereas the chamber fluxes calculated by the exponential flux calculation method (Rexp) did not significantly differ from the reference fluxes (p < 0.05). The flux under- or overestimations were chamber specific and independent of flux level. Increasing chamber height, area and volume significantly reduced the flux underestimation (p < 0.05). Also, the use of non-linear flux calculation method significantly improved the flux estimation; however, simultaneously the uncertainty in the fluxes was increased. We provide correction factors, which can be used to correct the under- or overestimation of the fluxes by the chambers in the experiment.Peer reviewe

Simulation of surface energy fluxes and stratification of a small boreal lake by a set of one-dimensional models

Peer reviewe