C.M.J. Jacobs

The Sensitivity of Regional Transpiration to Land-Surface Characteristics: Significance of Feedback

Abstract Several authors have determined the sensitivity of transpiration to different environmental parameters using the Penman-Monteith equation. In their studies the interaction between transpiration and, for example, the humidity of the air is ignored: the feedback with the planetary boundary layer (PBL) is not accounted for. Furthermore, surface-layer (SL) feedback (e.g., stability effects in the surface layer) is often neglected. In our study, both PBL feedback and SL feedback are accounted for by coupling the big-leaf model to a detailed model for the PBL. This study provides an analysis of the sensitivity of transpiration to net radiation calculated after an albedo change, aerodynamic resistance calculated after a change in the aerodynamic roughness, and surface resistance. It is shown that PBL feedback affects the sensitivity of transpiration to the tested variables significantly. The sensitivity of transpiration to surface resistance and to aerodynamic resistance, or aerodynamic roughness, is de...

Redefinition and global estimation of basal ecosystem respiration rate

Basal ecosystem respiration rate (BR), the ecosystem respiration rate at a given temperature, is a common and important parameter in empirical models for quantifying ecosystem respiration (ER) globally. Numerous studies have indicated that BR varies in space. However, many empirical ER models still use a global constant BR largely due to the lack of a functional description for BR. In this study, we redefined BR to be ecosystem respiration rate at the mean annual temperature. To test the validity of this concept, we conducted a synthesis analysis using 276 site-years of eddy covariance data, from 79 research sites located at latitudes ranging from similar to 3 degrees S to similar to 70 degrees N. Results showed that mean annual ER rate closely matches ER rate at mean annual temperature. Incorporation of site-specific BR into global ER model substantially improved simulated ER compared to an invariant BR at all sites. These results confirm that ER at the mean annual temperature can be considered as BR in empirical models. A strong correlation was found between the mean annual ER and mean annual gross primary production (GPP). Consequently, GPP, which is typically more accurately modeled, can be used to estimate BR. A light use efficiency GPP model (i.e., EC-LUE) was applied to estimate global GPP, BR and ER with input data from MERRA (Modern Era Retrospective-Analysis for Research and Applications) and MODIS (Moderate resolution Imaging Spectroradiometer). The global ER was 103 Pg C yr (-1), with the highest respiration rate over tropical forests and the lowest value in dry and high-latitude areas.

Fluxes of carbon dioxide and water vapour from a Sahelian savanna

Abstract Simultaneous measurements of atmospheric CO 2 flux, F c , and latent heat flux, E , from a shrub savanna in Niger, West Africa, were made by eddy correlation. The vegetation at the study site consisted of scattered shrubs with an understorey of grasses and herbs. The measurements made available some of the first data on CO 2 and H 2 O exchange for an semi-natural, mixed plant community, growing in the semi-arid tropics. Such data are necessary for the development of improved soil-vegetation-atmosphere models, able to describe the complex interplay between atmospheric CO 2 , vegetation conductance and the surface energy balance of the Sahel in global climate models. In this framework, the effect of the extreme and highly variable environment, represented by the saturation deficit, D , and integrated water content of the upper soil layers, Φ, was discussed. F c and E were measured throughout the transition from the wet to the dry season (September–October) during the HAPEX-Sahel Intensive Observation period, in 1992. At the same time, leaf stomatal conductances, g 1 , of the shrubs and two understorey herb species was measured. Daily totals of F c decreased by 50% (peak values declined from −10 to −5 μmol m −2 s −1 ) over 3 weeks following the last rainfall of the wet season. During the same period, g 1 decreased roughly four-fold for all sampled species. D appeared to be the main controlling parameter in the exchange of CO 2 and H 2 O. g 1 and ‘water use efficiency’, | F c E |, were well correlated with D , which also considerably influenced the response of F c to photosynthetically active radiation, Q p . Simultaneously, a decreasing Φ caused lower values of g 1 and F c . However, soil moisture had little effect on the empirical relationships found between atmospheric variables ( D or Q p ) and g 1 or F c .

Spatial variability of the Rotterdam urban heat island as influenced by urban land use

Novel bicycle traverse meteorological measurements were made in Rotterdam to assess the spatial variation of temperature during a tropical day. Nocturnal spatial urban temperature differences of 7?K were found to be related to city morphology. The coolest residential areas were green low-density urban areas. During midday measurements the downtown was up to 1.2?K warmer than the surrounding rural area while a city park was 4.0?K cooler than downtown. A regression analysis showed that the nocturnal measured urban heat island (UHI) can be linked to land use, namely plan area fraction of vegetation, built up area water and is most significant for vegetation. The vegetated area was derived from visible and near infrared aerial images. Neighbourhoods with vegetation (within an upwind radius of 700?m) had a significantly reduced UHI during the night. From the traverse observation data a multiple linear regression model was constructed and independently validated with 3-year summertime UHI statistics derived from 4 urban fixed meteorological stations. In addition, two fixed rural stations were used; a WMO station at Rotterdam airport and a rural station further away from the city. Wind rose analysis shows that UHI is strongest from easterly directions and that the temperature signal of the WMO station is influenced by an UHI signal from both the airport runways and urban directions. A regression model reproduced the nighttime spatial variability of the UHI within a fractional bias of 4.3% and was used to derive an UHI map of Rotterdam and surroundings. This map shows that high density urban configurations lacking greenery or close to large water bodies are vulnerable to high nocturnal temperatures during heat waves. This warming effect of water bodies is also evident for an urban weather station located in the harbor area, which had a similar nocturnal UHI frequency distribution as the downtown urban weather station. The UHI map can be used as a valuable planning tool for mitigating nocturnal urban heat stress or identifying neighborhoods at risk during heat waves.

Predicting Regional Transpiration at Elevated Atmospheric CO2: Influence of the PBL–Vegetation Interaction

Abstract A coupled planetary boundary layer (PBL)–vegetation model is used to study the influence of the PBL–vegetation interaction and the ambient CO2 concentration on surface resistance rs and regional transpiration λE. Vegetation is described using the big-leaf model in which rs is modeled by means of a coupled photosynthesis–resistance model. The PBL part is a one-dimensional, first-order closure model. Nonlocal turbulent transport is accounted for by means of a countergradient correction. The PBL model also describes CO2 fluxes and concentrations, which are driven by photosynthesis of the canopy. A number of sensitivity analyses are presented in which the behavior of rs and λE at an atmospheric CO2 concentration representative for the present-day situation is compared to their behavior under an approximately doubled CO2 concentration. The results reveal a positive atmospheric feedback on rs, by which an initial increase of rs, due to changes in ambient CO2 concentration, is magnified. The stomatal hu...

Assimilation of land surface temperature data from ATSR in an NWP environment--a case study

Directional thermal observations from the ATSR-2 satellite sensor were used to estimate separate vegetation and soil temperatures for a number of cloud free scenes covering south-east Spain over five days in 1999. Underlying methodology for this is a simplified radiative transfer scheme and a concurrent estimate of the fraction of ground covered with vegetation. The vegetation and soil temperatures were used together with near-surface relative humidity measurements to adjust the root zone soil moisture content and roughness length for heat in a newly developed multi-component land surface parameterization scheme, embedded in a regional weather forecast model. The ATSR surface temperature data have a strong influence on the modification of the thermal roughness length. The optimal roughness length gradually changes over the growing season, as can be expected from the dependence of thermal roughness on vegetation density. Application of the method to a grassland scene in The Netherlands resulted in a much smaller adjustment to the thermal roughness length. The distribution of the roughness over the Spanish test area appeared to be consistent in time, as correlation coefficients of roughness values between two subsequent acquisition dates were significantly positive. Small improvements in the calculated surface energy balance appear from independent near-surface air temperature observations in the Spanish area. The use of bi-angular thermal infrared observations seems useful to improve the description of aerodynamic roughness properties on regional scales.

Shallow cumulus rooted in photosynthesis

We study the interactions between plant evapotranspiration, controlled by photosynthesis (C3 and C4 grasses), and moist thermals responsible for the formation of shallow cumulus clouds (SCu). Our findings are based on a series of systematic numerical experiments at fine spatial and temporal scales using large eddy simulations explicitly coupled to a plant-physiology model. The shading provided by SCu leads to strong spatial variability in photosynthesis and the surface energy balance. This in turn results in SCu characterized by less extreme and less skewed values of liquid water path. The larger water use efficiency of C4 grass leads to two opposite effects that influence boundary layer clouds: more vigorous and deeper thermals due to the larger buoyancy surface flux (positive effect) characterized by less moisture content (negative). We find that under these midlatitude and well-watered soil conditions, SCu are characterized by a larger cloud cover and liquid water path over C4 grass fields.

Direct and Diffuse Radiation in the Shallow Cumulus–Vegetation System: Enhanced and Decreased Evapotranspiration Regimes

AbstractGuided by a holistic approach, the combined effects of direct and diffuse radiation on the atmospheric boundary layer dynamics over vegetated land are investigated on a daily scale. Three numerical experiments are designed that are aimed at disentangling the role of diffuse and direct radiation below shallow cumulus at the surface and on boundary layer dynamics. A large-eddy simulation (LES) model coupled to a land surface model is used, including a mechanistically immediate response of plants to radiation, temperature, and water vapor deficit changes. The partitioning in direct and diffuse radiation created by clouds and farther inside the canopy is explicitly accounted for. LES results are conditionally averaged as a function of the cloud optical depth. The findings show larger photosynthesis under thin clouds than under clear sky, due to an increase in diffuse radiation and a slight decrease in direct radiation. The reduced canopy resistance is the main driver for the enhanced carbon uptake by ...

Evaluation of European Land Data Assimilation System (ELDAS) products using in situ observations

Three land-surface models with land-data assimilation scheme (DA) were evaluated for one growing season using in situ observations obtained across Europe. To avoid drifts in the land-surface state in the models, soil moisture corrections are derived from errors in screen-level atmospheric quantities.With the in situ data it is assessed whether these land-surface schemes produce adequate results regarding the annual range of the soil water content, the monthly mean soil moisture content in the root zone and evaporative fraction (the ratio of evapotranspiration to energy available at the surface). DA considerably reduced bias in net precipitation, while slightly reducing RMSE as well. Evaporative fraction was improved in dry conditions but was hardly affected in moist conditions. The amplitude of soil moisture variations tended to be underestimated. The impact of improved land-surface properties like Leaf Area Index, water holding capacity and rooting depth may be as large as corrections of the DA systems. Because soil moisture memorizes errors in the hydrological cycle of the models, DA will remain necessary in forecast mode. Model improvements should be balanced against improvements of DA per se. Model bias appearing from persistent analysis increments arising from DA systems should be addressed by model improvements.

Effects of water diversion and climate change on the Rur and Meuse in low-flow situations

Water scarcity is one of the problems in water management that hinders European rivers in reaching a good ecological status as defined in the European Water Framework Directive. Water scarcity often coincides with high water temperature and low water quality. High water temperatures decrease the oxygen supply and often coincide with low flows that tend to increase the load of various compounds that affect the equilibrium in the ecosystem. The river Meuse regularly encounters situations of low water flow. In these situations, the river Rur, an important tributary of the river Meuse in Germany, contributes significantly to the Meuse discharge. Climate change can lead to more frequent periods of water scarcity. Moreover, plans exist to divert water from the Rur to former brown coal quarries in Germany. This exploratory study examines the relationships between discharge, water temperature and water quality under future climate change and water diversion scenarios in low-flow situations for the rivers Meuse and Rur. The results of the study confirm that rising air temperatures as a result of climate change will lead to higher water temperatures which will negatively impact the water quality of the Meuse in particular. Despite the fact that the contribution of the Rur has a positive impact on the water quality of the Meuse, the results suggest that effects of plans to divert water from the Rur may be small on average. However, the impact of the diversion may be stronger on individual hot summer days with low water levels when the Rur contributes significantly to the discharge of the Meuse.