B.G. Heusinkveld

A system to measure surface fluxes of momentum, sensible heat, water vapour and carbon dioxide

An eddy covariance system is described which has been developed jointly at a number of European laboratories and which was used widely in HAPEX-Sahel. The system uses commercially available instrumentation: a three-axis sonic anemometer and an IR gas analyser which is used in a closed-path mode, i.e. air is brought to the optical bench by being ducted down a sampling tube from a point near the sonic anemometer. The system is controlled by specially written software which calculates the surface fluxes of momentum, sensible and latent heat and carbon dioxide, and displays them in real time. The raw turbulent records can be stored for post-processing. Up to five additional analogue instruments can be sampled at up to 10 Hz and digitised by the sonic anemometer. The instruments are described and details of their operation and connection are presented. The system has relatively low power consumption and can operate from appropriate solar cells or rechargeable batteries. Calibration of the gas analyser needs to be performed typically every 2 or 3 days, and, given that the system requires minimal maintenance and is weather insensitive, it can be operated for the routine collection of surface flux data for extended periods. There are a number of corrections which have to be applied in any eddy covariance system and we describe the system of transfer functions which define our system. Some representative results showing the potential of the system are presented.

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 .

A simple model for potential dewfall in an arid region

It is not always easy to know, post-facto, whether both dewfall and fog may have occurred over a given evening period. Instrumentation limitations make it difficult to quantify dew deposition since they rely on artificial sensing surfaces that are either visually examined on a daily basis or recorded. In arid to Mediterranean regions, both dew and fog can play significant ecological roles as suppliers of moisture. Long-term observation records of dew and fog in such regions tend to be limited, however, due partly to a lack of interest and limited distribution of well-instrumented meteorological stations. Simple meteorological criteria are suggested here to calculate potential dewfall and to indicate whether fog was likely to have occurred over a given evening. A field campaign was carried out in the NW Negev desert, Israel, in September and October 1997, to collect meteorological data and carry out dewfall measurements.

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.

An extra large aperture scintillometer for long range applications

An incoherent scintillometer with 0.31-m aperture was testedalong a 9.8-km path over grassland. Scintillometer derived heat fluxes were comparedwith in situ eddy covariance measurements. Albeit with considerable scatter, the fluxescompared well during daytime. During nighttime credible fluxes are also obtained. The scintillometerfunctioned satisfactory for 96% of the 7-week period.

Observations of the radiation divergence in the surface layer and its implication for its parameterization in numerical weather prediction models

radiative heating is minimal at the evening transition, with a median value of � 1.8 K h � 1 between 1.3 and 10 m and � 0.5 K h � 1 between 10 and 20 m, respectively. After the transition, its magnitude gradually decreases during the night. For individual clear calm nights, a minimal radiative heating rate of � 3.5 and � 2.0 K h � 1 was found for the two indicated layers. The total radiative heating rate appears dominantly controlled by the upward longwave flux divergence. Surprisingly, at noon a radiative heating rate of � 1Kh � 1 was found between 1.3 and 10 m for clear calm days. The availability of these radiation divergence measurements enables evaluation of the model performance for the temperature tendency caused by radiation divergence. The mesoscale model MM5 performs poorly for the stable boundary layer, because it overestimates the surface temperature and wind speed, while it underestimates the magnitude of radiative cooling. Some computationally efficient methods based on physical modeling, statistical modeling, and dimensional analysis are proposed by examining the gathered data set. The physical modeling approach appears to perform best.

An automated microlysimeter to study dew formation and evaporation in arid and semiarid regions

Abstract The development of a simple and low-cost portable weighing microlysimeter that makes use of a load cell for automated recording and for studying daily dew formation, rate of accumulation, and subsequent evaporation in arid or semiarid regions during rainless seasons is presented. The sampling cup is 3.5 cm deep, with the load cell itself situated at 20-cm depth to minimize temperature effects. The device was tested in a sand dune experimental station situated near Nizzana, northwest Negev Desert, Israel, during which extensive micrometeorological measurements were collected. One microlysimeter was placed in a playa and a second was installed on the stabilized midslope of an adjacent linear sand dune. To assess the performance of the load cell microlysimeters (LCM), one pair of manual microlysimeters was installed next to each LCM. A third pair was installed at a point between the LCMs and a fourth pair above the midslope LCM. Sixteen overnight measurements were carried out within a 6-week period....

Temperature variation in a class A evaporation pan

Abstract The thermal behaviour of a class A evaporation pan has been studied by doing an outdoor experiment and by modelling the energy budget of the pan in detail. The observations showed that the temperature of the water remains more or less homogeneous throughout the daily cycle. This means that the water inside the pan is well mixed. This characteristic thermal behaviour was observed even under low wind ( u (0.5 m) −1 . The one-dimensional model simulations agreed well with the experimental evidence. However, in the model simulations corrections had to be carried out for the turbulence generated by the pan itself and in particular by the rim of the pan.

Systematic errors in ground heat flux estimation and their correction

Incoming radiation forcing at the land surface is partitioned among the components of the surface energy balance in varying proportions depending on the time scale of the forcing. Based on a land-atmosphere analytic continuum model, a numerical land-surface model and field observations we show that high-frequency fluctuations in incoming radiation (with period less than 6 hours, for example due to intermittent clouds) are preferentially partitioned towards ground heat flux. These higher frequencies are concentrated in the 0-1 centimeter surface soil layer. Subsequently, measurements even at a few centimeters deep in the soil profile miss part of the surface soil heat flux signal. The attenuation of the high-frequency soil heat flux spectrum throughout the soil profile leads to systematic errors in both measurements and modeling, which require a very fine sampling near the soil surface (0-1 centimeter). Calorimetric measurement techniques introduce a systematic error in the form of artificial band-pass filter if the temperature probes are not placed at appropriate depths. In addition the temporal calculation of the change in the heat storage term of the calorimetric method can further distort the reconstruction of the surface soil heat flux signal. A correction methodology is introduced which provides practical application as well as insights into the estimation of surface soil heat flux and the closure of surface energy balance based on field measurements.

Force-restore technique for ground surface temperature and moisture content in a dry desert system.

The level of the surface temperature as well as surface moisture content is important for the turbulent transports of sensible and latent heat, respectively, but this level is also crucial for the survival of shrubs, plants, insects, and small animals in a desert environment. To estimate the surface conditions, the force-restore technique for surface temperature as well as surface moisture has been applied under extreme dry desert condition. A simple physical model was constructed in which the force-restore technique for surface moisture has been included. This model simulates the accumulated dew amounts as well as the early morning drying process. A measurement program carried out in the northwestern Negev desert of Israel (Nizzana) gathered data on daily surface temperature, daily dew deposition amounts, and micrometeorological conditions during the dew formation and drying process. Actual dew amounts were assessed using microlysimeters. Model simulations agreed well with the observations. It appeared also that the reduction of the vapor pressure in the soils pores under extremely dry soil conditions is essential for the dew process.