Martti Heikinheimo

Meteorological data for agricultural applications

Abstract Many agricultural applications are based on meteorological data. For example estimation of surface moisture, crop yield or forecasting of outbreak of crop diseases or insect pests is largely dependent on meteorological conditions. To ensure that there continuously is a (in spite of changes in meteorological observing network) spatially and temporally adequate amount of meteorological data available a system was taken into use in Finland, where parameters measured at observation stations are first interpolated onto a 10 km ×10 km grid. The data is stored as grid square values in a database and calculations are made using this gridded data. The spatial interpolation method currently used at the Finnish Meteorological Institute is known as kriging. The interpolated data has successfully been used in several agricultural applications.

VARIABILITY OF THE STABLE AND UNSTABLE ATMOSPHERIC BOUNDARY-LAYER HEIGHT AND ITS SCALES OVER A BOREAL FOREST

Radiosondes releases during the NOPEX-WINTEX experiment carried out in late winter in Northern Finland were analysed for the determination of the height h of the atmospheric boundary layer. We investigate various possible scaling approaches, based on length scales using micrometeorological turbulence surface measurements and the background atmospheric stratification above h. Under stable conditions, the three previously observed turbulence regimes delineated by values of z/L (L is the Obukhov length) appears as a blueprint for understanding the departures found for the suitability of the Ekman scaling based on LE = u★/f (u★ is the friction velocity and f the Coriolis parameter). The length scale LN = u★/N (where N is the Brunt–Väisälä frequency) appears to be a useful scale under most stable conditions, especially in association with L. Under unstable conditions, shear production of turbulence is still significant, so that the three scales L, LN and LE are again relevant and the dimensionless ratios μN = LN/L and LN/LE = N/f describe well the WINTEX data. Furthermore, in the classical scaling framework, the unstable domain may also be divided into three regimes as reflected by the dependence ofu★/f on instability (z/L).

Momentum and heat fluxes over lakes Tamnaren and Raksjo ¨ determined by the bulk-aerodynamic and eddy-correlation methods

Micrometeorological measurements made concurrently over two boreal lakes, Lake Tamnaren (surface area 37 km 2 , depth 2 m) and Lake Raksjo ¨ (1.5 km 2 , 4 m), during the NOPEX observational campaigns provided long-term latent and sensible heat fluxes determined with the bulk aerodynamic method. The turbulence transfer coefficients were verified with short-term eddy correlation runs over Lake Tamnaren. The drag coefficient and the Stanton number attained a stability dependence close to the Businger-Dyer form. During weak or moderate wind conditions, the latent heat flux determined from eddy correlation was close to that calculated with the bulk aerodynamic method using typical values of the Dalton number. However, the independent verification of the Dalton number under conditions of moderate or strong wind suffered from the poor performance of the fast-response hygrometer due to movement of the float. Measurements of the surface roughness suggested a value of the Charnock constant near 0.02. On average, the stratification of the surface layer over both lakes was unstable, but stable conditions occurred almost daily due to advection of warm air from adjacent land areas. High evaporation rates occurred even during stable stratification. The variation of the latent heat flux on an hourly or daily basis was strongly correlated with wind speed. This was also demonstrated by the higher rates of evaporation on the larger Lake Tamnaren compared to the sheltered Lake Raksjoduring strong wind conditions. Towards the end of the summer of 1995, the lake surface temperature attained somewhat higher values on Lake Raksjo ¨, attributable to its larger depth. This also resulted in higher daily average sensible and latent heat fluxes compared to Lake Tamnaren in August-September. Thus, in the long-term, the observed daily deviations between the lakes nearly compensated each other. # 1999 Elsevier Science B.V. All rights reserved.

The Influence of Climate Warming on Soil Frost on Snow-Free Surfaces in Finland

The influence of the predicted climate warming on soil frost conditions in Finland was studied using a climate scenario based on a Hadley Centre (U.K.) global ocean-atmosphere general circulation model (HadCM2) run. HadCM2 results were dynamically downscaled to the regional level using the regional climate model at the Rossby Centre (Sweden). The future period this study focuses on is the end of the 21st century. The study was limited to ground surface conditions in which snow has been removed. The predicted air temperature rise was interpreted in terms of changes in soil frost conditions using an empirical dependence that was found between measured soil frost depths and the sum of daily mean air temperatures calculated from the beginning of the freezing period. On average the annual maximum soil frost depth will decrease in southern and central Finland from the present approx. 100–150 cm by about 50 cm. In northern Finland the change will be from depths of about 200–300 cm to about 100–200 cm depending on station. The annual maximum soil frost depth in the future would thus be about the same in northern Finland as it is in the current climate in southern Finland. In southern Finland after about 100 years the ground will seldom be frozen in December and even in January there will be no soil frost in about half of the years. In Central and northern Finland the probability of completely unfrozen ground in December–March is very small, even in the future.

Comparison of latent and sensible heat fluxes over boreal lakes with concurrent fluxes over a forest: implications for regional averaging

The seasonal and diurnal variation of latent, and sensible, heat fluxes above two boreal-zone lakes and an adjacent coniferous forest was studied using data from the NOPEX field campaigns in 1994 and 1995. Heat fluxes over the lakes were estimated with the bulk aerodynamic method and those over the forest with the eddy-correlation method. The night-time latent heat flux, while non-existent from the forest, was significant from the lakes during all summer months. The day-time flux from the lakes peaked, on average, in the afternoon, while the maximum flux from the forest occurred around noon. The differences in the latent heat supply between the two surface types varied within(60‐80) W m ˇ2 , depending on the month and time of day. The monthly mean latent heat flux from the forest was higher than that from the lakes in May, in June and July the fluxes were about equal; and in August‐September, the lake values exceeded those of the forest. The sensible heat flux reached its maximum values over the forest near noon, but over the lakes during the early morning. An excess supply of heat over the forest of up to 200 W m ˇ2 was found in comparison with the lakes during the mid-day hours. From May to August, the monthly mean sensible heat flux was higher from the forest than from the lakes. The difference in the heat exchange of lakes and forest was also estimated based from airborne measurements made from an aircraft flying at a height of 100 m. For flight legs consisting of a 15% portion over the lake, the leg-averaged latent heat flux was, on average, 2% lower and the sensible heat flux 9% lower, compared to a flight leg over fully-forested terrain. # 1999 Elsevier Science B.V. All rights reserved.

Test of a modified Shuttleworth-Wallace estimate of boreal forest evaporation

A modified version of the two-layer Shuttleworth‐Wallace approach was used to simulate evaporation from a mixed 50‐ 100 years old boreal forest stand in central Sweden. It was tested against eddy correlation flux measurements from the Northern hemisphere climate Processes land-surface Experiment (NOPEX). The soil regulation on canopy resistance was taken into account when estimating transpiration. The generality of the model performance was attained by using model parameters that represented typical boreal forest stand and not the specific stands at the site. The analysis was done for two growing seasons, 1994 and 1995 having quite different weather conditions. Despite the rough parameterisation, the model performance was good. Problems regarding representativity of the measured data used for the model validation are discussed. It was concluded that the model could accurately simulate evaporation components on a seasonal time scale and is suitable to be incorporated as a submodel in larger forest ecosystem models. # 1999 Elsevier Science B.V. All rights reserved.

Scale aggregation — comparison of flux estimates from NOPEX

Abstract The NOPEX two concentrated field efforts (CFEs) (June 1994 and April–July 1995) provide high quality data sets for the Boreal environment. The analysis of these data with traditional meteorological and hydrological approaches allow estimations of fluxes of latent and sensible heat, but these flux estimates are not directly comparable due to differences in temporal and spatial scales. The challenge here has been to overcome these difficulties so that the different estimates can be critically compared and evaluated in a systematical way. Five different approaches for the estimation of the regional flux of sensible and/or latent heat over the NOPEX area have been evaluated: (1) Direct aggregation — mixed layer evolution method, (2) Weighted averages of (a) aircraft measurements in the boundary layer and of (b) mast measurements, (3) Numerical models (a) ECOMAG — a distributed hydrological model and (b) MIUU — a mesoscale meteorological model. In general, good agreement was found between the regional estimates of the sensible heat flux, based on the mixed layer evolution method and land use weighted mast measurements. The aircraft measurements were found to be systematically smaller than the land use weighted mast estimates. For the latent heat flux good agreement was found between the regional latent heat flux derived from airplane measurements, land use weighted mast estimates, and the two mesoscale numerical models.

Estimation of Surface Solar Global Radiation from NOAA AVHRR Data in High Latitudes

Abstract A physical method for estimating the instantaneous global irradiance and daily cumulative insolation based on Advanced Very High Resolution Radiometer data was developed and tested at high latitudes in a boreal subarctic region. The satellite estimates were compared with ground-based pyranometer measurements at six stations in Finland. From the comparison of instantaneous satellite estimates with 15-min average irradiances measured by pyranometers, a high correlation coefficient (0.97 in July 1996 and 0.99 in March 1997) between these estimates was obtained under clear-sky conditions. A standard error of 8% and a zero value of bias were obtained in both months. Under cloudy conditions the correlation coefficient in July 1996 was in the range of 0.79–0.83; in March 1997 it ranged from 0.89 to 0.96. The standard error in cloudy cases varied from 27% to 39% in July 1996 and from 17% to 33% in March 1997. For daily insolation estimates, the correlation coefficient had an average value of 0.95. The st...

Latent heat flux from small sheltered lakes

The dependency of the latent heat flux on the over-water fetch on lakes surrounded by tall, dense forest was studied by making use of measurements made on two different-sized lakes. The measurements were made during the NOPEX (Northern Hemisphere Climate-Processes Land Surface Experiment) field campaign. It was found that, in the case of a typical Scandinavian lake with a size of less than 10 km2, the latent heat flux will increase as a function of over-water fetch due to the increase of wind speed and in spite of the increased air humidity. This also has implications on area-averaged fluxes: when two lakes having similar shorelines, lake water temperatures and solar radiation conditions are compared, then the evaporation per unit area is smaller from the smaller lake. When the lakes are large, with fetches of several kilometres, then the significance of sheltering is small. If point measurements are used for the estimation of area-averaged latent heat fluxes from lakes with short fetches and forested shorelines then the distance of the measuring site from the shoreline should be taken into account, otherwise errors of tens of percent may occur.

The Finnish Forest Fire Index Calculation System

Since the summer of 1996, the forest fire index calculation in Finland has been based on surface moisture estimation. The calculation is done on a 10×10 km grid. Air temperature, air humidity, wind speed, solar radiation and precipitation measurements made at observing stations are interpolated onto the grid with an objective interpolation method known as ‘krig-ing’. Potential evaporation is calculated for each grid-square based on the interpolated meteorological data using the Penman-Monteith equation. The moisture of a 60 mm thick surface layer is estimated using potential evaporation and precipitation data and, finally, the surface moisture is scaled to forest-fire index values (1–6) which are distributed to the fire authorities through the Internet. In the future, the inclusion of weather radar and satellite information in the spatial analysing system will further improve spatial analyses of precipitation and solar radiation.