Massimiliano de Franceschi1, Dino Zardi1, Lorenzo Giovannini1*
1 Gruppo di Fisica dell’Atmosfera, Dipartimento di Ingegneria Civile, Ambientale e Meccanica, Università di Trento, via Mesiano 77, 38123 Trento
Abstract
The project Ge.Pri. started in 2002 in Trentino, aiming at better characterizing the nighttime cooling over complex terrain. The experimental setup allowed the identification of different processes, such as radiative cooling and advection of cold air, which characterize the nighttime behaviour of air temperature close to the ground. In particular, a 6 m high thin mast was set up to measure temperature profiles in the surface layer, while a radiation measurement system was installed for analysing the radiation balance of solar and infrared radiation. Moreover, turbulent heat fluxes were measured with a three-axis sonic anemometer. The contribution focuses in particular on the description of an event in which great damages were caused by the activation of protection sprinklers in a night characterized by the advection of cold and dry air masses from north.
Parole chiave
Frost phenomena, radiative balance, surface layer, cold air advection, Adige Valley
Keywords
Gelate, bilancio radiativo, strato superficiale, avvezione di aria fredda, val d’Adige
Introduction
A relevant damage factor for crops is frost, which can cause severe destruction of fruit, vegetables and plants. The damage is characterized by explosive ice formation inside the cells, which ruptures cell membranes and kills the cells, and hence the tissue or perhaps the entire plant. In general, younger tissues have a greater water content, and tend to freeze at a higher temperature, hence shoot tips, emerging leaves and developing inflorescences tend to be most sensitive to frost. Late frost is increased by the enhancement of low temperature episodes in spring and the concomitant early-blooming consequent to higher winter temperatures induced by the earth global warming (Rosenzweig and Hillel, 1993; Ruttan, 1994). On the basis of the physical processes which determine it, frost events are usually distinguished in two main kinds: radiative and advective. Radiation frost is produced by heat loss for pure radiation during nighttime: starting at sunset, the ground surface radiates and its temperature decreases till the sun rises again the morning after. Under still air conditions, no turbulent exchange occurs and the air above the surface irradiates and loses heat as well, though less rapidly. As a consequence, air layers closer to the ground become cooler than those at higher levels. This produces nocturnal ground- based inversions, which are stronger when the air is still and the sky is clear. As opposed to radiation frost, advection frost is a non-local phenomenon produced by the advection of cooler air from local- and/or larger-scale winds.
The project Ge.Pri. (Gelate Primaverili, Spring Frosts) started in 2002 in Trentino, aiming at better characterizing the night-time cooling over complex terrain, by means of the analysis of data from targeted field campaigns, and at implementing numerical models to predict the minimum
temperatures. This contribution focuses on the description of an event in which great damages were caused by the activation of protection sprinklers in a night characterized by the advection of cold and dry air masses from north.
Materials and methods
The field campaign was performed in an apple orchard in the Adige Valley, in the eastern Italian Alps, at the Agricultural Research Area "Maso delle Part", in the River Adige Valley, near Mezzolombardo, (46.22° N, 11.08° E), at an elevation of 203 m a.s.l. The average height of the surrounding crests rise approximately 1200 m above the valley floor on the western side and 600 m on the eastern side. At the experimental site, an automated weather station (AWS) is operated since 1999 in support of agricultural practices. A second AWS was located in the middle of a cut-block, with sensors of the same type, except for the addition of a wind-vane. Moreover the averaging period was setup to 10 min instead of 1 h as usually adopted for standard AWS observations in the area. In addition, a 6 m high thin mast with a series of thermocouples at 6 height levels was set up to measure temperature profiles in the surface layer from 0.3 m to 5 m above the ground. A radiation measurement system was installed 2 m above the ground, between two rows of trees approximately 1.5 m apart, composed of a CNR1 radiometer (Kipp & Zonen), intended for separately analyzing the radiation balance of solar and long-wave radiation. A soil heat flux sensor (L.S.I. Lastem DPE260) was installed 1 cm below the ground surface near the radiometer, so as to complete the surface energy balance. Turbulent heat fluxes were measured by the eddy correlation technique at a height of 6 m above the ground, 3.5 m above the canopy top (hence 4 m above the displacement height estimated in 2 m). The
three wind speed component and sonic temperature fluctuations were measured with a three-axis sonic anemometer (Gill Solent Mod. HS Research). Water vapor and carbon dioxide fluctuations were measured with an infrared open path gas analyzer (LiCor LI-7500). Data were sampled at 20 Hz. The coordinate system of the sonic anemometer was locally aligned to the streamlines following Kaimal and Finnigan (1994), but limiting the procedure to a double rotation only.
Results and Discussion
From all the meteorological data collected during the experiment, one night (between 4 and 5 March 2004) unperturbed by clouds and characterized by high pressure conditions have been chosen for a more detailed description of the cooling processes which could lead to potentially dangerous late-frost events. This night is representative of a more general and frequent behaviour under same general meteorological conditions, hence the choice of limiting the analysis to this night only is not restrictive.
The local sunset, identified not only by the classical astronomical relationships, but also as the time when the direct downward shortwave equals zero, occurred about at 1530 LST (UTC+1), even if the diffused shortwave radiation remained until 1750 LST.
The observed temperature decrease can be divided into 4 steps. The first step lasts from 1600 LST to 1840 LST and is characterized by the persistence of a rather strong up- valley wind (around 3 m s-1). Hence the turbulent exchanges are quite high, but the two heat fluxes present a very different behaviour, with a positive latent heat flux and a negative sensible heat flux (it reaches -35 W m-2 at 1800 LST, a short time after the sunset). The up-valley wind ends at 1840 LST, when the intensity is less than 0.5 m s-1, and consequently the turbulent fluxes become almost zero. During this first period the air temperatures show a large reduction, of about 2.5°C per hour, the same at every height, thus inducing a temperature decrease from 14°C to 6°C. In this phase the cooling process displays a mixed behaviour with both radiative and advective effects which are difficult to separate, and it is also relatively fast because of the large turbulent exchange due to the high wind intensity.
The second step lasts from 1840 LST until 2050 LST and is characterized by a weak wind intensity from 250°N, from the mountain slopes to the West of the site. The temperature decrease is slower in comparison to the previous phase, and the air temperature stratification becomes stronger, with the lower height temperatures that decrease much more rapidly than the higher one: the temperature measured at 0.3 m above the ground is about 5°C lower than the one at 5 m.
The third step lasts only 30 minutes (from 2100 LST to 2130 LST), is characterized by a very fast decrease of the temperature at every height and is related to the arrival of cold air advected by a weak katabatic flow from the North. In this phase, the air temperature at 5 m decreases of about 3°C, while that at 0.3 m of about 1.5°C: the stratification is
still present but its intensity is reduced to 4°C between the uppermost and lowermost levels. At 2130 LST begins the fourth step (the last and the longest) that ends at 0650 LST the day after. During this period the temperatures decrease of about 4°C in 10 hours at every height, and the stratification intensity remains approximately constant (3°C). The wind is very weak, less than 0.5 m s-1, and the turbulent heat fluxes are negligible. The cooling process is exclusively radiative and long-wave net radiation balances the ground heat flux.
Fig.1 - Temperature a diverse altezze dal suolo nella notte tra il 4 e il 5 marzo 2004.
Fig.1- Temperatures at different heights above ground level in the night between 4 and 5 March 2004.
Conclusions
The data collected at a rather complex agricultural site in the River Adige Valley (Trentino, Italy) during the night between 4 and 5 March 2004 were analysed and reported here. The attention devoted to the nocturnal cooling processes under fair weather conditions showed the occurrence of a rather clear sequence of 4 distinct phases which substantially alternate advective and radiative effects. The analysis highlighted also the presence of a strong ground-based thermal inversion in the lowest atmospheric layers, leading to potentially dangerous situations for crops.
References
Kaimal JC, Finnigan JJ., 1994. Atmospheric boundary- layer flows: Their structure and measurement. Oxford University Press, New York, 289 pp.
Rosenzweig, C., Hillel, D., 1993. Agriculture in a greenhouse world: potential consequences of climate change. Nat. Geogr. Res. Explor. 9, 208-221.
Ruttan, V., 1994. Agriculture, environment, and health: sustainable development in the 21st century. University of Minnesota Press, Minneapolis, 416 pp.