Weather in wheat : An essay in micro‐meteorology

Abstract

AbstractData for three summers (1955‐1957) are presented, obtained from continuous recorders responding to temperature, humidity and wind in and above the wheat crop at levels up to about 2 m. Section 1 gives a brief account of the instruments (see Long, Q.J. 83, p. 202 for details). § 2 a description of the site, 70 × 70 m, which may have been too small for full development of profiles. § 3 (from 1955) shows that planting density affects the daily cycles of temperature and humidity in the crop, a thin crop having a lower average relative humidity and a shorter period of saturation on a dew night. Such a night is characterized by a humidity lapse between ground and canopy, and a humidity inversion above the canopy. On the exceptional occasion illustrated, both the up flux and the down flux may have reached a maximum rate of about 3 mg ground cm−2 hr−1. An adventitious air‐temperature ripple during a calm night is exploited to derive rough estimates of transport constant in a dense crop : near the ground it approached the molecular thermal diffusivity for air. § 4 (from 1956) displays anomalies in temperature and humidity profiles that can arise from interpolating readings within the 400 sec cycle of the recorders. It also gives soil temperature records from which estimates of soil thermal diffusivity, and soil heat flux, are made, indicating that on the first sunny June day after rain there may be a net heat flux into the ground of about 30 cal cm−2 : the equivalent of 0·5 mm of evaporation. From an energy balance further rough estimates of heat transport constant within the crop are found to be of order 2 to 20 cm2 sec−1 depending on height and external wind speed. The anomalies in profiles are attributed partly to periodicities in the air parameters almost identical with that of the recording instruments, and partly (§ 5, from 1957) to long‐persisting differences in temperature and humidity at constant level across the site. Within the crop an average temperature difference of 0·8°C may persist for four hours at positions 8 m apart (and a vapour‐pressure difference of 1·0 mb), and these ‘hot’ and ‘damp’ spots ‐ not always coincident ‐ move slowly during the day. The differences are smaller above the crop where mixing is more thorough. These differences swamp any that might be due to advective effects, and raise doubts about the meaning of ‘a profile’ over a tall crop.§ 6, introduces an analytical Part II which uses the data from a period of eight days (12–19 June 1957). of fine weather after rain, discusses the generalized profile equation (reducing to the standard logarithmic profile when α = 0) : measurements of temperature and humidity are not accurate enough to determine α for their profiles, and the swaying of the crop necessitates use of long‐period averages of wind speed before α can be determined for wind profiles. Working on 4‐hr periods (00‐04, 04‐08 and so on) it is found (§ 7) that profiles above the crop can be fitted either with α = 0, or a value of α chosen from the Deacon curve of 1 – α against Richardson number. Values for a crop height of about 60 cm, with heat transfer (Qs) and evaporation rates (E) from later sections, are: magnified image The discrepancy in u*/k during the first period corresponds to a 12‐fold estimated shearing stress.Wind profiles in the crop have a completely different shape in calm and windy weather, in the sense that near the ground the absolute wind speed tends to be constant: the crop behaves as though self‐sealing.