Tomasi, C. and Lupi, A. and Marani, S. and Vitale, V. (2003) Water vapour absorption effects on solar radiation in an Apennine valley from hygrometric measurements of precipitable water taken at various altitudes. Il nuovo cimento C, 26 (1). pp. 91-115. ISSN 1826-9885
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Abstract
Hygrometric ratio measurements were simultaneously taken on six autumn clear-sky days of 1981 and 1982 by employing four Volz sun-photometers and the FISBAT sun-photometer at five stations located at different altitudes along the western slope of the Leo Valley, in the Apennines (Italy). Due to the solar heating of ground, intense upslope breezes forming during the early morning caused the vertical transport of more humid air from the bottom of the valley toward the ridge of the mountain chain. Precise calibration curves of the hygrometric ratio were defined on the basis of criteria suggested by the atmospheric infrared hygrometry technique and using the calibration constants found through an accurate intercomparison procedure. Examining the sun-photometric measurements by means of these calibration curves, precipitable water was determined at all stations, with the frequency of one measurement every 15 minutes from the early morning to one hour after noon. Daily homogeneous time-patterns of precipitable water were defined at the various stations, showing that this quantity varies appreciably during the morning at all stations, sometimes presenting daily increases of more than 40% at the lower stations. Average values of absolute humidity were then determined within the four atmospheric layers defined by the station altitudes, finding that the convective transport of humid air along the valley slopes can produce important variations within the atmospheric layer below the 1.6 km height. For these moisture conditions of the atmosphere, calculations of the time-variations caused by water vapour absorption in the downwelling flux Φ1 of global solar radiation reaching the ground were made at the various stations, as well as of those in the upwelling flux Φ of solar radiation at the top-level of the atmosphere. The results indicate that: i) flux Φ1 can appreciably decrease due to water vapour absorption, by 10 to 20 W m −2 at the highest station of Mt. Cimone and by 70 to 80 W m −2 at the lowest station situated on the bottom of the Leo Valley, and ii) the changes caused by water vapour absorption in the upwelling flux Φ were estimated to range usually between about 5 W m−2 at the Mt. Cimone station and more than 25 W m−2 at the lowest station. In particular, as a consequence of the time-variations in both precipitable water and solar elevation angle, the change ΔΦ caused by water vapour in the instantaneous outgoing flux of solar radiation at noon was found to increase almost linearly as a function of precipitable water throughout the range from 0.8 to 1.8 g cm−2, with an average slope coefficient equal to 12.5 W m−2 per unit variation of precipitable water.
| Item Type: | Article |
|---|---|
| Uncontrolled Keywords: | Atmospheric optics ; Meteorology ; Water in the atmosphere (humidity, clouds, evaporation, precipitation) ; Solar radiation ; Climatology |
| Subjects: | 500 Scienze naturali e Matematica > 550 Scienze della Terra > 551.6 Climatologia e tempo atmosferico (Classificare qui i lo studio dei Cambiamenti climatici) |
| Depositing User: | Marina Spanti |
| Date Deposited: | 13 Mar 2020 09:55 |
| Last Modified: | 13 Mar 2020 09:55 |
| URI: | http://eprints.bice.rm.cnr.it/id/eprint/15102 |
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