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6.2 Evaluation of Alternative Spatial Models of Vapour Pressure in Canada

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6.2 Evaluation of Alternative Spatial Models of Vapour Pressure in Canada 6.2 EVALUATION OF ALTERNATIVE SPATIAL MODELS OF VAPOUR PRESSURE IN CANADA Daniel W. McKenney1∗, M.F. Hutchinson2, P. Papadopoll, D.T. Price3 1 Canadian Forest Service, Sault Ste. Marie, Ontario 2 Centre for Resource and Environmental Studies, The Australian National University, Canberra, Australia 3 Canadian Forest Service, Edmonton, Alberta 1 ABSTRACT 1) Water vapour pressure (the pressure exerted on any surface by gaseous water molecules); Spatial variation in measurements of atmospheric 2) Vapour pressure deficit (the difference between the humidity provide a basis for creating interpolated saturation water-vapour pressure and the actual estimates of humidity variables such as vapour water-vapour pressure); pressure. Here we compare several different thin plate 3) Relative humidity (the ratio of the actual water- spline models of 1961-90 monthly vapour pressure vapour pressure to the saturation water-vapour normals observed at Canadian climate stations pressure at dry-bulb temperature); including a model where observed vapour pressures 4) Absolute humidity, or vapour density (the mass were first adjusted to sea-level using standard concentration of water vapour per unit volume of barometric correction. Using a set of 30 stations air); withheld from the fitting procedure, we performed 5) Mixing ratio (the ratio of the masses of water vapour independent validations of each model. All models in to dry air, respectively); fact had good signals and would generally be 6) Specific humidity (the ratio of the masses of water considered as acceptable according to standard vapour to moist air, respectively); diagnostics. In general, withheld data could be 7) Dewpoint (the temperature to which the air must be predicted with mean errors of less than 10 Pa. The best cooled for the water vapour content to reach model was a square root transformation of the vapour saturation). pressure field and a trivariate spline with a spatially varying dependence on elevation as determined by the Saturation is a dynamic equilibrium state where data network. The maps generated as the final products water is constantly evaporating and condensing. The used all data points except for three stations identified intensity of this process is measured directly by vapour as potentially anomalous during the model-building pressure and indirectly by temperature. At saturation, phase. air holds the maximum amount of water vapour possible. Hence, at saturation, the dew point 2 INTRODUCTION temperature equals the air temperature. Further, the relative humidity is 100%, and the vapour pressure is Atmospheric vapour pressure, ea, is one of many equal to the saturation vapour pressure. climatic parameters of interest to meteorologists, Water vapour is generally introduced into the ecologists and agriculturists. Vapour pressure is bottom of the atmosphere by evaporation from a moist defined as the pressure exerted on any surface by surface. It becomes vertically distributed by turbulent gaseous water molecules (Linacre et al., 1977). Water mixing processes including free convection of warm vapour obeys Dalton’s Law, which states that the total surface air and forced advection due to wind, as well as pressure of a mixture of gases equals the sum of the synoptic-scale circulation of air masses (see Stull, 1988 partial pressures exerted by each constituent gas (List for details). 1949). Hence, vapour pressure is the partial pressure The unit measurement of vapour pressure is the exerted by water vapour in a mixture of other kilopascal: one kilopascal is equal to 10 millibars, atmospheric gases (which together make up “dry air”) 0.2953 inches of mercury, or 0.145 pounds per square to yield the total barometric pressure. inch. Accurate measurement of atmospheric humidity is Thus vapour pressure is the pressure exerted by as much art as science (Weast, 1973), with the moisture in the air, and increases with both atmospheric preferred instrument being a high quality aspirated temperature and moisture content. It is related to psychrometer, although for most practical purposes relative humidity in that the latter is the ratio of the dew-point hygrometers are considered more reliable. actual vapour pressure to saturation vapour pressure at Vapour pressure deficit (VPD), defined as the same temperature. Water vapour in the air can be e*(Ta) – ea, where e*(Ta) denotes saturation vapour described directly or indirectly in several ways pressure at air temperature Ta, is another measure of including: atmospheric humidity often used to model evaporation of water from moist surfaces, because it approximates ∗ Corresponding author address: Daniel McKenney, Canadian Forest Service, Sault Ste. Marie, Ontario [email protected] 2 with the Clausius-Clapyron and Goff-Gratch equations while being computationally much faster than the latter. Our objective is to describe the development of a national monthly mean vapour pressure (VP) model for Canada and hence provide some baseline vapour # pressure reference data for Canada away from the # # R # stations for which it is calculated. This VP field is # # R # R # available as one of several climate fields available from R # R # # R R R R R R # R the Canadian Forest Service, Landscape Analysis and # # R # R R RR R # R R # R R RR R R R R R R # RR R R R R R R R R R# Applications Section – Sault Ste. Marie RR R R RR # R R # R R #R R #R R RR RRR R R R R# R R R R#R RRR R # R R R R R R RR R#R R R RR R R R RR R R 1030426 R R R R RR R # R (http://www.glfc.cfs.nrcan.gc.ca/landscape/climate_mod 1018610 R R R R R R RR R R R RRRR # R R RR R R R RR RR R # 3 suspect stns # RR R #R # R R # R 8206240 els_e.html). It was derived using the ANUSPLIN thin R 146 model stns # 30 withheld stns plate smoothing spline algorithms of Prof. Michael Hutchinson, Centre for Resource and Environmental Studies at The Australian National University (http://cres.anu.edu.au/) and station data from Environment Canada’s Climate Data Centre in Figure 1. Geographic locations of 176 vapour pressure Downsview, Ontario. stations: 146 stations used for surface fitting and 30 stations withheld for validation. All but three suspect stations were used in the final model. 3 DATA AND METHODS Mean hourly data by month from 176 stations were available from Meteorological Service of Canada’s the water potential gradient between saturated air at or Monthly Climate data base. Figure 1 shows the just above a moist surface and the vapour pressure of locations of the stations including those withheld from the “free air”. The VPD multiplied by an appropriate the initial model-building phase for validation purposes. transfer coefficient then allows the evaporative flux to The data are a synthesis of hourly vapor pressure be estimated. In practice, however, it is often important values derived from dew point temperature through to calculate this gradient more precisely by replacing Ta conversion tables. For air temperatures colder than – with the surface temperature. In the case of plants, 37o C where dew-point temperatures were not transpiration is a process that evaporates water available, a value of 10 Pascal (Pa) was assumed. The molecules from a moist cell surface (at leaf monthly values were averaged for the 1961 to 1990 temperature) and transfers them to the free air, period using any station with five or more years of although the pathway may be restricted by structures records, regardless of missing year-month totals or such as stomata embedded in the leaf surface. Hence counts. The values range from 0.07 kPa in winter atmospheric vapour pressure is often used as an (northern Arctic) to 1.78 kPa in summer (southern important driving variable for many models of plant B.C.). transpiration and canopy evapotranspiration. Table 1 illustrates, from 5 selected stations, the Here, we report the development of a high seasonal changes in vapor pressure across Canada, resolution spatial vapour pressure data set suitable for which are very much influenced by air streams from driving spatialized process models applied to Canada. three distinct sources: the Pacific, Atlantic and Arctic Rasmusson (1967) provides discussion and maps of Oceans. Over land, atmospheric humidity is invariably the spatial nature of water vapour in North America. higher in summer, because the warmer air can hold Castellvi et al. (1996, 1997) describes some methods to more moisture and higher surface temperatures develop regional vapour pressure deficit models. increase evaporation and transpiration from lakes, Castellvi et al. developed models for estimating daily rivers and vegetation. Hence, for much of the country, vapour pressure deficit at weather stations in which the humidity of air near the surface is influenced by a only temperature and precipitation are measured. There combination of synoptic processes and seasonal are a range of expressions for estimating saturation changes in vegetation, as well as proximity to inland vapour pressure as a function of temperature at water bodies. standard pressure. They include the Goff-Gratch (1946) In winter, atmospheric humidity is highest on the formulations as presented in the Smithsonian Tables two coasts, particularly the Pacific, with values of 0.67 (List, 1949) or one of the many empirical equations kPa in Vancouver BC and 0.40 kPa in Halifax. In the which have appeared in the literature (e.g., Tetens mid-continental regions, the low temperatures, often 1930; Murray, 1967; Richards, 1971; Tabata, 1973; due to outflows of frigid Arctic keep the interior humidity Campbell 1977; Lowe, 1977; Rasmussen, 1978), all of very low, with values of 0.16 and 0.18 kPa in Winnipeg which use the Goff-Gratch formulation as a standard for and Saskatoon respectively. Humidity increases rapidly comparison. The Goff-Gratch approach is generally in spring, and high throughout the country recording considered the most accurate, suitable for generating values of 1.44 kPa and 1.66 kPa on the east and west tables, but it is computationally inefficient when used for coasts respectively and 1.57 kPa in Portage La Prairie.
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