Human activities are highly impacted by the changes and variations in the physical properties of the layer of air situated near the surface of the Earth. Those changes are of interest at daily, seasonal and climate scales. Table 3.1 describes the seven Essential Climate Variables identified by the Global Climate Observing System (GCOS) for worldwide monitoring, in the category of Surface Air Variables.
Table 3.1 Essential Climate Variables identified by GCOS for worldwide monitoring, in the category of Surface Air Variables (Source: https://www.climate.gov/maps-data/primer/air-atmospheric-climate-variables )
| No. | Name | Definition and description of basic measurements at meteorological and climate stations |
|---|---|---|
| 1 | Near-surface temperature | Near surface air temperature is the temperature of the air around us, generally measured at a height of around two metres above the surface (also named 2 m temperature), reported in degrees Celsius (°C). Measurements are made using thermometers, shielded from direct solar energy. The most common type of thermometer is the liquid-in-glass thermometer. More precise thermometers measure air temperature by checking how much electricity can pass through a sample of pure metal. |
| 2 | Total precipitation | Total precipitation is water in liquid or solid form that falls to Earth's surface from clouds. It can be in the form of drizzle, snow, ice, freezing rain, or hail. Rain gauges are the most common instrument used to measure rainfall (liquid precipitation). A rain gauge is an open-at-the-top container that is calibrated to measure the depth of liquid caught, which is reported in depth units (volume/unit area) of millimetres (mm), metres (m) or inches (in). |
| 3 | Surface wind speed and direction | Wind is air in motion relative to the Earth's surface. It is a vector quantity, meaning it is described in terms of both speed and direction of motion. Anemometers are used to measure wind speed, which is reported in metres per second (ms-1). Wind vanes and windsocks measure wind direction. Wind directions refer to where the wind is coming from; for example, a north wind is coming from the north and blowing towards the south. Winds are most commonly described using the eastward and the northward components (the horizontal component of wind moving towards the east and towards north). |
| 4 | Water vapour | Water vapour is water in the atmosphere in its vapour (gaseous) form. Half of the water vapour in the atmosphere is found within two kilometres of the Earth's surface. Absolute humidity reported in grams of water vapour per kilogram of air (g kg-1) or specific humidity, in grams of water vapour per kilogram of dry air (g kg-1), represent common measures of the amount of water vapour in air. Relative humidity reported as percent (%) of water vapour pressure compared to a saturated (condensed) vapour pressure, tells how much water vapour is in the air relative to the amount it has the potential to hold at a given temperature. The instrument used to measure water vapour content in the air is called a hygrometer. The simplest type of hygrometer is made from human hair, which swells and lengthens as it absorbs water vapour from the air. |
| 5 | Atmospheric pressure | Atmospheric pressure is the weight-per-unit area of the column of air above it. As gas molecules are always moving in every direction, air pressure is the same in all directions. Barometers measure air pressure. The most common type of barometer is a sealed flexible container of air. When the air pressure outside the container changes, the container responds by contracting or expanding. This change is registered by a needle or digital readout. These values are expressed in millibars (mb), which is a unit of pressure commonly used in aviation and meteorology that is equal to 1 hectoPascal (hPa) or 100 Pascal (Pa), where one Pa is one newton per square metre (Nm-2). Standard sea level pressure is 1013.25 mb, or a nominal value of about thousand millibars. Changes in atmospheric pressure can indicate a change in weather. |
| 6 | Surface longwave radiation | Surface longwave radiation is defined as the flux density of radiation emitted by the gases, aerosols and clouds of the atmosphere to the Earth's surface and it is measured in watts per square meter (W/m²). The long wavelength radiation (or the infrared-range radiation) returned to the surface is mainly measured by a pyrgeometer. |
| 7 | Surface shortwave radiation | Surface shortwave radiation is defined as the flux density of the solar radiation at the Earth's surface. The unit of measurement for radiation is that of irradiance, watts per metre squared (Wm-2). On the ground, an instrument called a solar pyranometer measures the amount of incoming solar radiation that reaches Earth. |
Although all variables in the table are essential for climate analysis, the four first variables in the table (2 m temperature, total precipitation, surface wind speed and direction, and surface water vapour) are considered as essential for climate change adaptation studies at local scales in northern Canada, and constitute the focus of this chapter. For each of the four variables we will present first the historical datasets estimated from observations. Descriptions of the modelled datasets (historical and future projections) are presented in Chapter 4.
The previous table also presents the classical instruments used in the measurement of those variables at meteorological and climate stations. As mentioned in Section 2.1, those historical records are local, the period that they cover varies with the location and may include missing values over the period of record. Station measurements are unevenly distributed over the land and their total number has changed over time with an important decline in the number of manual stations after 1990 (Mekis et al., 2018). For the Canadian North, most of the meteorological and climate records started in 1950’s, the number of stations is much smaller than in the southern Canada and unevenly distributed (see Figure 2.1 that shows the locations of stations from several surface networks in Canada, as of September 2016).
Characteristics of station data, gridded observational data, and reanalysis data were discussed in Section 2.1. In addition, for meteorological variables, estimates from satellites have recently become available for some of the meteorological variables over the recent past. Some hybrid datasets, combining station measurements with reanalyses or satellite data, also exist. Consequently, the historical observation-based sections for meteorological variables will present the following type of datasets:
a) Station data (presented in blue)
b) Gridded observations (presented in yellow)
c) Reanalysis (presented in green)
d) Re-gridded reanalysis with or without bias corrections (presented in orange)
e) Satellite data (presented in pink)
f) Hybrid data (presented in violet)
Historical meteorological variables are necessary as a first step in Climate Change Vulnerability and Risk Assessments to define the baseline conditions that describe the climate over the historical period. Historical data is also used to evaluate climate models skill over the historical period, to bias correct climate projections. Some of the meteorological variables are also used as input for local permafrost models and regional land surface models.
The following explains some considerations that were taken into account when selecting and documenting the existing meteorological datasets for the historical period.
- Only datasets covering the Canadian North in whole or in part are considered.
- A 30-year period of record is indicated to meet international best practice for climate analyses in order to allow for a sufficient period to identify important, human-caused changes and trends, and for comparison and validation of climate model simulations. To the datasets that comply with this criterion, we have added some datasets of strategic importance. For example, some new datasets were developed recently, and while presently there are not 30 years of data available for that specific dataset, there are plans to extend the data to a longer period, or for stations, they are considered as important because no other records are available for that specific region and they are valuable in the development of gridded products.
- Regarding the use of gridded datasets, the climatological and monthly means of various variables have generally large-scale patterns but are strongly controlled by local effects. This is especially seen for precipitation field and wind, and in a shorter measure for humidity and temperature. However, the local effects for all variables are stronger on fields with smaller time steps, as the daily means and hourly values. Some of the factors that produce local effects are the proximity to large water bodies and the topography. Consequently, the gridded datasets should have a good spatial resolution in order to capture the local characteristics. For this reason, in this report only those gridded datasets with a nominal spatial resolution finer than about 0.6°, covering northern Canada or parts of northern Canada are considered. Some of the reanalyses or gridded datasets have evolved in time and several versions are available for public use. Only the most recent versions are presented here as those are considered to use the most advance models and technic, therefore are susceptible to provide the best results.
Each of the four variables are presented separately. The section is ending with supplementary datasets from stations that are not integrated into the Meteorological Service of Canada (MSC) network and can be of interest for local applications.