ERA5 is the fifth generation global atmospheric reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) , providing a consistent reconstruction of weather and climate over the past eight decades. The dataset is available from 1940 onwards and replaces the earlier ERA Interim reanalysis.
In this report, two ERA5 datasets are used. The first consists of single level reanalysis variables obtained from the Climate Data Store ERA5 Single Level . From this dataset, we use the boundary layer height (blh) and surface pressure (sp), which are defined in the ERA5 documentation as follows:
Boundary layer height - blh () - This parameter is the depth of air next to the Earth's surface which is most affected by the resistance to the transfer of momentum, heat or moisture across the surface. The boundary layer height can be as low as a few tens of metres, such as in cooling air at night, or as high as several kilometres over the desert in the middle of a hot sunny day. When the boundary layer height is low, higher concentrations of pollutants (emitted from the Earth's surface) can develop. The boundary layer height calculation is based on the bulk Richardson number (a measure of the atmospheric conditions) following the conclusions of a 2012 review.
Surface Pressure - sp () - This parameter is the pressure (force per unit area) of the atmosphere at the surface of land, sea and inland water. It is a measure of the weight of all the air in a column vertically above a point on the Earth's surface. Surface pressure is often used in combination with temperature to calculate air density. The strong variation of pressure with altitude makes it difficult to see the low and high pressure weather systems over mountainous areas, so mean sea level pressure, rather than surface pressure, is normally used for this purpose. The units of this parameter are Pascals (Pa). Surface pressure is often measured in hPa and sometimes is presented in the old units of millibars, mb (1 hPa = 1 mb= 100 Pa).
The second dataset consists of ERA5 variables on pressure levels, obtained from the Climate Data Store ERA5 Pressure Levels . From this dataset, we use geopotential (z) and the zonal (u) and meridional (v) wind components, which are defined in the ERA5 documentation as follows:
Geopotential - z () - This parameter is the gravitational potential energy of a unit mass, at a particular location, relative to mean sea level. It is also the amount of work that would have to be done, against the force of gravity, to lift a unit mass to that location from mean sea level. The geopotential height can be calculated by dividing the geopotential by the Earth's gravitational acceleration, g (=9.80665 m s-2). The geopotential height plays an important role in synoptic meteorology (analysis of weather patterns). Charts of geopotential height plotted at constant pressure levels (e.g., 300, 500 or 850 hPa) can be used to identify weather systems such as cyclones, anticyclones, troughs and ridges. At the surface of the Earth, this parameter shows the variations in geopotential (height) of the surface and is often referred to as the orography.
U component of wind - u () - This parameter is the eastward component of the wind. It is the horizontal speed of air moving towards the east. A negative sign indicates air moving towards the west. This parameter can be combined with the V component of wind to give the speed and direction of the horizontal wind.
V wind speed - v () - This parameter is the northward component of the wind. It is the horizontal speed of air moving towards the north. A negative sign indicates air moving towards the south. This parameter can be combined with the U component of wind to give the speed and direction of the horizontal wind.