Boundart Layer

Commonly known as Planetary Boundary Layer Height (pblh), we use the bldepth variable in our codes. From wikipedia:

the planetary boundary layer (PBL), also known as the atmospheric boundary layer (ABL) or peplosphere, is the lowest part of the atmosphere and its behaviour is directly influenced by its contact with a planetary surface. On Earth it usually responds to changes in surface radiative forcing in an hour or less. In this layer physical quantities such as flow velocity, temperature, and moisture display rapid fluctuations (turbulence) and vertical mixing is strong. Above the PBL is the "free atmosphere", where the wind is approximately geostrophic (parallel to the isobars), while within the PBL the wind is affected by surface drag and turns across the isobars.

Essentially this is the layer of the atmosphere we are interested in. Notice that the model will calculate vertical levels quite further than the top of the BL. For some reason I don't understand yet, it is recommended have levels until the P=50mbar height, which is somewhere over 25000m.

Wind

Naturally the wind has three components: wind = (u,v,w). The WRF returns each of the three components for every point in the grid. However, the intensity is usually only reported in the plane, neglecting the third component: |wind|=sqrt(u²+v²). The third component plays a major role in another property explained later on.

Surface wind

BL average wind

This is the weighted average of the wind across the full Boundary Layer. It is weighted by the vertical extend of each layer and only until the top of the BL

BL top wind

BL Max.Up/Down Motion

The wblmaxmin variable in the code. This value is the maximum or minimum vertical velocity of the wind along the BL. I think it'd better to explain the calculation: For each horizontal node in the calculation grid (i,j) we take all the w component of the wind for every vertical node within the BL and extract either the maximum or minimum value, depending on which one is bigger.

I deem this property is useful in three ways. First it shows the leeward side in the mountains and obstacles. Second, regions with high wblmaxmin where soaring will be easier even without much thermal activity. Finally, wblmaxmin is also high when two flows of air collide rising in a convergence line.

Thermal strength

Max vertical speed of the air rising due to thermal activity. It is calculated using the Deardorff velocity

Height of Critical Updraft Strength

hcrit from DrJac's Documentation: it is height at which the average dry updraft strength drops below 1.14m/s (225f/m). This can be considered as the effective maximum height a paraglider can reach, although in the presence of clouds the cloud base would be the limit (lower than hcrit).

Technicalities

The WRF solves the equations of motion for several atmospheric properties in a 3d grid. Let us focus first in the first layer of this 3d-grid. The horizontal components of the 3d-grid form a regular rectangular grid in Lambert Conformal projection (in other projections it appears to have curved edges). Notice that these points are placed at the height of the terrain, so each one will have a different height above mean sea level. The more dense this grid is, the better description of the topography the model will be able to see, but it comes at a (high) computational cost.

The vertical levels are created parallel to the terrain at first but transitioning to flat levels in the upper layers in a hybrid model.

This means that reading any given layer in the WRF output is not the same as reading the property at a fixed height above sea level and is not the property above ground level either since the model transitions from terrain-parallel to "flat" levels.