Basic concept
We first note that PowderGrams are provided for the models ICON-EU and its ensemble ICON-EU-EPS with a forecast period of five days, and for ICON-D2 and ICON-D2-EPS with a forecast period of two days. We also remind you that the ICON models are calculated operationally by the German Weather Service (DWD).
The designation EU refers to a domain over Europe, with a grid spacing of 6.5 km. Whereas D2 covers Germany and the Alps and is "nested" in ICON-EU with a grid spacing of 2.2 km.
The PowderGrams therefore differ in the forecast period and for ICON-D2 we also offer statistics on precipitation and new snow density resolved for altitude. All other content is virtually identical.
Added value through mountain ▲, valley ▼ and circumference ●
A meteogram traditionally represents weather forecasts as a time series for a specific location. The special feature of the PowderGram concept is the combined representation of the weather forecast for a mountain, labeled ▲ in the time series, and a nearby valley town, labeled ▼.
In addition, mean values are calculated for all grid points within the circumference around the mountain with the boundary point valley town. These are particularly useful for variables that are intended to describe regional conditions, and we represent these with ●.
Contents of the PowderGram
We now systematically go through all the elements of the PowderGram from top to bottom. In the title, we find the initialization time and the models used. Then we find the name, the coordinates, the actual altitude and the altitude in the model for mountain ▲ and valley ▼.
After all, the model orography is only an approximation of reality. This is why mountains are always lower in the model orography and valley locations are always higher than in reality. To the right of this is a legend for the "box plots".
Below this is a map showing the exolabs snow depths as well as the location of the mountain and valley location and the circumference. The radius of the circumference and the number of grid points are also shown.
Temperatures and snow line
The first time series on the left shows the air temperature at an altitude of two meters, which has been corrected (dry-adiabatically) to the actual altitude - for the mountain (▲), valley location (▼) and circumference (●). The average of the circumference of the snow line is also displayed. Snowflake symbols appear here if the snow line is below the model orography.
Clouds, humidity and solar radiation
The second time series on the left shows the mean values of cloud cover, relative humidity at two meters above the model orography and the (radiant power of) solar radiation. The dashed line represents the maximum possible radiant power and serves as a reference.
This describes the regional weather conditions. By using the mean values for the circumference area, a meaningful assessment is possible, as it is independent of individual model grid points.
Snowfall and rain
The central time series 3, 4 and 5 are dedicated to precipitation and use the ensemble model (EPS). In each case, snowfall is shown in the upper half for the mountain ▲ and rain in the lower half for the valley ▼.
The statistics for the ensemble members are shown here for snowfall and rain rates (time series 3), as well as for amounts of new snow and rain (time series 4), using the boxplots: Minimum, Maximum and Median, as well as 10th and 90th percentiles, as shown in the legend on the top right.
In the 5th time series we then show the probabilities for significant precipitation rates in bars (>15mm per day for rain rate and >15cm per day for snowfall rate) and for precipitation amounts in lines (>15mm for rain amount and >15cm for new snow amount).
Wind conditions
In the 6th time series we show the wind conditions for the mountain and the circumference area: wind speeds, maximum gust speeds and wind direction.
Surface energy balance and snow depths
Finally, in the bottom row 7 on the left, we show the ICON snow depths for the mountain, valley and circumference area. We also calculate and display the surface energy balance, which is made up of the energy fluxes of thermal and solar radiation as well as latent and sensible heat fluxes.
We choose the signs such that negative (red) values indicate warming of the snow cover, if there is snow, and positive (blue) values indicate cooling. Here, we again choose the circumference area average, whereby we consider whether the calculation on the mountain would not be more relevant. Nevertheless, we believe that the surface energy balance shown allows a valuable assessment of the snow conditions, especially with regard to firn.
Statistics of precipitation and new snow density
On the right-hand side, there are two more histograms of the ensemble for the PowderGrams of ICON-D2 and ICON-D2-EPS, resolved by model height. The grid points are sorted by model height and the amount of fresh snow and rain for certain height intervals is shown statistically with boxplots.
At the bottom right, the color coding shows the probability of the occurrence of snowfall with the corresponding new snow density. This is calculated according to the empirical estimate by Jordan et al., 1999, (see Helfricht, Hartel, et al., 2018) as a function of wind speeds and temperatures near the ground.
The red line indicates a new snow density of 100 kgm-3, which corresponds to the usual assumption that 1 mm of snowy precipitation results in 1 cm of new snow. We would also like to point out that we now also offer the new snow depth on map.powderguide.com, which is based on the above-mentioned estimate of new snow density.
