Precipitation Type Diagnostic

The optional diagnostic ptype provides a categorical precipitation type at the surface for ICON with NWP physics. It follows the modified Bourgouin approach (Bourgouin 2000, Birk et al. 2021) and uses ICON-native thermodynamic functions with vertical temperature, pressure, humidity and height profiles together with surface precipitation and ground temperature. Explicit ICON surface hydrometeor rates reclassify the diagnostic to hail or graupel when either dominates, to a rain/snow mix when both are substantial, or to wet snow for snow at a near-0 °C surface. For ice pellets, a Bourgouin surface-warm-layer melting check attenuates their survival and treats melted ice pellets as an alternate rain candidate when a final warm layer is strong enough to melt them before they reach the surface.

Although the modified Bourgouin/Birk relations yield a probability for each thermodynamic type (snow, rain, freezing rain, ice pellets, freezing drizzle), the ICON implementation reports only the single most likely of these (the highest-probability type). Exact ties are resolved by a fixed priority in the categorical diagnostic. Beyond the rain/snow and wet-snow refinements explained above, simultaneous mixes, such as snow with ice pellets or freezing rain, are not represented.

For complex profiles with several melting and refreezing layers, the diagnostic uses the retained near-surface wet-bulb-energy areas following the Zukanovic/Bourgouin reduction used by the reference implementation for the rain, freezing-rain and ice-pellet decision logic. The Birk snow-probability relation uses all positive wet-bulb melting energy in the diagnosed column.

The diagnostic is computed when ptype is requested in an output_nml and NWP grid-scale microphysics is active. The precipitation amount entering the diagnosis is the difference of accumulated total precipitation since the previous ptype diagnosis. Nonzero precipitation types are emitted only where this interval precipitation exceeds a conservative trace threshold scaled with the elapsed diagnosis interval (0.01 kg m-2 h-1, with a 1.e-6 kg m-2 amount floor). Grid points below this threshold are set to code 0.

The previous-precipitation state is stored in the restart field ptype_tot_prec0 and is shared by all output streams on a domain. If ptype is requested in more than one output stream, the interval is therefore the time since the previous global ptype diagnosis rather than the time since the previous output in a particular stream.

The output is an integer two-dimensional surface field using GRIB2 precipitation type Code table 4.201:

Code	Meaning
0	no precipitation
1	rain
3	freezing rain
5	snow
6	wet snow
7	mixture of rain and snow
8	ice pellets
9	graupel
10	hail
12	freezing drizzle
13	freezing rain on ground

Code 13 is a local extension used to distinguish freezing rain at ground temperature below the diagnostic threshold. Code 9 is based on ICON’s graupel/snow-pellet-like surface precipitation rate and may include graupel-like snow pellets depending on the active microphysics scheme.

Codes 6, 7, 9, and 10 are conservative refinements of the Bourgouin-based column diagnosis. They are only emitted when the available surface precipitation rates provide sufficient supporting evidence; otherwise the thermodynamic Bourgouin category is retained.