1. Three-category ice scheme
WP 3.4.1
Three-category ice scheme
Thorsten Reinhardt, Deutscher Wetterdienst

2. Motivation:
In convective precipitation events, which are intended to be simulated
explicitly in LMK, there occur not only unrimed or slightly rimed ice
particles, but also more densely rimed particles like graupel or
hailstones which have different properties compared to snow.
At same mass concentration
Graupel has a larger sedimentation flux
Graupel grows less efficiently by vapour deposition and riming
Graupel melts less fast
than snow.
For this reason, we want to consider in the LMK microphysics
parameterization - besides snow - also graupel as an additional
precipitation ice particle category.

3. Precipitation ice particles in LM microphysics parameterization:
Up to now:
Snow
Now additionally:
Graupel

4. Initiation of graupel:
from freezing of raindrops
from conversion of snow to graupel due to riming (if a cloud water threshold of 0.2 g/kg is exceeded)
Additional conversion rates
(formulated analogous to those of snow):
water vapor deposition / sublimation
riming
collision with cloud ice particles (aggregation)
melting

6. Additional Changes: For autoconversion from cloud water to rain
a threshold of 0.2 g/kg is introduced.
Coefficients for deposition and melting
depend now from pressure and temperature; rates re-fitted
Sublimation of cloud ice according to sublimation rate
Sublimation of snow/graupel considered also at T>0 ºC.

7. Idealized simulation: 2-d warm bubble (after 72 min)
Standard scheme
Graupel scheme
yellow: cloud ice
red: snow
green: graupel
light blue: cloud water
dark blue: rain
(units: g/kg)
Mesh size: 2 km

8. Real 3-d simulations with LMK: Cross-sections of hydrometeor distribution
Stratiform snowfall event (left) & convective event (right)
Graupel scheme simulates snow/graupel in a plausible way.

9. IMPROVE-2 Testcase
Observation.
Cloud water content (and also graupel, not shown) in modified version (increased snow-graupel conversion) agrees better with observation, but modified version seems to overpredict graupel in LMK real-weather simulations
Graupel scheme
Graupel scheme modified
g/kg
Mesh size: 1km

10. Cloud water content: IMPROVE-2 Case
Observation
2-moment scheme
Graupel scheme LM 3.13
Graupel scheme modified
x=1km
g/kg

11. LMK Testsuite July to September 2004
Synop verification reveals:
Graupel testsuite scores are quite similar
to parallel testsuite without graupel
Total precipitation slightly (few percent) reduced
Positive frequency bias for precipitation (especially mm/h)
is reduced

12. Outlook:
• Testing whether a graupel category with more hail-like properties
instead of those of low-density graupel might be more appropriate
• Evaluation of simulated cloud microphysical quantities against
cloudnet data
Graupel scheme is standard for ongoing LMK testsuites now.