Evaluation of hydrometeor types and properties in the ICON-LAM model
with polarimetric radar observations
Abstract
A direct comparison of hydrometeor types (HMT) from state-of-the-art
hydrometeor classification schemes (HMC) with modelled hydrometeors
(ICOL-LAM, operational weather predictions model of the German Weather
Service) is challenging, e.g. due to different HMT definitions and
numbers and difficulties to identify dominant types in mixtures of
hydrometeors. A comparison of published HMCs even revealed significant
differences between the membership functions used for the same
hydrometeor types (Figure 1), emphasizing again the high uncertainty in
scattering simulations for ice hydrometeors because of their complex
geometries, dielectric properties, and largely unknown size and
orientation distributions. The HMCs were applied to perturbed
polarimetric variables observed by the X-band Radar in Bonn (BoXPol) to
test their robustness against measurement errors and show that
especially in the regions with solid precipitation misclassification in
hydrometeor typing occurs often. Thus, a dual strategy to evaluate the
hydrometeor type representation in ICON-LAM is presented: i)
Classification after clustering of the data is assumed to reduce the
sensitivity of the decision to the uncertainty of scattering
simulations. First an agglomerative hierarchical clustering of the radar
pixels based on their similarity in multi-dimensional polarimetric
signatures is applied, and afterwards for each identified cluster a
comparison of the distributions of polarimetric moments with scattering
simulations or membership functions for different HMT is performed. ii)
A direct comparison of multivariate simulated and observed distributions
of polarimetric moments. These comparisons will be performed for
different heights and/or space-time subsets, and for clusters with
similar HMT in the model and the observations as identified with the
advanced radar-based hydrometeor classification scheme. Results for a
set of case studies observed with the polarimetric X-band radar
composite in Bonn, Germany, will be presented.