Abstract
Conventionally, radar-based nowcasting methods have been conducted by
shifting or extrapolating the most recent estimated precipitation field
along the estimated motion field to lead times up to 3 hr. However, such
methods assume that these fields do not evolve during the lead-time
period. A more recent nowcasting approach is the short-term ensemble
prediction system known as STEPS. STEPS aims to filter small spatial
scales of rain as they have an expected short life-time compared to
large spatial scales. Besides, it perturbs the precipitation field
during extrapolation so that the prediction uncertainties related to the
evolution of precipitation is considered, resulting in an ensemble
nowcast. In this work, the configuration, implementation, and
performance of STEPS are studied for its radar-based ensemble nowcasting
application in Germany. Attention is given to the spatial localization
(i.e., the adjustment of parameters that control the spatio-temporal
evolution of precipitation in large domains). For such purpose, the
capability of STEPS is analyzed using multiple rain events collected by
the German radar network. Preliminary analysis regarding the spatial
scale filtering of STEPS (without perturbation) shows an improvement
against conventional extrapolation of 20 to 30 % on the critical
success index starting at lead times of 30 min at a rainfall threshold
of 0.1 mm hr-1. In terms of the spatial structure,
STEPS (without perturbation) provides a consistent nowcast for lead
times up to 3 hr. Although small spatial structures are filtered, the
spatial structure at scales of 32 km or larger is maintained. The skill
of a probabilistic nowcast (STEPS with perturbation) was also analyzed.
It is shown that for the probability of exceeding the 5.0 mm
hr-1 threshold, the predictability of the nowcast is
compromised for lead times on the order of 1 hr. However, the capability
of identifying rain and non-rain areas is still valuable: the hit rates
are still larger than the false alarm rates. Our preliminary results
highlight aspects needed in the configuration of STEPS such as the
evolution model and the perturbation model. In this manner, this study
can serve as a basis for an improved nowcasting system in Germany and as
a reference for forecasters to understand the characteristics of the
examined nowcast system.