 |
| Titel |
Nowcasting and very short range forecasting of wind gusts generated by deep convection |
| VerfasserIn |
André Simon, Alexander Kann, Michal Neštiak, Ingo Meirold-Mautner, Ákos Horváth, Kálmán Csirmaz, Olga Ulbert, Christine Gruber |
| Konferenz |
EGU General Assembly 2011
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250052486
|
|
|
|
|
|
| Zusammenfassung |
| The study examines several parameterizations of convective wind gusts developed for the
INCA (Integrated Nowcasting and Comprehensive Analyses) and MEANDER nowcasting
systems. The analysis and forecasts of INCA and MEANDER were tested in several cases of
wind gusts produced by both isolated thunderstorms and widespread convective
systems. Observations and experiments were provided for several countries of Central
Europe, including Austria, Hungary, Slovakia and Czech Republic. The results were
compared to forecasts of non-hydrostatic, high-resolution numerical models MM5 and
WRF, which enabled explicit simulation of deep convection. The gust algorithms
of the nowcasting systems are based on empirical methods, where the maximum
possible wind gust in the neighbourhood of thunderstorm cells is related to the rate
of cooling caused by pseudoadiabatic descent. The inputs are radar reflectivity
measurements and temperature/moisture profiles of the thunderstorm environment derived
from LAM (Limited Area Model) forecasts and adjusted by surface observations.
These methods work well in situations with isolated thunderstorms and enable to
assess the potential for downbursts and wind gusts generated by local thunderstorm
outflows. In the parameterizations of the MM5 and WRF models, wind gusts are
proportional to the wind speed in level, from which the momentum can be transported
downward to the surface layer. The depth of this transport depends on the ratio
of the potential and turbulent kinetic energy of the air parcels at evaluated model
levels. It is shown that in contrast to empirical methods, numerical models have
better performance by predicting wind gusts induced by rear-inflow-jets associated
with large convective systems or with supercell thunderstorms. Gusts exceeding 30
m/s were forecasted in cases of severe thunderstorms, when such wind speed was
observed or wind damage was reported. For convective systems, the enhancement
of the low level wind seems to be often a consequence of large pressure gradient
and mesoscale pressure perturbations, which form in the vicinity of the gust front.
Forecasting of these mesoscale features requires 2D or 3D numerical models with
non-hydrostatic dynamics, which are computationally expensive and their results are
spatially/temporaly not enough accurate for nowcasting purposes. A combination of
wind gusts forecasted by high-resolution numerical model (treated as a first guess)
with simple methods based on radar and surface observations is suggested for the
nowcasting period (approximately 3 hours). A smooth transition is applied to reach
consistency of the nowcasting system outputs and model forecasts after the end of the
nowcasting period. It is concluded that correction of model forecasts is necessary
to get more realistic distribution of wind gusts, above all in situations with local
thunderstorms producing intense downdrafts but relatively small-scale outflows. |
|
|
|
|
|
|