P1.12
Upper level trough with tropopause dynamic anomaly from South on Venetian Alps. Difficulties for unsettled weather forecast between two sectors: Dolomites and Pre-Alps
Thierry Robert-Luciani Sr., Agenzia Regionale per la Prevenzione e la Protezione Ambientale, Arabba, Italy
The eastern Italian Alps are often the scene of severe thunderstorms during summer. While most of meteorological situations, forecast does not present problems of prognosis, there are special cases with synoptic flow from Southwest or South which generate difficulties for forecast predictions. At times, after very hot period, when dynamic tropopause anomalies affecting Venetian Alps with trajectories from South, there is high risk of severe thunderstorms, systematically on Dolomites, while the weather conditions are more unpredictable on the Pre-Alps.
De facto the trajectories from South to North of upper level low or dynamic anomaly of tropopause facilitate the warm and Mediterranean air advection and gradually more moist air stagnation in the alpine valleys, when the through reaches near or over eastern Italian Alps. But if weather becomes unsettled on the Dolomites, in the same times on the Pre-Alps the warm air advection crossing upper Planet Boundaries Layer tends to block condition of instability and often fine and sunny weather persists, whereas on the northernmost relieves (40/50 km far) severe thunderstorms occur. Why we observe this weather diversity with the same dynamical atmospheric conditions on the limited mountain area and why often exists this border line between stable weather and unsettled one?
To improve prediction the first step is to refer to checklist of key parameters, but it isn't a better method because for the same weather indicators and the same numerical values doesn't allow to differentiate weather condition between Pre-Alps and Dolomites. In the first cases the sky is almost clear with presence of light cumuliform cloudiness, while in the second one growing cumulonimbus with heavy showers or thunderstorms. If usual indicators of weather prognosis like instability index or others classic numerical parameters may not allow to discriminate two meteorological sectors. How to do and what parameters to apply to get better the local forecast disparity.
The conceptual dynamic meteorology can be considerate like a second and more pertinent step with upper troposphere dynamic with maxima of potential vorticity (tropopause dynamic anomaly) associated with significant local up motion. The atmosphere reacts to these upper deformations by creating vertical motion; this ones can have repercussions on the low levels where the pressure drops and the convergence increases
But upper atmospheric analysis is only partial and it is necessary to join them with low levels of Venetian Alps to explain local differences: in the first area (Pre-Alps), where dynamic field cannot stimulate instability, while in the second one (Dolomites) severe thunderstorms might occur, Why this discrepancies between Dolomites and Pre-Alps?
The following and last step is the interaction between upper (dynamic or latent anomaly) and lowest levels to predict different weather evolution. In case of flow from Southwest the upper level forcing passes over the Alps without trigger effects on the Pre-Alps because the warm air advection in the medium atmosphere tends to inhibit up motion generated by convection of low levels without unit them with upper dynamic tropopause, except cases of low jet and local orographic convergences. On the Dolomites the more intense convective effect (highest mountains) can join low convective ascendances with upper up motion connected to tropopause dynamic anomaly, consequently unsettled weather often can occur.
Considering the imperfections of prognosis numerical models, the experience appears as a fundamental step to improve forecast on the Venetian Alps. The more important steps are: 1- understand how much different orographic effect and Planetary Boundary Layer conditions can interact with upper dynamic level, 2- individualize precursor elements which can produce a favourable situation of unsettled weather on Pre-Alps. In spite of experience it isn't easy to make a good evaluation between inhibition convective effect, upper dynamic level and local forcing factors, that extends the unsettled weather so on the Pre-Alps.
Poster Session 1, Monday Poster Session
Monday, 30 August 2010, 5:30 PM-7:00 PM, Alpine Ballroom B
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