Climatological characterisation of the Ora del Garda wind in the Alps

Transcription

1 Climatological characterisation of the Ora del Garda wind in the Alps L. Giovannini 1, L. Laiti 1, D. Zardi 1, M. de Franceschi 1,2 1 Atmospheric Physics Group, Department of Civil, Environmental and Mechanical Engineering, University of Trento, Trento, Italy 2 Major Seminary, Diocese of Bolzano-Bressanone, Bressanone, Italy Kranjska Gora, 4 June nd International Conference on Alpine Meteorology 2013

2 Study area Dataset Ora del Garda statistics Lake s temperature Air temperature Synoptic wind Conclusions Study area

3 Study area Dataset Ora del Garda statistics Lake s temperature Air temperature Synoptic wind Conclusions Dataset I Two surface weather stations I 10-year period ( ) I April-September I I I I Hourly average wind speed (WS) and direction (WD) Maximum hourly wind gust (sampled every 10 s) Global solar radiation Air temperature I Hourly observations of lake water temperature at 10 m depth at RIV I Daily values of WS and WD at 850 hpa from NCEP reanalysis (1200 UTC)

4 Criteria for selection of lake breeze days Global solar radiation until noon >30% of the maximum radiation in the same time period for that month WD reverses from offshore to onshore at sunrise ± 2 h WD becomes offshore or WS <1 m s 1 after sunset WD is offshore or WS <1 m s 1 for most of the hours between midnight and sunrise 135 N<WD<255 N for at least 3 h consecutively and at the same time WS>2 m s 1

5 Frequency of lake breeze days on the lake s shore The lake breeze at RIV occurs in 68% of the days analysed. Higher frequencies in the central months of the period: April: 57% May: 71% June: 71% July: 75% August: 72% September: 63%

6 Lake breeze intensity on the lake s shore

7 Timing of the lake breeze on the lake s shore

8 Monthly variations on the lake s shore: onset Progressive delay in the onset from April to September

9 Monthly variations on the lake s shore: cessation Later cessation in May-July

10 Frequency of lake breeze days in the Adige Valley Lake breeze days at GAR: WD = 240 N ± 30 N and at the same time WS > 2 m s 1 for at least 1 h between the onset time at RIV and 2 hours after the cessation time The lake breeze breaks up at GAR in 89% of the days in which it develops at RIV. High frequencies in all months: April: 88% May: 85% June: 88% July: 92% August: 88% September: 91%

11 Lake breeze breakup intensity in the Adige Valley

12 Timing of the lake breeze breakup into the Adige Valley

13 Onset-breakup timing

14 Monthly variations in the Adige Valley: onset As at RIV, progressive delay of the onset from April to September

15 Monthly variations in the Adige Valley: cessation Earlier cessation in August and September

16 Influence of the lake s temperature cf. Telisman Prtenjak and Grisogono (2007) Theor. Appl Climatol.

17 Influence on air temperature The arrival of the lake breeze front is marked by a temperature drop at the lake s shore

18 Influence on air temperature The temperature drop tends to be smaller and smaller from April to September

19 Influence of the synoptic wind Data were divided into 6 classes based on WS and WD at 850 hpa: Onshore (On): 135 WD 255 Onshore Strong (OnStr): 135 WD 255 and WS 4 m s 1 Offshore (Off): WD 75 or WD 315 Offshore Strong (OffStr): WD 75 or WD 315 and WS>4 m s 1 Parallel East (E): 75 < WD < 135 Parallel West (W): 255 < WD < 315

20 Influence of the synoptic wind on the lake s shore

21 Influence of the synoptic wind in the Adige Valley

22 Conclusions Very regular circulation, marking significantly local climatic conditions: High wind intensities at both stations and significant temperature drop at the passage of the front Monthly variations: Later onset from April to September Stronger intensities in spring at GAR No clear relationship between water/air temperature contrasts and the lake breeze strength Influence of the synoptic wind: Synoptic wind influences the timing and the stregth of the lake breeze Opposite effects on lake breeze strength between RIV and GAR

23 THANKS FOR YOUR KIND ATTENTION!