COMPLEX PROPERTIES OFTHE BORA WIND

I. Stiperski 1 , B. Ivančan-Picek 1, V. Grubišić 2 and A. Bajić 1

1Meteorological and Hydrological Service, Croatia2University of Vienna, Vienna, Austria

HyMeX, 8-10 June 2010, Bologna

Overview

Aim and motivation

Data and model

Complex Bora flow

Air-sea interaction

Conclusions

HyMeX, 8-10 June 2010, Bologna

Outline

HyMeX, 8-10 June 2010, Bologna

Overview

Effects of BoraNE

- huge Bora variability

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1 61 121 181 241 301 361 421 481 541

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d sp

eed

(m/s

)Vsec V10min23. 12. 2003. form 04:10 UTC to 04:20 UTCMASLENICA BRIDGE

04:10 04:12 04:14 04:16 04:18

13.9 m/s

58.6 m/s

- maximum Bora speed on the Adriatic coast – 69 m/s

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d sp

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d di

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V 1 0 m i n

V m a x

D i r e c t i o nM A S L E N IC A B R ID G E

1 3 . 1 2 . 1 4 . 1 2 . 1 5 . 1 2 . 1 6 . 1 2 . 1 7 . 1 2 . 1 8 . 1 2 . 2 0 0 1 .

Overview

HyMeX, 8-10 June 2010, Bologna

HyMeX, 8-10 June 2010, Bologna

Overview

Major advances in the understanding of Bora flow :

ALPEX and MAP

Basic severe Bora dynamics:

- presence of the upstream low-level critical level and/or u pstreamtemperature inversion

- 3-D structure (hydraulic jumps, mountain wave breaking , gap jets , wakes, rotors, pulsation)

List of recent publications:Grubiši ć, 2004; Gohm and Mayr, 2005; Gohm et al. 2008; Beluši ć, 2007; Horvath et al., 2009; Grisogono and Beluši ć, 2009

Motivation and aimN

W E

S

2005-2006

V E L E B I T

HyMeX, 8-10 June 2010, Bologna

Motivation and aim

- Most of the Bora studies of the past have focused on the nort hernpart of the Dinaric Alps

- Zadar area is highly vulnerable to weather conditions

(tourism, transport and communication, natural environment, ...)

- Special sodar measurements during the winter - spring 2004/5 season

- Investigate small-scale characteristics and spatial variabi lity ofthe strong Bora flow in the lee of Southern Velebit and identif yreasons for weak winds in the city of Zadar

HyMeX, 8-10 June 2010, Bologna

Data and model

- severe Bora event 20 December 2004

HyMeX, 8-10 June 2010, Bologna

sea level pressure

- COAMPS (non-hydrostatic, fully-compressible model with multiple nesting capability)∆x = 1 km

∆x = 333 m

Data and model

HyMeX, 8-10 June 2010, Bologna

20 December 2004 – time series of measured (black) and modeled (orange) 10-minute wind speed, measured gusts (grey) and wind direction (dots)

Data and model

HyMeX, 8-10 June 2010, Bologna

Sodar dataTime-height diagrams

wind speed (m/s) and wind vectors vertical velocity (m/s)

Complex Bora flow

HyMeX, 8-10 June 2010, Bologna

Wake vorticies

ZD

Wind field retrieved byWiSAR from ASAR data

23 Jan 2008, 09 UTC

Courtesy byAlperts, Ivanov, Horstmann, 2009Mon.Wea.Rev.

Complex Bora flow

HyMeX, 8-10 June 2010, Bologna

Jets and wakes

Complex Bora flow

HyMeX, 8-10 June 2010, Bologna

Undular Hydraulic Jump Rotor

ZD

Air-sea interaction

HyMeX, 8-10 June 2010, Bologna

Daily averaged ALADIN 10m wind (blue) and station measurements (red) for 12 February 2003 (left); Model predicted sea level and transports for 13 February 00UTC (right).(Kuzmić et al., 2005)

Modeling and observational studies of the Adriatic Sea: MAP, DOLCEVITA, EUROSTRATAFORM

Air-sea interaction

HyMeX, 8-10 June 2010, Bologna

Taken from Pullen et al. (2007)

Bora induced gyre in northernAdriatic Sea

shown are mean ocean surfacecurrents, both in contours andvectors

14UTC 31 Jan to 06UTC 2 Feb 2003

Conclusions

HyMeX, 8-10 June 2010, Bologna

• significant time/space variability of the wind speed and di rection during thesevere Bora

• the Bora flow in the lee of southern Velebit is shown to be h ighly complex, characterized by hydraulic jumps, wakes, wake vorticies and rotors

• the wake, responsible for the occurrence of lighter than av erage Bora winds in theZadar area, is induced by the highest peaks of southern Vel ebit and formsdownwind of a stong hydraulic jump

• the wake is unstable and transitory

• strong horizontal and vertical wind shear

• Bora and Adriatic Sea interaction – with timescales of a fe w days or even less(oceanic gyre, sea surface temperature)

• need for improvement our understanding of the air – sea proce sses using in-situobservations and high resolution modeling particularly sh ort-duration feedbackeffects

HyMeX, 8-10 June 2010, Bologna

Thank you!