Bora flow over the complex terrain of the mid- Adriatic area Kristian Horvath 1, kristian.horvath@cirus.dhz.hr, Željko Večenaj 2, Branko Grisogono 2 1 Research Department, Meteorological and Hydrological Service, Croatia 2 Geophysical Institute Andrija Mohorovičić, Croatia
Wind speed Wind [m/s] speed [m/s] 0 20 40 60 Wind speed [m/s] 0 20 40 60 Introduction:: bora winds Bora gusty NE downslope windstorm all along the eastern Adriatic coast Hurricane wind speeds/gusts: 40/70 m/s Large temporal (GF~3) and spatial variability Cross-cutting issue: hard to measure, simulate and predict 60 40 MASL - 1 MASL - 2 20 gusts wind speed (70.000 additional observations) gusts wind speed 0 3 4 5 Hour 6 7 8 9 12 00 12 12 00 12
Introduction:: pulsations Downslope windstorms: quasi-periodic behavior, so-called pulsations Three mechanisms proposed: 1. Vortex tilting in the wave-breaking region (Clark and Farley, 1984) 2. The effect of propagating lee waves (Clark et al., 1994) 3. Kelvin-Helmholtz instability (KHI) between low-level shooting flow and wave-breaking aloft (Scinocca and Peltier, 1989, P&S1990, Smith 1991) Pulsations in bora (e.g. Petkovšek, 1976; Belušić et al., 2006) Proposed mechanism of bora pulsations in the n. Adriatic is KHI (Belušić et al., 2007) Power spectrum [m/s] 2 10 min 5 min 1 min MASLENICA Pulsations~7min Frequency [Hz] Belušić et al.
WINDEX feat. BORA measurement campaign 1) 60-m wind tower on Pometeno brdo 3 sonics at 10 m, 22 m and 40 m (5Hz) 2 cup & vanes at 30 m and 60 m 2) SODAR on Split Airport 10 m vertical resolution (up to 300 m) Period: 3 months (Feb-May 2010, extended May 2011)
Anticyclonic bora episode 28 Apr 2010 28Apr2010 00UTC 29Apr2010 00UTC 28Apr2010 00UTC MSLP&wind10 29Apr2010 00UTC MSLP&wind10 Geo&wind500 Geo&wind500
Wind tower measurements on 28 Apr 2010 Medium to strong bora event: near-surface winds reaching 15 m/s Streamwise wind speed component 22 m AGL
Measurements:: pulsations The evolution of pulsations (running spectra) Different temporal scales, far from periodic! 8 min
Numerical modeling The WRF model setup: 4 one-way nested domains (dx=9 3 1 0.333 km) D04 Upstream UP 40 vertical levels IC&LBC ECMWF_OA MYJ, KF, Thompson Tower Noah LSM True horizontal diffusion SODAR Lee L1 Mnt. Top MT Lee L2
Structure and point verification Large spatial wind speed variability due to individual mountains Fair representation of wind speed at tower location wind speed @ 22 m AGL [m/s] 14 12 10 8 6 4 2 MEAS_22 WRF_22 0 0 3 6 9 12 15 18 21 24 27 30 forecast range
Vertical structure 06 UTC CONTROL 18 UTC 06 UTC NODINARA 18 UTC
Numerical simulations:: Representation of pulsations Pulsations at tower location are represented to an extent, but simulated pulsations have less energy than observed MEAS WRF maxima
0 Height [km] 4 0 Height [km] 4 Daytime flow&pulsations Daytime flow is unstationary Pulsations: Non-local The most intensive beneath the primary breaking mountain wave P Wsp.&Theta Travel far away from the origin R<0.25 generally not found near the primary gravity-wave breaking region KHI mechanism questionable Cross-mnt. wsp.&ri
0 Height [km] 4 0 Height [km] 4 L~3-4km Nighttime flow&pulsations Nighttime flow more stationary More frequent flow separation Pulsations: Non-local P Wsp.&Theta More sporadic 87654 3 2 1km Appear beneath the breaking mountain wave Ri<0.25 Dissipate quickly downstream of the origin point Ri<0.25 found near the primary gravity waves Pulsations point to KHI Cross-mnt. wsp.&ri
Propagation of pulsations:: large spatial variability Background flow Pulsations (8-15min) UP Formation L1 Pulsations develop for T~ 3-8min MT L2 Formation Pulsations (8-15min) Pulsations develop for T~ 3-8min
Pulsations:: upstream variability Less certaity on the origin of non-local daytime pulsations of T~8-15 min Are these daytime pulsations caused by background flow properties? daytime nighttime
Power spectrum Pulsations:: upstream variability during daytime Scorer parameter of the background flow during daytime 700 s Scorer 952 m ASL upstream Where does variability of the daytime upstream Scorer parameter come from? Period [s]
Pulsations:: upstream variability during daytime The effect of surface fluxes 700 s Scorer 952 m ASL upstream CONTROL NO HEAT FLUXES
Pulsations:: no fluxes from the surface CONTROL L2 NOFLUXES L2 Reduced intensity of pulsations during daytime, primarily for T>~8 min
Conclusions Bora flow in the mid-adriatic more complex than in the north (effects of the primary and secondary orography, 3D, ) Two modes of bora streamwise pulsations are found: A) Shorter pulsations (T<8 min) predominantly point to KHI and may occur regardless of the period of day B) Longer pulsations (8-15 min) point to effects of upstream variability and occur predominantly during the daytime Future: analyze cross-streamwise pulsations, perform multi-scale WRF simulations, study TKE budget