Bora flow over the complex terrain of the mid- Adriatic area

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1 Bora flow over the complex terrain of the mid- Adriatic area Kristian Horvath 1, 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

eastern Adriatic coast Hurricane wind speeds/gusts: 40/70 m/s Large

measure, simulate and predict 60 40 MASL - 1 MASL - 2 20 gusts wind speed

2 Wind speed Wind [m/s] speed [m/s] Wind speed [m/s] 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 MASL - 1 MASL gusts wind speed ( additional observations) gusts wind speed Hour

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.

3 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.

4 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)

5 Anticyclonic bora episode 28 Apr Apr UTC 29Apr UTC 28Apr UTC MSLP&wind10 29Apr UTC MSLP&wind10 Geo&wind500 Geo&wind500

6 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

7 Measurements:: pulsations The evolution of pulsations (running spectra) Different temporal scales, far from periodic! 8 min

8 Numerical modeling The WRF model setup: 4 one-way nested domains (dx= 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

9 Structure and point verification Large spatial wind speed variability due to individual mountains Fair representation of wind speed at tower location wind 22 m AGL [m/s] MEAS_22 WRF_ forecast range

10 Vertical structure 06 UTC CONTROL 18 UTC 06 UTC NODINARA 18 UTC

11 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

12 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.

13 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 km 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

14 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

15 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

16 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]

17 Pulsations:: upstream variability during daytime The effect of surface fluxes 700 s Scorer 952 m ASL upstream CONTROL NO HEAT FLUXES

18 Pulsations:: no fluxes from the surface CONTROL L2 NOFLUXES L2 Reduced intensity of pulsations during daytime, primarily for T>~8 min

19 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

A Numerical Simulation of Convectively Induced Turbulence (CIT) above Deep Convection

A Numerical Simulation of Convectively Induced Turbulence (CIT) above Deep Convection Jung-Hoon Kim and Hye-Yeong Chun Department of Atmospheric Sciences Yonsei University, Seoul, Korea 1. Introduction