1 / 17

Airflow over and around hills and mountains

Discover the significance of Nh/U, non-dimensional mountain heights, and orographic wind flows with statistical products and physical insights. Explore neutral flows over hills, stable flows over mountains, and the impact of the Coriolis force on airflow patterns. Learn how surface friction affects mountain waves and the implications for wind forecasting.

robertan
Download Presentation

Airflow over and around hills and mountains

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Airflow over and around hills and mountains Haraldur Ólafsson University of Iceland Icelandic Meteorological Office haraldur@vedur.is

  2. WHY INTERESTING FOR FORECASTERS? • STATISTICAL PRODUCTS • EXPERIENCE / PHYSICAL INSIGHT LOGICAL OROGRAPHIC WINDS ARE IN GENERAL POORLY OR NOT AT ALL RESOLVED BY THR NWP MODELS FORECASTERS MUST RELY ON

  3. Key non-dimensional number Nh/U (Non-dimensional mountain height or inverse Froude nember) N={g/T ∂T/∂z } ½ Brunt-Väisälä frequency or stability N is often 10 -2 s -1 h : mountain height u : upstream wind speed U h Flow over mountains

  4. What is Nh/U? • Non-dimensional wavelenght • Potential energy / kinetic energy Next slides: Flow with Nh/U = 0 Flow with 0 < Nh/U < 1 Flow with Nh/U >1.5

  5. NEUTRAL FLOWS (OVER HILLS)Nh/u=0 GENTLE SLOPES STEEP SLOPES U/UO U/UO Boundary-layer separation STREAMLINES STREAMLINES

  6. WIND SPEED IN MOUNTAIN WAVES ρn-5 ρn-2 ρn Umax where the air descends Nh/u > 0

  7. UMAX Z Nh/U = 0 UMIN UMIN NEUTRAL FLOW (HILLS) T Nh/U > 0 UMIN UMAX Z T STABLE FLOW (MOUNTAINS)

  8. FLOW OVER MOUNTAINS { Nh/U } > { Nh/U } C { Nh/U } C = (mountain shape,∂/∂ z (U,N),...) ≈ 1.5 => Blocked flows FAST Dense Air SLOW WAKE SLOW L H L BLOCKING B = ½ ρ u2 + P + gρz = constant In blocking: P ↑ => U ↓

  9. For L<100 km (Ro>1) Corner wind DV/dt =1/ρVP-fV + F H L Gap wind L H Tip jet / corner wind

  10. Two examples of simulated wind in the vicinity of Reykjavík • Weak ENE • Strong NE

  11. Eastnortheasterly winds in Reykjavík – moderate wind speed (ca. 6 m/s) Nh/U ~2.0 Veðurstofa

  12. Eastnortheasterly winds in Reykjavík – moderate wind speed (ca. 6 m/s) Nh/u ~0.5 Veðurstofa

  13. If the airmass is stable almost all the time and mountain (gravity) waves form in a stable airmass that impinges a mountain why are there not strong waves all the time?

  14. Surface friction is wave-destructive! No surface friction Surface friction

  15. Isentropes in two numerical simulations (K) Very amplified waves Little wave activity in the troposphere A stable layer

  16. Impact of scale / the Coriolis force Ro = U/fL U : upstream wind speed f : the Coriolis parameter L : length scale If L = 100 km, U = 10 ms-1 and f = 10-4 s-1 Ro = 1 => Significant impact of the Coriolis force on the flow very fast deceleration fast Ro = 1: L L H slow less deceleration fast

  17. The mountain wind forecasting diagram Red=speed upBlue=slow down T 10 Ro=U/fL 1 0.1 0.1 1 10 ĥ=Nh/U

More Related