1. Chapter 12.4 (Houze 2014): effects of terrain on precipitation The wind began to blow steadily out of the west and pour the water of the distant seas on the dark heads of the hills … J.R.R. Tolkien, The Lord of the Rings introduction to this chapter (Robert Houze’s Symons gold medal lecture, RMS, May 2015) introduction to this chapter Also check out this Meted module: Dynamics & Microphysics of Cool-Season Orographic StormsDynamics & Microphysics of Cool-Season Orographic Storms, by Jim Steenburgh, U Utah

2. 1. mechanisms in which terrain affects precip stratified flow release of potential instability elevated sfc heating, anabatic convergence, and convection ocean katabatic flow / land breeze reduced sub-cloud evaporation seeder-feeder trapped lee waves blocked flow, cold air damming low hill usually embedded in stratiform precip sfc heating, lee convergence stratified flow, lee plunging & hydraulicjump capped lee convergence around isolated mtn convergence over lee ridge Precipitation primarily due the triggering of convection by orographic flow. Precipitation mainly due to stratiform orographic ascent.

3. orographically triggered convection: (b) potential instability release buoyancy vertical displacement

4. convection is often embedded in broad stratiform regions orographically triggered convection: (b) potential instability release L

5. orographically triggered convection: (m) convergence over lee ridge Sierra de Córdoba frequency of super-deep convection (>40 dBZ up to 10 km) as seen by TRMM radar, DJF Andes Deep convection initiation in areas with much CAPE & CIN is common over lee ridges, e.g. Palmer Divide, Cheyenne Ridge, Black Hills broad ascent

6. pre-existing squall line moving across a terrain barrier

7. stratiform orographic precip (i) blocked flow Rotunno and Houze (2007) Fr > 1 Fr < 1 southnorth blocked flow example Alps

8. stratiform orographic precip (i) blocked flow terrain map showing drainage flow in Alpine valleys deep moisture flux

9. stratiform orographic precip (i) blocked flow barrier jet cross-mountain flow IMPROVE experiment (2000), Oregon Cascades

10. stratiform orographic precip (i) blocked flow Andes

11. 2. frontal systems passing over mountains barrier jet into page out 6 hours later21Z on 9 Dec 1995

12. precip evolution during frontal cyclone passage prefrontal postfrontal LLJ

13. atmospheric rivers intersecting terrain

15. 3. Tropical cyclones intersecting terrain

16. deep convection near TC eye height (km)