Radars in the Mountains

Germann, MCH

Atmosphere is very layered

3 flow regimes evident but really 5?

5 layers?

1

2345

Vertical profile of reflectivity

Orographic Precipitation

Rain shadow

FOEHN, no rain

H3km

The Alps

Lee Side Suppression

Two Conceptual Models of Orographic Precipitation

Medina and Houze, 2003

Precipitation is on the flow when the flow is blocked.

Radial velocity Reflectivity

Precipitation is on the mountain crest when the flow is “up and

over”.

A little of both!

Precipitation is very sensitive to evaporation and sublimation – no precipitation reached

the ground

Melting is obvious but what is magical about 20 dBZ?

Snow processes can be very shallow!

Examples of very shallow precipitation

VPR/Process Classification Hourly sampling/manual

classification

Summary

• Shallow Weather– Focus on drizzle as an example to explain

detectability and measurability– Observability is a function of the radar too (MDS,

beamheight, beamwidth)– A few simple but key calculations to explain (not

calibration, not attenuation) – A little insight into “radar meteorology”

• A few case examples– Drizzle, snow, lake effect snow, orographic

Freezing Level and Mountain Sited Radars

Time-Height TemperatureFreezing Level from Radiosondes

March 2008 Payerne

July 2008 Payerne

Most of the time, the radar sees snow!

Erzgebirge

Ore m

ountains, grey

Doppler

No rain

No rain

1h accumulation

No rain

Accumulations

Stable Case: Blocked Flow from South

North South

ALPS

Precipitation on plains, Italy

Flow is blocked, flow is towards the south

MAP IOP-8 Medina and Houze, 2003

Radial Velocity - Blocked Flow

Medina and Houze, 2003

Unstable Case: Up and Over

Precipitation is on the first range

No blocked flow. Flow is from the south

Medina and Houze, 2003

Radial Velocity – Up and Over

Medina and Houze, 2003

Lee Side Troughing

On a very small terrain feature

Southern Ontario is relatively flat. Elevations rise from about 200m on Lake Huron to about 400m just south of Georgian Bay.

Lake

Huron

Lake Ontario

Lake Erie

Georgian Bay

Topography of Southern Ontario

Topography of Southern Ontario

Average of six GOES images over a 3-hour period

Moderate WSW flow

Dissipation of thin SC cloud in downslope flow

SC cloud formingbeneath an inversionat 1500 m above ground

Dissipation of Clouds in a SW Flow

Topography of Southern Ontario

Niagara Escarpment

Average of six GOES images over a 3-hour period