METR 2413 20 February 2004 Thermal Advection Since

METR 2413 20 February 2004 Thermal Advection Since we do not directly measure vertical motions, the analysis of thermal advection on maps will provide a very useful tool for determining the vertical motions currently occurring in the atmosphere.

Why use Temp. Advection? • The temperature at a location may change in two ways: – The air parcel which is being sampled might change its thermodynamic state. For example, sunlight might increase its internal energy, and hence its temperature will rise. – The air parcel might be replaced by a different parcel with a different thermodynamic state as the wind blows past the station. This process is called advection. • In practise, both processes will operate. However, on the synoptic scale, temperature changes on timescales less than a few days are dominated by advection effects.

Thermal Advection The advection term consists of a wind velocity component and a temperature gradient component. The spatial relationship between these two is important.

Thermal Advection Spatial relation between wind and temperature gradients • (Geostrophic) wind is parallel to isobars. • Temperature gradients are represented by isotherms. • The magnitude of the pressure gradient and temperature gradient and angle between the two, isobars (wind) and isotherms, determines the strength of advection.

Solenoids • When the wind crosses the temperature gradient at nearly a 90 degree angle the “boxes” formed on the weather map are called Solenoids analyzed on a weather map indicate the presence of strong advection and vertical motions.

Solenoids • Thermal advection Solenoids can be identified on 850 mb charts by comparing isotherms and isohypses. • Also by comparing 1000 -500 mb thickness and surface pressure isobars, which have historically been plotted together on weather charts (MSLP/1000500 thickness chart)

500 -1000 mb Thickness • In addition to isotherms on a constant pressure surface, we can look at thickness compared to surface pressure • Remember the hypsometric eqn? • Thickness between 2 pressure surfaces is directly related to mean layer temp! – Increase mean temp, increase thickness – Decrease mean temp, decrease thickness – This can be used as an additional tool when analyzing thermal advection…

Why use thickness? • Heard of the Thermal Wind? – Not really a wind at all, but a vector difference between the geostrophic wind at different heights – Thermal Wind is always parallel to contours of thickness, with cold air to the left and warm to the right – If we plot thickness along with surface pressures, and assume that surface winds are somewhat parallel to surface isobars, then we have 2 pieces of information… • 1) Surface wind vector • 2) Thermal Wind vector The difference between the two is the geostrophic wind above the surface, so now we know how the geostrophic wind changes with height

Thermal Wind No thermal advection: Thermal wind is parallel to low level wind, so geostrophic wind at lower and upper levels are parallel Cold Air Advection: Thermal wind is to the left of the low level wind, so geostrophic wind must back with height => CAA Warm Air Advection: Thermal wind is to the right of the low level wind, so geostrophic wind must veer with height => WAA