A brief note on temperature advection

[knocker]

The following is an extract from Synoptic Analysis and Forecasting by Shawn Milrad

Temperature Advection

Definition

In Chapter 7, we discussed two upper-tropospheric mechanisms associated with vertical motion: jet streak divergence/convergence and 500-hPa geostrophic vorticity advection. Here, we introduce the third synoptic-scale mechanism for vertical motion: lower-tropospheric geostrophic temperature advection. Along the same lines as geostrophic vorticity advection (Chapter 7), geostrophic temperature advection is the transport of temperature by the geostrophic wind. Temperature advection tends to be quite small in the upper troposphere, where the isohypses and isotherms are mostly parallel to each other, so we will focus on the lower troposphere.

In Chapter 6, we paid special attention to regions on 850-hPa charts where the isohypses and isotherms were nearly perpendicular to each other; this is a key clue to recognize regions of temperature advection. Recall that the geostrophic wind blows parallel to isohypses (on an isobaric chart) and parallel to MSLP isobars on a surface chart. To identify temperature advection, the analyst or forecaster should ask “what type of air is the geostrophic wind transporting into a given region: warm or cold?”

Fig. 8.4A shows a schematic of the geostrophic wind parallel to the isotherms (thickness lines); in this case, there is no temperature advection. However, in Fig. 8.4B, the geostrophic wind is perpendicular to the isotherms (thickness lines) and blowing from warm to cold; this is defined as warm-air advection (WM). In contrast, Fig. 8.4C shows the geostrophic wind perpendicular to the isotherms (thickness lines), but blowing from cold to warm; this is defined as cold-air advection (CM).

Now that we understand how to recognize WM and CM, let us explore how each relates to vertical motion. During WM, the lower troposphere warms as warm air is advected into a region from somewhere else. This disrupts thermal wind balance, which the atmosphere immediately tries to restore. To do this, the atmosphere must cool adiabatically, which is accomplished through ascent. Through the mass continuity principle, ascent must be accompanied by surface convergence (Chapter 7). In summary, lower-tropospheric WM is associated with ascent and surface convergence

During CM, the lower troposphere cools as cold air is advected in from elsewhere. This also disturbs thermal wind balance, which the atmosphere immediately tries to restore through adiabatic warming by descent. Through the mass continuity principle, descent will be accompanied by surface divergence (Chapter 7).  In summary, lower-tropospheric CM is associated with descent and surface divergence

Edited December 9, 2019 by knocker