Lower-Tropospheric Frontogenesis, the Air Mass, and Extreme Precipitation

Lance Bosart’s earliest research included his seminal papers on both mid-tropopsheric and coastal frontogenesis processes. His subsequent research often included analyses of these frontogenesis processes in his synoptic-dynamic studies of extreme cyclogenesis and extreme precipitation. A few examples of such seminal research include his analyses of the Presidents’ Day Cyclone of 18-19 February 1979, the Megalopolitan Snowstorm of 11-12 February 1983, and the extratropical transformation of Hurricane Floyd in 1999.

The purpose of this presentation is to examine the role of the air mass as a modulator for extreme precipitation in the presence of lower-tropospheric frontogenesis.

The role of the air mass in modulating the strength of a precipitation event is addressed with an analysis of the most basic of expressions, namely P = RD, where P is the total precipitation, and R is the precipitation rate, averaged through the duration, D, of the event.  Though appearing simple, this expression includes R, which incorporates thermodynamic and dynamic factors driving the air’s ascent.  This ascent is associated with a change of water vapor mixing ratio, as an air parcel’s moisture condenses following a moist adiabat.  Thus, the analysis of this deceptively simple expression involves non-linear interactions between the parcel’s ascent, associated with the frontogenetical thermally direct vertical circulations, and its air mass.

We focus on the pertinent aspect of an extreme precipitation event:  the details of the associated air mass, and the mesoscale lifting mechanism.  The precipitation rate, R (assumed to be same as condensation, with an efficiency of 1, may be expressed as the product of vertical motion and the change of saturation mixing ratio following a moist adiabat, through the troposphere.  This expression for R includes the essential ingredients of lift, air mass temperature, and static stability (implicit in vertical motion).  We use this expression for precipitation rate to examine extreme precipitation events in the extratropical latitudes to document the associated air masses and their physical impacts on the precipitation rate. We find that extreme extratropical precipitation events occur throughout all seasons in the presence of a subtropical air mass, as defined by the lower-tropospheric equivalent potential temperature. Furthermore, the trigger for such extreme events is typically lower-tropospheric frontogenesis.

Implications of this air mass modulation on precipitation rate are discussed in the context of longer-term global climate change.