Exploring Post-Cold Frontal Moisture Transport in an Idealized Extratropical Cyclone Study

November 10, 2015

Amanda Sheffield

Committee: Sue van den Heever, Sonia Kreidenweis, Dick Johnson, Richard Eykholt (Physics)

Abstract

Moisture transport in extratropical cyclones (ETCs) has been studied in the past in the context of the warm conveyor belt (WCB), a 'conveyor belt' transferring moisture from the warm sector boundary layer to the free troposphere both eastward and poleward of the warm front. Recent research has highlighted a different, potentially important mechanism of transporting water vapor in ETCs by post-cold frontal (PCF) clouds. PCF clouds are typically boundary layer cumulus clouds located in the cold sector of an ETC that transfer moisture to the free troposphere through convective-evaporative processes. Recent studies have suggested that these PCF cumuli may vertically transport nearly equivalent amounts of moisture as the WCB. Therefore, not only are these PCF cumuli important in venting the PCF boundary layer, they also play a role in limiting the amount of moisture available for convergence in the source region of the WCB. This limitation can have important consequences for regional weather and climate through its impact on the timing and location of precipitation, the three-dimensional redistribution of water vapor, and the distribution of clouds within ETCs.

The goal of this study has been to investigate the role of PCF clouds in the moisture transport of an ETC, and the impacts of environmental factors such as SST and aerosol loading on this transport role. We have achieved this goal through the use of numerical simulations of such a storm system. Previous studies have utilized model simulations with relatively coarse grid resolutions and convective parameterization schemes. Here, we simulate a wintertime ETC over the Pacific Ocean using high spatial and temporal resolution, advanced microphysics and explicitly resolved convection.

The results of this research demonstrate that PCF cumuli are found to vertically ventilate BL moisture over an expansive region behind the cold front. The free tropospheric moisture contents and stability profile of the cold sector exert a strong control over the size, depth and frequency of the PCF clouds, and varies with distance from the cold front.

Increased aerosol loading results in the invigoration of the PCF clouds. This is associated with an increase in the upward vertical moisture flux, increased cloud condensate formation, and reduced precipitation rates. Sea surface temperature is found to be a significantly more important factor in the development of PCF cumuli than aerosol loading, where increasing SSTs are associated with increased cloud fraction, cloud top heights, and precipitation rates. The impact of PCF clouds on vertically redistributing water vapor from the cold sector is found to depend in varying degrees on the large-scale advection of water vapor by the ETC system, the surface evaporation rates, the updraft velocities, the precipitation rates, and the cloud fraction within the PCF region.

The pathways of the vertically redistributed water vapor within the ETC were then examined through the use of massless, passive tracers. The results of these experiments show that the water vapor lofted out of the PCF BL by the cumulus clouds is advected hundreds of kilometers eastward within 8-12 hours of release of tracers in the PCF BL. Furthermore, cross frontal transport from behind the cold front to the WCB source region appears to be small, in contradiction to previously hypothesized results. This is due to the fact that the cold frontal boundary provides a zone of strong vertical lifting that does not allow tracers to converge further east.