Narrow Cold-Frontal Rainbands


The surface cold front in cases of rearward sloping ascent tends to be sharper than that for katafronts, particularly when there is a strong southerly low-level jet (LLJ) ahead of the front. The presence of the LLJ provides strong relative flow of warm air toward the front at low levels, which upon encountering the cold front, is forced to rise rapidly in a narrow plume. This updraft jet, which is only 2-3 km deep, produces a narrow band of very heavy rain at the surface cold front termed the Narrow Cold Frontal Rainband (NCFR). Following this rapid ascent, the air undergoes a brief period of descent within the strong convective downdraft before renewing a more gradual slantwise ascent up and over the frontal surface in a typical anafront-like fashion. Thus, two distinct patterns of precipitation are present in anafronts containing a strong LLJ - a broad region of mostly stratiform, post-frontal precipitation, and a narrow core of heavy rain within the NCFR.


Schematic model of narrow cold front rainband (NCFR) along an anafront. Thick lines represent the cold frontal zone and the top of the convective boundary layer ahead of the cold front. Front-relative circulation system, low-level jet, and regions of intense rainfall within the NCFR and of weaker stratiform rain produced by slantwise ascent are shown. The strong southerly low-level jet (LLJ) is found ahead of the surface cold front, the Narrow Cold Frontal Rainband (NCFR) exists above the frontal updraft jet, and a thermally direct circulation is present beneath the upper-level jet (ULJ) and in association with the anafront circulation.

This band of nearly vertical, two-dimensional convection displays a leading edge akin to a density (gravity) current. Its passage is heralded by a characteristic rapid cooling and pressure jump, and in many instances by a wind surge or at least a sharp windshift. These phenomena are rarely evident in satellite imagery, but can be seen clearly by radar as a narrow line of intense, yet relatively shallow, convection. The motion of the front towards the LLJ results in strong inflow, which when considered together with the vigorous ascent core, can be understood as being integral components of a transverse circulation centered about the LLJ. This circulation is frontogenetically driven by the horizontal temperature differences resulting from diabatic forcing within a region of very strong horizontal deformation: (1) the slantwise ascent region produces precipitation that falls into quite dry, cool air, whereupon it evaporates and generates a cool pool of air; and (2) the air within the convective updraft is diabatically warmed by the release of latent heat. Characteristically strong vertical wind shear accompanies the strong inflow; hence, there is a positive vorticity about a horizontal axis ahead of the front. There also exists negative vorticity about a horizontal axis produced by solenoidal forces (diabatic cooling) behind the front. When these two circulation systems possess equal but opposite vorticity strengths, then an optimal state of lifting is made possible. It is believed that this may explain the long-lived vigorous nature of NCFR events. See Koch and Kocin (1991) for additional information.

We will find much use to be made of the anafront with LLJ model in the two real-case applications presented in this online tutorial.

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