Resonance and Low-Dimensional Modeling of the Low-Level Jet

The Low-Level Jet (LLJ) is a phenomenon of considerable practical and theoretical interest. Of practical interest is its strong role in advecting moist and unstable air northward into the Great Plains and in establishing favorable wind profiles for severe weather. Of theoretical interest is the observation that the LLJ is commonly supergeostrophic during its nocturnal phase. The LLJ theory of most general importance was first proposed by Blackadar in 1957. In this theory, the supergeostrophic nocturnal jet is the result of an inertial oscillation. During the day, the low-level winds are retarded due to friction caused by strong vertical mixing. At night, the vertical mixing ceases and the layer of air near the surface decouples, becomes frictionless, and accelerates under a synoptic pressure gradient with supergeostrophic winds being reached due to the inertial oscillation of the Coriolis force. The natural frequency of this response is related to the Coriolis parameter whereas the frequency of forcing is related to the 24 hour surface heating cycle. Where these two frequencies are the same (near 30 degrees north or south latitude), a particularly strong supergeostrophic jet can be expected due to resonance.

In order to test the Blackadar theory and look for resonance, the simplest possible models of the LLJ which still contain the essential physics were constructed. The simplest model is a 0-dimensional parcel model which balances acceleration, the Coriolis force, the pressure gradient, and damping from surface physics. The next level of complexity (1-dimension) implements vertical or meridional dependence. These models were compared with full 4-dimensional gridded model output and observations. The development and analysis of these low-dimensional LLJ models has been very valuable in clarifying the important physics responsible for the LLJ. In particular, we have found the the resonance effect can be an important mechanism in enhancing the jet. We have also found that the Blackadar mechanism appears to be dominant in LLJ dynamics. Indeed, we have found it to be pervasive. In addition, we have found that the LLJ is highly dependent on surface physics. We have found low-dimensional modeling of the LLJ to be a powerful check on the surface parameterizations of more complex numerical models.