Mesoscale Circulations (Atmo Sci 460)

Take-home Test on Synoptics


Correct, expand and/or refute the answers to the following four questions. Please hand in at the beginning of class on Monday, February 27. I will look particularly favorably on answers that are TYPED!


1. (10pts) Construct a scenario in which synoptic-scale sinking air (subsidence) might increase the likelihood of upright convection.


The sinking air warms the surface through frictional contact with the ground. The sinking air also draws cold air down from aloft, creating steep lapse rates and increasing CAPE. Convection can then be produced given the existence of a sufficiently strong triggering mechanism.


2. (30pts) Identify the physical processes responsible for extratropical cyclone development (cyclogenesis). Extraordinarily rapid development (storm central pressure falls of more than 1 mb per hour for one day or more) can occur anywhere on the globe, but preferentially occurs over the oceans. The most intense storms over the oceans are 40-50 mb deeper than their equivalents over the continents (North Atlantic fisherman report that things get a bit dicey out there with 100 knot winds and 60 foot waves!) Explain.


Any process that produces ascent over a cyclone center will result in surface convergence and cyclone development. That means the laplacian of temperature advection, differential cyclonic vorticity advection, latent heat release from condensation, friction, ascent in the left exit or right entrance region of an upper jet, ascent along the nose of a low-level jet, and frontogenesis all can act to intensify a storm.


The main difference between the continental and oceanic systems is the reduced friction at the surface over the oceans. This means that over the oceans all the winds are stronger, leading to stronger advections, stronger ascent from frictional convergence at the cyclone center, stronger jets, and stronger frontogenesis (through the stronger cross thermal gradient flows). Reduced friction working in concert with the forcing mechanisms available to all storms is thus the primary factor in contributing to the relatively more intense cyclones over the oceans.


3. (30 pts) On the morning of 16 February 2006 (last Thursday), Milwaukee experienced a band of convective storms embedded within a larger region of synoptic-scale ascent. Soundings indicated that we were positioned on the cool side of a strong surface front, a location that is characterized by a saturated, surface-based inversion. Temperatures were in the mid-thirties Fahrenheit, and we experienced a mix of snow, rain and ice pellets (aka thunderslop). How is this possible?


ItÕs not possible, it was an illusion brought on in a case of mass hysteria.


Okay, it really did happen. HereÕs how. A parcel of air is lifted from the surface by the front. Because it is saturated, it cools more slowly than the dry adiabatic rate, and because it is winter and the air is cold, it is easy to lift it to a level where it is warmer than the surrounding air. Then, it is positively buoyant and convection ensues.


4. (30pts) Suppose that it is early Spring. At mid-day, an intense cold front is approaching Milwaukee from the northwest. Skies are clear in the warm sector, but cloudy above and trailing the surface position of the cold front. The cross-frontal surface flow is strong, with weak winds in the warm sector. Stating any necessary assumptions, predict the evolution of surface temperature, clouds and precipitation at Milwaukee.


Because there is only weak flow in the warm sector, the front will weaken, which will result in the air sinking along the position of the cold front. Clouds will dissipate along the front. Temperatures will be steady through the day, with clear skies. Then, the cold front will pass and the temperature will fall quickly, but less quickly than it would have earlier as the front continues to weaken. Skies will remain clear.