101: Conveyor Belt Model II

This is the image I ended with last time. It is an idealized model of a mid-latitude cyclone (MLC) as described by the Conveyor Belt Model (CBM), a concept to help resolve many of the issues that have arisen with the Norwegian Cyclone Model (NCM). Developed over the past few decades, the CBM considers the atmosphere in three dimensions, as opposed the NCM which mainly depicts the atmosphere in two dimensions. The key difference between the NCM and CBM is that the latter downplays the role of fronts as driving mechanisms of the system; instead it emphasizes the movement of the air masses involved in the system. The CBM also identifies several key air flows within the air masses referred to as conveyor belts, hence the name of the model. There are three belts (which are all depicted above in color): the Dry Tongue (green), the Cold Conveyor Belt (blue), and the Warm Conveyor Belt (red). It is these air flows that help define the fronts and also account for some of the features the NCM missed.

The Dry Tongue

This flow is responsible for much of the relative lack of cloudiness behind the cold front in the cold air mass. The dry tongue is a jet of air from high in the troposphere that dives down in the general direction of the low center, becoming very dry in the process. Right as nears the low center, the flow abruptly ascends back up and joins the upper level flow, which is generally towards the east or northeast. This dry air flow is often easy to locate on visible satellite images based on the effect it has on cloud cover. Being a dry jet, this conveyor belt inhibits cloud formation, thus it will often scour out higher level clouds in the large cloud shield associated with the MLC. This effect is often noticeable in imagery, since the clouds adjacent to the dry flow will often cast a noticeable shadow on the low clouds beneath the dry air, as seen in the visible satellite image below of a developing MLC in the Northwest Pacific on March 5, 2013.

For the other two conveyor belts I’ll be using data from the October, 2005 storm that was introduced in the first “101: Conveyor Belt Model” post. The most important product of this data are air parcel trajectories. This type of display, show here in animations, uses wind data to calculate the path an arbitrary parcel of air would take over some period of time. Following a few air parcels allows the general air flow to be inferred, and thus the orientation of the cold and warm conveyor belts can be identified. All of these animations feature a few parcels that begin at lower levels, although each animation begins at a different time and are of different durations in order to best capture air flow that best exemplifies the feature being discussed. On the “ground” of each animation is the corresponding infrared satellite image. The MODIS image (reproduced below) of the storm that was used previously along with the corresponding surface map shows the relative placement of the fronts, thus a basic sense of the locations of the fronts can be estimated throughout most of the animations. The colors in these animations gives a sense of the temperature of the air, so air near the surface will be warmer (red and orange) and the air high up will be much colder (green and blue).

The Cold Conveyor Belt

Its best to being this explanation by showing the trajectories of air in the cold air mass near the cold front, which, for the purpose of this discussion, is defined in the same way as it has been traditionally defined by the NCM. Notice how most of the parcels remain relatively low and all of them stay just behind the dense line of clouds, which happens to contain the cold front.

Starting from an earlier time shows air moving roughly towards the center of the low from the east. This air is quite chilly and like the air near the cold front, it stays pretty low. An important detail is that all of these parcels remain north of the large fan of clouds that contain the warm front. In fact, this conveyor belt generally slips in right under the warm front and just north (or nearly north) of the surface warm front.

The Warm Conveyor Belt

It is the depiction of the warm air mass that really gets expanded upon in the CBM. In the animation below, air parcels have been placed directly in front of the cold front, which is the dense line of clouds. The display itself is looking roughly to the south and the cold front is moving from west to east, with the cold air mass on the right (west). Notice how at first the parcels seem to essentially stay at the same location relative to the cold front, but ascend rather abruptly. Because this air is from the warm air mass, it is less dense than the cold air behind the cold front, so it is being forced upward. Eventually the parcels level off as they reach an altitude in which there is no longer a well-defined front and begin to track above the cold air mass. This part, at least, is pretty consistent with the NCM.

This final animation shows the trajectory of warm conveyor belt air parcels that begin the warm air mass and ascend up the warm front. It is this flow that might be the most significant change made by the CBM. In the classic NCM, the warm front is usually depicted as the boundary between cool air to the north and warm air from the south gliding over the dense cool air. Such a definition tends to paint the warm front as a relatively low energy boundary, after all, there is no cold air plowing through the warm air, it is simply sliding up a “ramp”. How things have changed. The CBM’s depiction of the warm air flow near the warm front is less of a glide, and more like a rocket. In this new concept, the air in the warm conveyor belt abruptly ascends as it reaches the warm front and levels back off at a very high altitude. From here, it follows the upper level flow (i.e. jet stream) along with the other two conveyor belts, although they are at a somewhat lower altitude.

To summarize all of this, I’ve included the image below. This is the same model that was used at the beginning of the post, but in a different view, and with some of the features labeled.