Why fronts die
Some notes to explain the processes involved in the weakening (or 'frontolysis') of otherwise active frontal zones.
For significant precipitation to occur, two conditions need to be satisfied.
- There must be a mechanism to produce upward motion through a deep layer of the troposphere.
- There must be sufficient moisture in depth - and for mid-latitude baroclinic systems (i.e. fronts), this must extend for some way above the zero degree (or freezing) level - i.e. clouds should have supercooled water droplets and ice crystals coexisting.
Taking each necessary condition in turn:
1. Upward motion:
In the atmosphere, vertical (upward) motion, i.e. ascent, is achieved in two ways.
(a) Release of convective potential. [ Showery type ]
(b) Dynamic/warm advection forcing. [ Mass ascent type ]
Type (a) is often a factor in frontal rain situations, and can be the means of enhancing precipitation within a frontal belt, but it is type (b) that we are primarily concerned with here, and which I will discuss.
Dynamic forcing is achieved by, for example, ascent forward of a short-wave trough; broad scale ascent in the warm/entrance or cold/exit regions of a strong and well defined jet stream; or a jet streak moving quickly through the main jet stream aloft. It can also be achieved by strong/focused warm advection, indeed in many frontal situations, it is usually a combination of the two (dynamic + thermal warm advection) that needs to be considered.
2. Moisture availability:
There needs to be a source of moisture of course, but perhaps more importantly for significant rainfall/snowfall, there needs to be a continuous supply of moisture, via the low level warm 'conveyor' model. In this model, warm/humid air enters a system at low levels ahead of the driving upper trough, and rises slantwise upwards and either forwards (forward sloping front ===> split upper cold ====> dynamically weak surface cold/kata front), or rearwards (rearward sloping front ====> dynamically intense surface cold/ana front), cooling by adiabatic expansion, and feeding/maintaining the thick layer cloud structure in the process.
Any change that cuts off the supply will cause the front to die; precipitation will form from cloud water/ice droplets, which are not replenished, and the total water (vapour+liquid+ice) content will fall - leading to a decrease in precipitable water content.
(Note that the warm conveyor model also allows for the transfer northwards -- in the northern hemisphere -- of warm air and westerly momentum. These are also important for maintenance of a well defined front, with vorticity and thermal contrast/baroclinicity considerations.)
Looking now at a typical example of cold frontal events, where the cold front is very active over NW British Isles, depositing large amounts of rainfall/snowfall, but by the time it gets to the south of England, it is relatively weak, despite having a good 'Norwegian model' structure across the surface front: wind shift/temperature drop etc.
*** Active: In the NE Atlantic, a well marked upper trough is moving eastwards with strong ascent on its forward side, and with the low level flow from the SSW, with a long maritime fetch, gathering and transporting copious amounts of water vapour off the relatively warm North Atlantic, the combination of upward motion, plus ready availability of vapour into the cloud decks, produces marked rainfall western and northern areas, enhanced over mountains by orographic forced lifting, and encouraging the release of embedded convective potential - if present. The driving upper trough is relatively sharp.
*** In-active: By the time the front arrives across SE Britain, the driving upper trough has 'relaxed' away north-eastwards, removing the source of strong upward motion. At the same time, the act of relaxation implies descent (rising contours), which in turn leads to subsidence warming through a great depth of the troposphere, warming of the cloud, and evaporation of cloud layers, diminishing the rainfall amounts, and squashing the significant sub-zero degC cloud potential.
In addition, and equally importantly, with no ascent through a deep layer, there is no means to replenish the moisture content as the humidity is used up in the precipitation process. A combination of descent (due trough relaxation), and non-resupply, leads to a decaying frontal structure -- the front 'dies'. All we are left with is the low level cloud structure: thick stratocumulus and stratus, with thinning altocumulus and altostratus on top, where precipitation forms primarily by coalescence - insufficient for 'proper' rain, but can produce 'drizzly' conditions. However, even these 'weak' fronts can have some elements of embedded instability to catch out the unwary forecaster!
Now, why does all this happen. Upper troughs don't maintain their character for ever. In the particular case under consideration, the trough is confluent -- this means that the wind speeds in the leg ahead of the trough axis were markedly stronger than those in the leg behind the trough. Under the right conditions, such troughs lose identity quickly. Warm air ahead of a following Atlantic system floods forward into the trough, with warming out of the cold air that caused the trough to form, and maintain its shape. Warm the air out in this case, and the trough loses its shape - it relaxes. Whether a trough relaxes or not depends upon the relative position of one trough to another, the exact arrangement of isotherms in the westerly flow, and developments upstream in the wave-train: hence the interconnectivity of synoptic meteorology.
This process can usually be seen as a 'splitting front'. When the trough is sharp, the rain area is usually closely associated with the surface frontal position. As the trough runs ahead and relaxes ("over-runs" the surface front, hence an 'over-running trough'), the upper cloud moves more quickly than the surface front/associated low level cloud, with incursion of dry/descending air over the top of the original front. On satellite imagery, particularly Infra Red (IR), a 'split' can be seen: the high/cold cloud of the upper front (ana-front) bearing the rain shears away eastwards, as the greyer/warmer cloud of the trailing cold front follows slowly behind -- often lying from NE to SW -- slowing further as the mass descent leads to rising pressure, and the gradient across the front dies - this cold front, the one drawn on surface charts, is now a cold kata front.
The dew point/air temperature across the front can be still marked, but all it really marks now is the low level discontinuity. It has no relevance to the upper structure, where the real action is, and which has gone marching away east, or east-northeastwards into the general upper flow. The best way to judge how potentially active a front is is to look at thickness (TTHK) charts.
There are other rules developed over time, before the advent of NWP models, which are still useful. For example, consider the lower-tropospheric flow as characterised by the 700 hPa wind direction. If the flow is between 180 and 220 degrees, larger amounts of rain are likely than if the flow is from 230 or further to the west. This ties in with the relaxing upper trough model: sharp troughs have a high southerly component to the 700 hPa wind; relaxing troughs have an increasingly westerly component.
Why a particular train of troughs relaxes before reaching UK/near continent, or whether such troughs do the reverse, and sharpen, is all tied up with the broad-scale flow, and consideration of the jet patterns: strength, direction, location and variability. These are governed by large-scale patterns of oceanic/continental temperature contrasts, and inter-action of airflow with significant mountain chains: e.g. the Rockies. If we change the temperature patterns of the globe, we will change the factors governing the jet stream, and perhaps change the distribution of mid-latitude rainfall; but that would be the subject of another missive.
In passing though, the southeast of England has always suffered from highly variable rainfall, and analysis of fronts crossing the UK shows that of the two types (ana/active and kata/weak), the latter is (and perhaps always has been), most common. This may have something to do with frictional effects as fronts try to cross the land mass of the British Isles -- leading to disassociation of the lower and upper frontal structures. Certainly split fronts account for more occasions of the cold frontal type than many people (including meteorologists) realise ... its just that you don't hear about them on the BBC!