Meteorological Physical Background

Cold fronts and cloudiness: conveyor belts

The physical mechanism by which cold fronts develop is the relative movement of cold air against warm air. The warm air rises over the baroclinic zone separating the two air masses while the cold air moves underneath. According to the moisture content, the rising of air can cause cloudiness and precipitation.

To understand cloudiness or precipitation associated with frontal zones the concept of conveyor is useful. These are streams or relatively narrow stripes of air flowing along tilted isentropic surfaces (Θe · Θw). They are defined as flows relative to the cyclone itself, thus representing the flow of air through the during its movement and evolution (Relative streams).

There are three types of conveyor belts:

  • Warm Conveyor Belt (WCB): an air stream that originates in warm air and carries warm, moist air from low to high levels, usually with a path towards the pole. Characterized by high values of Θe or Θw.
  • Cold Conveyor Belt (CCB): an air stream that originates at low levels ahead of the warm front. It transports cold air from the low and mid-levels, usually towards the SW, and is part of the cloudiness associated with the occlusion cloud spiral. Initially it is cold and drier than the WCB (low values of Θe or Θw).
  • Dry Slot (DS): located west of the WCB and the CCB and transports very dry air that originates in the upper troposphere. This current can enter the circulation system of a low pressure resulting in a "dry tongue" or "dry slot", which is a cloud-free region spiraling around the CCB.
Schematic distribution of conveyor belts. Numbers correspond to pressure levels expressed in hPa.

Cold fronts can be divided into two categories: Kata Cold Fronts and Ana Cold Fronts, which can be described in terms of their conveyor belts. The main feature that distinguishes these types of cold fronts is the orientation of the jet in the middle and upper levels of the troposphere:

  • In Ana Fronts, the jet axis and dry intrusion are parallel to the cloud band which promotes their development behind the cold front surface. The warm air ascends along the front to higher latitudes, and can result in post-frontal rainfall.
  • In the case of Kata Fronts, the jet axis crosses the cloud band. The warm air descends along the front, and can result in rainfall ahead of or along the front.
Schemes of the different conveyor belts associated with Kata Cold Fronts (left) and Ana Cold Fronts (right) in the Southern Hemisphere. CA: cold air, WA: warm air.

While there are not many studies on ana and kata fronts in Argentina, the observational climatology indicates that ana fronts are more common. Moreover, we cannot always distinguish the two types clearly.

Some characteristics of Kata and Ana Cold Fronts are presented below:

Feature Kata Cold Front Ana Cold Front
Cold air
  • Moves slowly relative to the warm air, generating moderate low-level convergence.
  • Moves fast relative to the warm air, generating strong low-level convergence.
Warm Conveyor Belt
  • Ascends parallel to the frontal zone with a forward component higher up.
  • Determines the forward position of cloudiness and precipitation.
  • Ascends parallel to the frontal zone with a rearward component higher up.
Cloudiness
  • Predominates over baroclinic zone and ahead of the surface cold front.
  • The cloud band is tilted rearward of the cold front at the surface, following the slope of the baroclinic zone.
Dry intrusion
  • Descends from highest levels of the troposphere and crosses the front from behind.
  • Restricted to the CCB and tends to dissipate the clouds at high levels.
  • Parallel to the WCB.
  • A well-defined rear edge of the cloud marks the transition between the two relative streams.
Precipitation
  • Mostly ahead of the surface cold front.
  • Mostly behind the surface cold front.



Typical movement of cold fronts over South America

The dynamics of cold fronts in the southern South America is highly influenced by the presence of the Andes mountains. The mountain range extends from equatorial latitudes till approximately 60°S, reaching an average height of over 3000 m.

Topography of South America (Height in km)
At mid-latitudes, synoptic perturbations generally move from west to east. In particular, in southern South America, these perturbations are affected by the presence of the Andes range, blocking their propagation towards the east. Consequently, cold fronts behave differently on different sides of the mountain range as they move to the north.

West of the Andes, cold fronts only reach 30°S at low levels, while to the east they penetrate up to tropical latitudes, which happens more frequently in winter.Garreaud (2000) explains the dynamic processes involved in the high meridional displacement of cold fronts in winter on the east of the Andes. This is mainly due to the presence of an intense pressure gradient produced by the interaction between the migratory post-frontal anticyclone and the low pressure system related to the cold front which moves along the southern coast of Argentina.

Then, as the anticyclone enters the continent, an important ageostrophic flow to the north is developed due to the blocking of the zonal component of the wind on the western slope of the Andes. This generates causes the flow to accelerate towards the north, parallel to the mountain range. In this way, frontal systems crossing the continent on the east side of the Andes are channeled towards the north over the central part of Argentina and may reach subtropical and even tropical latitudes.

Seasonal dynamic of cold fronts

There are differences in the behavior of cold fronts between summer and winter.

a) Summer

  • Strong warm and moist advection (WA) at low levels related to the presence of the South American Low Level Jet, which generates great instability.
  • Weak post-frontal cold advection (CA) which does not produce a notable temperature decrease.
  • Organized convective activity at the leading edge of the cold surface front.

b) Winter
  • Major meridional outbreaks, reaching tropical latitudes in the most intense cases.
  • Important post-frontal cold advection (CA) which generates negative temperature anomalies over the central and northern Argentina.
  • Establishment of a post-frontal anticyclone over the central part of Argentina fostering clear skies and strong radiative cooling that may lead to the occurrence of frost in some cases.
  • Dense stratiform cloudiness over northwest Argentina can form behind the cold front in a southeasterly flow due to adiabatic ascent over higher terrain.

Cold fronts and the interaction with upper level jet

There is a wind maximum (jet stream) related to the cold frontal system. It lies around 250 hPa and is located to the south of the system. This maximum presents transversal circulations as a result of ageostrophic components generated in the entrance and exit regions (four quadrant model, Uccellini and Johnson, 1979). In the entrance region, at high levels, the acceleration of air due to the flow confluence generates ageostrophic wind from the north, creating divergent and convergent zones on the northern and southern side of the jet streak, respectively. This, in turn, helps establishing a direct circulation cell, upward motion on the warm side of the surface front and downward on the cold side. However, in the exit region of the jet streak, deceleration caused by flow diffluence favors a convergence and divergence pattern, opposite to the one at the entrance region. This configuration promotes the establishment of an indirect ageostrophic cell.

Thus, the cold front zone, which is in phase with the high level jet's entrance region, will tend to move towards the north, favored by these ageostrophic circulations (Vera and Vigliarolo, 2000).


Cold fronts and the interaction with SACZ

The activation of the South Atlantic Convergence Zone (SACZ), which usually affects the region during the monsoon season in South America (from the end of October till April), modifies the behavior and dynamic of cold frontal systems which cross this part of the continent.


03 February 2015/12:00 UTC - GOES 13 IR 10.7; green: geopotential height at 200 hPa, yellow: isotachs at 200 hPa

When SACZ is in its active phase, cold fronts may propagate towards the northeast of Argentina and central and south ern parts of Brazil, where they settle. As a new front pushes north, it reinforces the old baroclinic boundary (stationary front), blocking moisture to the north. In the cases in which SACZ is not active, fronts begin to weaken slowly when arriving to southern Brazil, and then they begin to move towards the Atlantic Ocean without ever reaching the SACZ region (Nieto-Ferreira, 2011).

The episodes of increasing or decreasing of convective cloudiness over central Argentina are highly influenced by the existence of a dipolar structure which may be observed in the outgoing long radiation, with one center to the north of the La Plata river and the other over the SACZ. In cases with increased convection over Argentina, the circulation is determined by the presence of a strong anticyclone over southern Brazil which weakens convection over the SACZ, a SALLJ which channels humidity from southern Amazonia towards the region, and an intense south tropical jet at high levels (Diaz and Aceituno, 2003). On the other hand, when strong convection over the SACZ occurs and there is no strong SALLJ over the north of Argentina, convection in the central part of the country is not expected to be intense.

Summer conditions with an active SACZ (left) and an inactive SACZ (right). Convection is reinforced or inhibited over northeastern Argentina according to how the warm humid air from Amazonas is channeled.