Cloud Patterns Associated with Cold Fronts in Central Argentina

Appearance in Satellite Data

Different types of clouds associated with cold fronts in southern South America can be distinguished by the combined use of information given by the different channels of the GOES-E satellite.

• Visible (VIS): thick frontal clouds are very bright white.
• Infrared (IR): areas with convective clouds are shown in brighter tones than clouds at lower levels which are in shades of grey.
• Water Vapor (WV): the frontal zone is seen as a darker band associated with dryness in the mid- and high levels of the atmosphere. At the leading edge of this band, there are cloud areas of greater thickness which are shown in lighter tones.
• RGB: the advantage of these images is that different types of clouds can be identified easily. Yellow colors show low level clouds. Thick clouds are shown in white, bright shades and the high level translucent cirrus are shown in shades of blue.

Cloud patterns associated with cold fronts differ between the warm and cold seasons. Two examples are given below, to compare the cloudiness in each case. The schematics show the general features and do not necessarily correspond fully to the case studies presented.

a) Summer

There are thick convective clouds ahead of summer cold fronts, generally due to the occurrence of Mesoscale Convective Systems (MCS) when the South American Low Level Jet (SALLJ) is present.

20 December 2014/17:45 UTC - GOES 13 VIS 0.65 image	20 December 2014/17:45 UTC - GOES 13 IR 10.7 image
20 December 2014/17:45 UTC - GOES 13 WV 6.75 image	20 December 2014/17:45 UTC - GOES 13 RGB image (0.65, 0.65 and 10.7)

The following animation shows the passage of a cold front over Argentina on 6 February, 2014. It shows how the cold front reaches Argentina from the south Pacific and moves rapidly to the east towards the Atlantic Ocean.

Press "Play Button" to see the loop; 5 February 00 UTC - 6 February 21 UTC - GOES 13 IR 10.7

b) Winter

Cloudiness related to cold fronts in winter possesses the following characteristics:

• The presence of extensive bands of stratiform cloudiness with a NW-SE orientation.
• In some cases deep convection is embedded in the stratiform clouds.
• When cold fronts reach lower latitudes (~20°S), the persistence of low-level clouds (Stratus) is frequent in the northwest of Argentina, close to the eastern slopes of the Andes mountain range.

05 July 2014/14.45 UTC - GOES 13 VIS 0.65 image	05 July 2014/14.45 UTC - GOES 13 IR 10.7 image
05 July 2014/14.45 UTC - GOES 13 WV 6.75 image	05 July 2014/14.45 UTC - GOES 13 RGB image (0.65, 0.65 and 10.7)

The following animation shows the passage of a cold front over Argentina on 22 August, 2013. In this case, it can be seen that the cold front reaches further north than in summer, up to Paraguay and Bolivia, triggering deep convection around 00 UTC on 23rd August.

Press "Play Button" to see the loop; 22 August 00 UTC to 22 August 21 UTC - GOES 13 IR 10.7

c) Cold air cloudiness behind Argentinean cold fronts

There are often open cell-type convective clouds within the cold front and behind it. These clouds mainly affect the coastal regions of Argentina in winter, bringing ice pellets or sleet, snow or rain showers.

	26 July 2014/18.00 UTC - Aqua/MODIS RGB image (0.65, 0.56 and 0.47)

The following time-lapse shows the cloud top temperature development of the open cell cloud tops over Argentinian coast on 11 September, 2015.

Press "Play Button" to see the loop; 22 August 00 UTC to 22 August 21 UTC - GOES 13 IR 10.7

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 (Θ_e · Θ_w). 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 Θ_e or Θ_w.
• 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 Θ_e or Θ_w).
• 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:

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.

Key Parameters

• Sea level pressure: frontal trough and associated low pressure system and post-frontal anticyclone.
• Advection of equivalent potential temperature (EPT) and humidity convergence at 850 hPa: maximum values of both variables at the leading edge of the cold surface front.
• Thermal front parameter (TFP): maximum value of thermal front parameter in the frontal zone.
• Cyclonic vorticity advection at 500 hPa: maximum values ahead of the trough.
• Isotachs and streamlines at 250 hPa: maximum wind associated with the jet stream at high levels. Jet streak orientation is mostly parallel to the CF cloud band

Sea level pressure

	06 February 2014/00:00 UTC. GOES 13 IR 10.7 image; magenta: sea level pressure

Advection of equivalent potential temperature and humidity convergence at 850 hPa

	06 February 2014/00:00 UTC. GOES 13 IR 10.7 image; magenta: Equivalent Potential Temperature at 850 hPa, yellow: humidity convergence at 850 hPa (gr/kg day), green vector (arrows): wind at 850 hPa.

Thermal Front Parameter

	06 February 2014/00:00 UTC. GOES 13 IR 10.7 image; green: equivalent thickness, blue: TFP.

Cyclonic vorticity advection at 500 hPa

	06 February 2014/00:00 UTC. GOES 13 IR 10.7 image; cyan: geopotential height of 500 hPa, green: positive vorticity advection at 500 hPa, blue: negative vorticity advection at 500 hPa.

Isotachs and streamlines at 250 hPa

	06 February 2014/00:00 UTC. GOES 13 IR 10.7 image; green: streamlines at 250 hPa, yellow: isotachs at 250 hPa.

Typical Appearance In Vertical Cross Sections

• Equivalent potential temperature: maximum gradient tilted downward in the frontal zone.
• Frontal slope: depends on the temperature contrast of air masses on both sides of the cold front. A higher/lower thermal contrast means lower/higher slope.
• Relative humidity: high on the front slope and low behind it.
• Temperature advection: maximum warm advection on top of the frontal zone, cold advection behind and below the frontal zone.
• Divergence: maximum surface convergence ahead of the front.
• Upward movement: maximum vertical motion on top of the frontal zone, these being higher in summer than winter. Downward motion below the frontal zone.
• Brightness temperature (IR images): minimum values (below -20°C) related to cloudiness in the frontal zone. In the region of deep convection the values are below -60°C. In the stratiform cloudiness values range between -30 and -40°C.

The following examples show two situations associated with the presence of cold fronts in central Argentina. The main difference is the steepness of the frontal zone slope, which is determined by the thermal contrast between both air masses (Margules, 1906). The slope of the cold fronts in winter/summer could be shallow/steep because of the high/low thermal contrast.

6 February 2014/00.00 UTC - GOES 13 IR 10.7 image	10 September 2015/00.00 UTC - GOES 13 IR 10.7 image

Isentropes and Relative humidity

	Lon: 63°W. Equivalent potential temperature (K) in black, relative humidity (%) in blue and brightness temperature (°C) in yellow. 6 Feb 2014/00.00 UTC.
	Lon: 63°W. Equivalent potential temperature (K) in black, relative humidity (%) in blue and brightness temperature (°C) in yellow. 10 Sep 2015/00.00 UTC.

Temperature advection

	Lon: 63°W. Equivalent potential temperature (K) in black, temperature advection (10 -4 °C/s) in red and brightness temperature (°C) in yellow . 6 Feb 2014/00.00 UTC.
	Lon: 63°W. Equivalent potential temperature (K) in black, temperature advection (10 -4 °C/s) in red and brightness temperature (°C) in yellow . 10 Sep 2015/00.00 UTC.

Divergence

	Lon: 63°W. Equivalent potential temperature (K) in black, divergence (10-5 s-1) in magenta and brightness temperature (°C) in yellow. 6 Feb 2014/00.00 UTC.
	Lon: 63°W. Equivalent potential temperature (K) in black, divergence (10-5 s-1) in magenta and brightness temperature (°C) in yellow. 10 Sep 2015/00.00 UTC.

Vertical motion

	Lon: 63°W. Equivalent potential temperature (K) in black, omega (Pa/s) shaded and brightness temperature (°C) in yellow. 6 Feb 2014/00.00 UTC.
	Lon: 63°W. Equivalent potential temperature (K) in black, omega (Pa/s) shaded and brightness temperature (°C) in yellow. 10 Sep 2015/00.00 UTC.

Temperature advection

Weather events are highly variable and can differ from season to season. Presented here are weather events related to the passage of a cold front during summer and winter in central Argentina.

Summer

Winter

6 February 2014/00.00 UTC - GOES 13 IR 10.7 image; weather events (green: rain, blue: drizzle, red: thunderstorm, yellow: fog, brown: dust, black: no actual precipitation or present weather report)	10 September 2015/00.00 UTC - GOES 13 IR 10.7 image; weather events (green: rain, blue: drizzle, red: thunderstorm, yellow: fog, brown: dust, black: no actual precipitation or present weather report)

6 February 2014/00.00 UTC - SYNOP surface temperature report	10 September 2014/00.00 UTC - SYNOP surface temperature report

References

General Meteorology and Basics

• Díaz, A. and Aceituno, P., 2003: Atmospheric Circulation Anomalies during Episodes of Enhanced and Reduced Convective Cloudiness over Uruguay. J. Climate, 16, 3171-3185.
  http://dx.doi.org/10.1175/1520-0442(2003)016<3171:ACADEO>2.0.CO;2
• Garreaud, René D., 2000: Cold Air Incursions over Subtropical South America: Mean Structure and Dynamics. Mon. Wea. Rev., 128, 2544-2559.
  http://dx.doi.org/10.1175/1520-0493(2000)128<2544:CAIOSS>2.0.CO;2
• Margules, M., 1906: Über Temperaturschichtung in stationär bewegter und ruhender Luft. Met Z, Hann-Bd.
• Nieto-Ferreira, R., Rickenbach, T. M. and Wright, E. A., 2011: The role of cold fronts in the onset of the monsoon season in the South Atlantic convergence zone. Q. J. R. Meteorol. Soc. 137: 908-922. doi: 10.1002/qj.810
• Uccellini, L. W., and Johnson, D. R., 1979: The Coupling of Upper and Lower Tropospheric Jet Streaks and Implications for the Development of Severe Convective Storms. Mon. Wea. Rev., 107, 682-703.
  http://dx.doi.org/10.1175/1520-0493(1979)107<0682:TCOUAL>2.0.CO;2
• Vera, C. S. and Vigliarolo, P. K., 2000: A Diagnostic Study of Cold-Air Outbreaks over South America. Mon. Wea. Rev., 128, 3-24.
  http://dx.doi.org/10.1175/1520-0493(2000)128<0003:ADSOCA>2.0.CO;2