Key Parameters

All the NWP products provided here were generated by using ERA-Interim reanalysis and these data are available from The parameters used to describe tropical lows are:

  • Equivalent potential temperature (ET)

    Equivalent potential temperature is a measure of the moisture content of the surface air as well as the environmental lapse rate. The change of ET with height is sued to determine if the atmosphere is convectively (potentially) unstable. If ET decreases with height the atmopshere is convectively unstable.
  • Precipitable water

    Precipitable water is often used to determine atmospheric moisture content. Precipitable water (W) is the total mass of water contained in a vertical atmospheric column if all the water vapor in the column were to condense.
  • Relative vorticity

    Vorticity is a measure of the rotation of a fluid and is said to be the curl the wind velocity field. Relative vorticity in the isobaric coordinate system is the horizontal component of vorticity vector (a scalar). In the Southern Hemisphere low pressure systems have negative vorticity and high pressure systems positive vorticity. The advection of upper tropospheric cyclonic relative vorticity is associated with the development of surface low pressure systems.
  • Total static energy (TSE)

    An alternative to the equivalent potential temperatures is the total static energy (TSE) (also referred to as moist static energy). TSE is defined as the total energy in an atmospheric parcel of air, neglecting the small amount of kinetic energy. TSE is computed by:

    where CP is the specific heat of dry air, z is the geopotential height in meter (gpm), L the latent heat of condensation and q the water vapour mixing ratio. The first term on the right-hand side of the equation represents the enthalpy, the second term the geopotential and the third term the latent energy.

    The vertical profile of TSE can also be used to investigate the potential instability of the atmosphere. In this image TSE values decrease to the mid troposphere.

    A typical vertical profile of total static energy associated with deep convection. The dashed line is the upward projection of the surface energy value.
  • Wind divergence

    Horizontal wind convergence describes the inflow of air while divergence describes the outflow of air. Negative values are convergence and positive values divergence
  • Wind shear

    Magnitude of the vector difference between the components of the wind at two pressure levels

Continental Tropical Lows

The Model for the Identification of Tropical Weather Systems (MITS) was developed with two aims in mind. First a set of criteria is proposed to identify the tropical character of the CTL and 15 January 2013 1200 UTC MSG IR 11-13 Image secondly a list of atmospheric variables are suggested which aids in the identification of heavy rainfall areas associated with these CTL. Several very heavy rainfall events over southern Africa has been associated with CTL. Some examples of these flood events are: the Free State floods in 1988, the floods over the north-eastern interior of South Africa in Mozambique in 2000 and the heavy rainfall in the Kruger National Park in January 2013.

This conceptual model deals with Continental Tropical Lows (CTL) which occur from late December to early March over the southern sub-continent of Africa. These lows are very slow moving and can exist for up to a week and may cause very heavy rainfall.

MITS consists of the 5 criteria listed below and these will be illustrated by using ERA data and by investigating a weather event which occurred from 17-22 January 2013 over southern Africa.

Criteria of MITS

  1. The low pressure system must stand upright from 850 to 400hPa and should be displaced by a ridge of high pressure at the 200hPa level.
  2. A core of high average column temperatures should be present in the 500 to 300 hPa layer above or near the surface low pressure system.
  3. Precipitable water values in the 850 to 300 hPa layer should exceed 20 mm and be in the same geographical position as the 200hPa ridge or high-pressure system and must be accompanied by upper tropospheric wind divergence.
  4. Average total static energy (TSE) in the 850 to 300 hPa layer should exceed 330 x 103 J * kg-1.
  5. Upward motion to be present from 700 to 400hPa, the atmosphere should be conditionally unstable up to 400hPa and precipitable water values should also exceed 20 mm.

1. Vertical integrity of low and high pressure systems

On the 19th of January 2013 the low pressure was standing upright from 850-400 hPa and above that a strong high pressure system was located. On the IR 10.8 image the cyclonic circulation into the low situated over Botswana is clearly visible but also the anticyclonic curvature of the cirrus stretching out into the Indian Ocean.

The 850 (shaded), 700 (dotted) and 500 (solid) hPa geopotential heights on the 19th of January 2013. The low over Botswana is clearly visible at all levels
19 January 2013 1200 UTC - MSG IR 10.8 Image and 700 hPa winds (in knots)

The 200 hPa high pressure systems is located in the same position as the low at lower levels over Botswana. Mid-level cloud (magenta shades) is visible over southeastern South Africa with a band of cirrus above that stretching into the Indian Ocean. This cirrus lies on the southern side of the upper level high. Active and high and fast developing convective cloud is present over western Zimbabwe (orange colours) with older decaying convective cells over northern South Africa.

The 400 (contour) and 200 (shaded) hPa geopotential heights on the 19th of January 2013. The low is visible as a trough at 400 hPa with a 200 hPa high above that.
19 January 2013 1200 UTC - MSG Day Microphysics RGB Image and 300 hPa winds (in knots)

This figure indicates that very little directional wind shear exists in and near the low and therefore the thermal wind will be close to zero indicating the barotropic nature of this low. To further illustrate this the 850-300 hPa wind shear was less than 5 knots over central Botswana. Note the large wind shear values in the band of cirrus stretching out over the Indian Ocean.

Longitudinal vertical cross section (15 °E to 35 °E) of relative vorticity (values multiplied by 105) at 21 °S and winds (in knots). The green shades are anticyclonic vorticity and the yellow shades cyclonic vorticity. The tropical low standing upright with height is clearly visible at 25 °E.
The 850-300 hPa wind shear (m/s) on the 19th of January 2013. The wind shear values was less than 5 m/s over central Botswana.

2. Average column temperatures in the 500 to 300 hPa layer

Due to the large amounts of latent heat release in the tropical low a warm core of temperatures exists in the 500-300 hPa layer above the surface low.

The average 500-300 hPa temperature field (shaded) shows a warm core in the same position of the 850 hPa low

Temperatures at 500 hPa was as warm as -2 C in the center of the low with weak temperature gradients over most of Botswana. To the west and southwest of the tropical low temperatures decrease rapidly and temperature gradients increase indicating a change in the properties of the airmass. On the Airmass RGB the brown colors to the west of the low confirms that drier air is present in the west with moist tropical air over the eastern parts.

The 500 hPa temperatures (contours) and the magnitude of the temperature gradient (shaded) * 105 K/m
19 January 2013 1200 UTC - MSG Airmass RGB Imag

3. Moist diverging high in the upper troposphere

Values of precipitable water, greater than 20 mm in the same geographical position as the 200hPa high-pressure system helps to indicate a tropical system in MITS. The WV 6.2 image indicates the high water vapor content of the higher levels of the Tropopause over Botswana and Zimbabwe.

The precipitable water (850-300hPa) shaded, 300 hPa wind divergence (dotted) and 200 hPa geopotential heights on the 19th of January 2013. A moist divergent high was present over Botswana.
19 January 2013 1200 UTC - MSG Airmass RGB Image and 300 hPa Geopotential heights. The upper level high over Botswana has high levels of humidity.

The Carnot engine is not quite visible in this vertical cross section of vertical velocity. What is evident however, is the upward motion through the entire troposphere in and east of the low and subsidence flanking the strong upward motion. In the vicinity of the low wind convergence is present from the surface to 300 hPa but this is replaced by strong divergence close to 200 hPa.

Longitudinal vertical cross section (15° E to 35° E) of vertical velocity at 21° S and winds (in knots). The blue shades are upward motion and the red shades downward motion.
Longitudinal vertical cross section (10° E to 40° E) of divergence (brown shades are divergence and magenta shades convergence) and vertical velocity (contours) at 14° S on 8 January 2013 at 0000 Z.

4. Average total static energy (TSE)

Values of precipitable water,In MITS the average TSE is computed for the 850 to 300hPa layer.Experimental results indicated that TSE values greater than 330 x 103 J*kg-1 is indicative of tropical circulation over Southern Africa.In this example the TSE values over Botswana is higher than 444 J/kg. The vertical profile of TSE indicates the convective instability of the atmosphere over the continent and close to the center of the low. Also note the influence of the colder drier mid-level air over Namibia causing the TSE values to decrease significantly.

The average column TSE values (divided by 1000) on the 19th of January 2013. Values over Botswana are higher than 440 J/kg
Longitudal vertical cross section (15° E to 35° E) of TSE at 21° S. The lower values in the mid troposphere is indicative of a potentially unstable atmosphere.

5. Deep cumulus convection and vertical motion

In an attempt to predict areas where heavy rainfall is likely from continental tropical weather systems some of the critical values of the parameters associated with MITS are isolated. MITS identifies heavy rainfall areas where the following criteria co-exist

  • Precipitable water values > 20 mm
  • Average TSE values > 335*103 J/kg
  • Level of potential instability up to 400 hPa
  • Upward motion should exist from 700-400 hPa
The precipitable water (850-300hPa) larger than 20 mm (shaded) and 300 hPa wind divergence on the 19th of January 2013.
Average column TSE values (divided by 1000) larger than 335 J/kg (shaded) and LPI up to at least 400 hPa on the 19th of Janu
Upward motion present in the 850-400 hPa column.
The shaded grids shows where heavy rainfall is predicted.

MITS was capable of identifying the heavy rainfall over southeastern Botswana but failed to identify the very heavy rainfall over northern South Africa. The rainfall in these areas were most likely enhanced by the topography of the area.

Daily TRMM rainfall for 19 January 2013. For more information on TRMM data see: Acker and Leptoukh (2007).