Meteorological Physical Background

Introduction

It is only during late summer when the ITCZ moves southwards to the northern extremes of South Africa that tropical easterly lows and continental tropical low pressure systems develop. In this section a brief background about each of these three synoptic scale phenomena is provided. First some general information about southern Africa and the appearance of convective cloud in a low shear environment is discussed.

The physical background of the three synoptic scale phenomena is provided separately.



Southern African location map and topography

Map of Africa: © 2006 UNEP/GRID-Arendal

Southern Africa is characterized by a moderately elevated plateau, for the greater part rising to over 1000 m above sea level and to more than 1500 m over extensive areas. For this reason it is the practice in the forecasting offices in South Africa to convert surface pressure measured at synoptic weather stations to the height of the 850hPa pressure level measured in geopotential meter (gpm). In this conceptual model the surface therefore refers to the 850hPa pressure level.



Convective cloud in low shear environments

Convective cloud which form in low shear environments do not have an anvil and appear circular. In A large wind shear exists and an anvil develops while in B no wind shear exists and the clouds develop without an anvil and can grow vertically to the Tropopause.

http://faculty.luther.edu

Compare the severe storm RGB on the 12th of January 2013 (on the left) with the 30th of November 2013 (on the right). In January over southern Botswana in a low shear environment the convective cloud had a circular structure while in November in a high shear environment over Lesotho the anvil is clearly visible. The vertical profile of the winds (in knots) at King Shaka International Airport in KwaZulu-Natal shows the wind shear as well as the relatively strong winds in the mid troposphere on the 30th of November 2013.



Continental Tropical Lows

From time to time the easterly waves develop into CTL and the Model for the Identification of Tropical Weather Systems (MITS) deals with some of the characteristics of these lows.



Overview

MITS consists of five circulation and thermodynamic criteria. They are

  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.

A barotropic atmosphere is one in which the density is a function of pressure alone. This means that isobaric surfaces are also surfaces of constant temperature. If the horizontal gradient vector of the average column temperature is zero then the thermal wind will be zero. The thermal wind is defined as the vertical shear of the geostrophic wind. When the thermal wind is equal to zero the geostrophic wind will not change with height. This also means that in a barotropic atmosphere the horizontal component of the gradient of geopotential remains constant with height. Synoptic scale high- and low-pressure systems will in this ideal situation “stand upright” with height.

Barotrophy provides very strong constraints on the motion in a rotating fluid and is in fact never achieved in the atmosphere. Nevertheless in a tropical atmosphere the circulation will tend towards this ideal situation. The vertical integrity is also detailed up to 400 hPa, of low-pressure systems, associated with the precipitation zone of equatorial wave disturbances in the western Pacific Ocean. MITS was designed to look for a low-pressure system, which stands approximately upright with height from the surface (850hPa) up to the 400hPa level.

In the real tropical atmosphere strong surface and middle tropospheric convergence occurs in association with the "upright" low-pressure system. This convergence in turn results in upward motion and if adequate water vapor is available, convective cloud or so-called "hot towers" will develop. The process by which the hot towers act as energy tubes through which energy from the lower troposphere is transported to the upper troposphere where it is distributed horizontally by the upper air divergence. The condensation releases a large amount of latent heat, which is in turn responsible for above-normal upper tropospheric temperatures. The geopotential thickness of a layer is directly proportional to the average column temperature with the result that an upper tropospheric high-pressure system forms above the surface low. The latent heat release therefore, quickly results in a warm core (high pressure) developing above the lower level low.

This process where the latent heat release causes warm temperatures and a upper tropospheric high is only known as Convective Instability of the Second Kind (CISK). CISK is a cooperative interaction between small-scale cumulus convection and a larger-scale disturbance where:

  • The large-scale convergence organizes the cumulus convection
  • Condensation heating in the clouds in turn supplies energy to the larger-scale system



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

As detailed above latent heat release leads to above average column temperatures and an upper tropospheric high above the surface low pressure system. The average column temperatures in the 500 to 300 hPa layer are fundamental for the identification of tropical weather systems. No specific threshold is set for the average 500 to 300 hPa temperatures in a tropical circulation system but the column temperatures should be compared to the surrounding areas in order to identify a warm cored system. It should be remembered that this field alone does not identify a tropical circulation system as an upper tropospheric high pressure system will necessarily be associated with warm temperatures. In addition the absence of a horizontal temperature gradient of the 500 to 300 hPa average column temperatures indicates a barotropic atmosphere. However true barotrophy never occurs in the real atmosphere. A weak upper tropospheric temperature gradient can nevertheless be used to identify a "tropical atmosphere".



3. Moist diverging high in the upper troposphere

A fundamental component in the development of the tropical upper tropospheric high-pressure system is the release of latent heat through condensation of water vapour. Maintenance of an upper tropospheric high therefore requires a high moisture content below the upper high pressure system. 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 Values of precipitable water, greater than 20 mm in the same geographical position as the 200hPa high-pressure system indicate a tropical system.

Dines compensation requires that the upper tropospheric high maintain significant horizontal wind divergence a few kilometers below the tropopause i.e. the 200 to 300hpa levels. This divergence removes air horizontally in the upper troposphere. It is because of this wind divergence that the vertical integrity of the tropical system can be maintained. Without the upper tropospheric divergence the system would fill up because of the lower level convergence and latent heat release. This process is known as the Carnot Engine and the following schematic provides an explanation for the process.


(1) isothermal inflow of near-surface air (A-B)
(2) moist adiabatic ascent in the eye of the Carnot cycle - all convection and outflow just below the tropopause (B-C)
(3) sinking of cooled air in the environment far from the tropical cyclone center (C-D). To close the system
(4) the cooled air is assumed to return to the tropical cyclone environment adiabatically (D-A)


4. Average total static energy (TSE).

In an atmosphere where higher than normal upper tropospheric temperatures occur, as is the case of an air mass with tropical characteristics, the contribution of the enthalpy will increase the TSE values. Higher geopotentials, generally found in the upper troposphere in tropical weather systems will also increase the TSE values. The high moisture content found in the lower troposphere in tropical weather systems increases the TSE values through the latent heat term. 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



5. Deep cumulus convection and vertical motion.

The interior of South Africa is approximately 1 500 m above sea level and this means that the 700hPa level is approximately 1 500 m above the land surface and can still be considered representative of the lower troposphere. The 700hPa level is also close to the summer convective cloud base level. An important function of MITS is to isolate areas where upward motion exists from 700hPa to 400hPa.

The vertical profile of TSE values is used to determine the potential instability of the atmosphere. A potentially unstable atmosphere is required for deep cumulus convection. To determine the level of potential instability (LPI) the following formula is used:

LPI=TSE(850)-TSE(i)

where i varies between 700hPa and 400hPa. For the atmosphere to be considered conditionally unstable up to 400hPa (a requirement of MITS) the LPI must remain positive for i = 700hPa to 400hPa.

Heavy rainfall from continental tropical weather systems is possible when:

  • The precipitable water values in the troposphere should exceed 20 kg * m-2.
  • Wind divergence must be present in the upper troposphere.
  • Average TSE values in the troposphere should exceed 335 * 103 J * kg-1 .
  • The atmosphere must also be conditionally unstable up to at least 300hPa.
  • Upward motion must exist from 700hPa to 300hPa.
  • Maximum upward vertical motion should occur below the 300hPa level.


Easterly lows

The African Easterly Wave, associated with the West African Monsoon has been studied extensively, and much less research results are available for the easterly waves and lows which develop over southern Africa in austral summer. Some notable exceptions are also the waves as inverted V troughs. He identified the following characteristics of these tropical waves.

  • They are cool but very humid below 700 hPa
  • Above 500 hPa the waves are warm cored
  • The high humidity in the lower troposphere originates from the ITCZ

Furthermore, the Angola Low is a semi-permanent feature over southern Africa in summer. He explains that a confluence zone exists east of this low stretching towards the ITCZ. South of this band easterly waves develop.

Characteristics of the African Easterly Wave:

  • Wavelength of 2000 to 4000 km
  • Period of 3-5 days
  • Move westward at speeds of 7-8 m s-1, about 6-7 degrees longitude per day
  • Latitudinal extent of 10 to 15 degrees
  • Maximum amplitude in the low to mid-troposphere
  • Exists apart from the ITCZ, although they may extend into that area

This conceptual model will show that easterly low have similar characteristics over southern Africa with some deviations.

  • Over southern Africa the low is strongest at 500 hPa
  • Preferred areas of convection is to the east of the low
  • The low moves only at about 3 degrees longitude per day



Intertropical Convergence Zone (ITCZ)

The Intertropical convergence zone (ITCZ) is defined as that area near the equator where the northeast and southeast trade winds converge. This area is known as the doldrums as winds can be calm for weeks trapping sail-powered boats.


Overview

The ITCZ is found in the area of low level wind convergence of the southeasterly and northeasterly trade winds close the equator. Over the oceans the large areas of convective cloud indicate the location of the ITCZ. Over Africa the position of the ITCZ ranges from 10-40 E and it extends to around 20° S in late summer (January and February) and the seasonal displacement of the ITCZ is sun-synchronous (Suzuki, 2011). However, one should be cautious to use the position of the ITCZ in short term forecast of precipitation over Africa as strong equatorial heating plays a fundamental role in the development of convection (Vasques, 2009). Nevertheless there is a good associated between the seasonal position of the ITCZ and rainfall. In order for the ITCZ to be maintained enough water vapor is required to sustain deep convection.

The ITCZ has the following characteristics

  • It occurs near the equator in areas of near surface wind convergence and may be associated with large convective cloud
  • The surface winds are very light
  • Abundance of low and mid-level moisture

In January moths the ITCZ extends southward over the continent to about 20 S while in July the ITCZ is located over northern Africa.

http://www.meteoweb.eu/2012/11/litcz-scivola-a-sud-dellequatore-nellafrica-australe-prende-vita-la-stazione-delle-piogge-levoluzione-per-i-prossimi-mesi/166037/
Cross section of the ITCZ in January. Over the ocean the rainbelt is often associated with the position of the ITCZ but over the continent the winds and boundaries are much weaker. Blue arrows indicate relatively cool dry air from the winter hemisphere. http://www.weatherwise.org/Archives/Back%20Issues/2009/Nov-Dec%202009/full-Intertropical-Converge.html

The ITCZ is characterized by a zone of deep convection with only shallow convection on either side. The demarcation zone lopes pole-ward with height and all convergence into the ITCZ occurs in the boundary layer in a region of strong cyclonic shear vorticity. In the middle to upper levels northerly winds blow through the (SH) ITCZ.

On a daily basis the ITCZ exhibits an amazing degree of variation. On one day it may be defined by a long east-west zone of deep convection which twenty-four hours later will have broken up into cellular clouds or may have disappeared entirely.

There is cyclonic wind shear across the ITCZ, with westerly winds on the equator side and easterly winds polewards.

Westward propagating waves may be found along the ITCZ. These synoptic and mesoscale systems have their own individual vertical circulations and zones of horizontal convergence and divergence. Consequently there are regions of intense upward motion and of downward motion all along the ITCZ on a planetary scale.

Generally there is tropical maritime air on the equatorward side of the ITCZ, while on the poleward side there is air from the higher latitudes which suffered much subsidence in descending while moving northwards and is thus relatively warm, dry and stable. Hence the air on the equator side is generally cooler. However the temperature differences between the two sides are generally too small to be easily discernible.

The ITCZ is thus usually recognized on synoptic charts through the wind discontinuity and as a trough of low pressure, or on the satellite images in terms of maximum cloudiness.

Clouds form in the moist layer on the equatorward side of the ITCZ. If the moist layer is deep, large cumulus and cumulonimbus formation is possible. Maximum cloudiness is therefore on the equatorward side of the ITCZ at a distance of 200 to 500 km from it.