For nearly all conceptual models numerical derived parameters show up as key parameters. In the case of convective developments the key parameters are relevant observations and additional artificial satellite channels. The following material will be used:

Satellite imagery with an appropriate cold cloud top enhancement

• Typical circular and oval shape of the MCS
• Cloud top temperatures below approximately -30° C
• Typical grey shade distribution within the MCS (see Cloud structure in satellite images)

Weather reports

• Cb, thunderstorms, showers (see Weather events)

Lightning reports

Vertical distribution from radio soundings

Wind profile

The wind direction in the lower and mid-levels of the troposphere, especially in front of and within the area of the right leading edge of the cell, is characterized by a turning to the right with height.

Stability analysis

In operational forecasting instability indices are widely used. In this manual the Showalter and the Boyden index are used, not as the result of a single radiosonde measurement but from model parameters. They cover broader regions and are, therefore, also discussed in the relevant chapters of the "Convective cloud fetaures under typical synoptic environment".

Vertical distribution of parameters from Local Area Models: ALADIN

Due to the small number of available radiosonde stations, one can use pseudo-TEMPS from the ALADIN LAM model in order to derive a better resolution of the vertical structure of the atmosphere. Pseudo-TEMPS can also be derived from other LAM data, such as HIRLAM.
The ALADIN model offers a high spatial horizontal resolution and an acceptable vertical resolution, when comparedn with the radio soundings. The following example shows the grid - point of Aigen/Austria. With the pseudo - TEMPS from ALADIN a similar presentation of the parameters is possible. The stability analysis shows a conditionally unstable stratified air mass up to 500 hPa (yellow), and unstable air in the ground layers (red).

Development images, indicating new development or decay of cloudiness:

A typical system life cycle can be detected in development images (see Basics - Satellite Channels: Artificial and Combination Channels ):

• Developing stage: nearly the whole MCS cloudiness (in the IR) appears as a white area in the development image, indicating new cloud pixels.
• Mature stage: the largest area of MCS cloudiness in the IR is a grey area in the development image, indicating no further strong development and not decay; at this stage only the MCS boundaries show up as white in the development image, revealing a typical circular to oval shape of grey areas surrounded by white circles.
• Dissipating stage: black areas initially develop mostly at the rearward edges of the MCS but later on, black areas develop in several areas within the whole MCS.

The brightest values in the development image can be found at the stage when completely new cloud develops; if the tops of an existing MCS become colder, or a sub-cell within the MCS grows to produce colder tops, the development image shows only light grey shades. In these cases, the image features themselves also have to be observed since they show, e.g. the circular pattern of the rising sub-cell.
The investigation of development images is promising but relatively new and, consequently, they have still to be studied and tested in more detail.

Radar

Another very prominent observation tool is radar (see Meteorological physical background). Although this manual concentrates on satellite imagery, the following provides an introduction to radar. More details may be found in relevant radar literature. One important fact is that with radar small scale features within the MCS at different heights can be observed.

The following paragraphs introduce and illustrate different radar reflectivity products:

Plan Position Indication (PPI)

One elevation (usually close to zero), all azimuths. PPI is the fastest of all radar products and therefore suitable for studying the fast-developing mesoscale storms.

Constant Altitude Plan Position Indication (CAPPI)

Traditional PPI has two problem areas: near the radar site there is often ground clutter. In the lowest beam PPI clutter is often so strong that filtering also removes the weather signals (a gap in the middle). Secondly, far from the radar source the beam overshoots precipitation, partly or totally. CAPPI is an attempt to avoid the problems of PPI. Constant altitude data are picked and interpolated from different elevation scans. Eventually, beyond a certain distance, the lowest available data are selected and the result is called PseudoCAPPI.

Echo Top

Echo top is the tool to follow a developing convective system. It is used together with the temperature profile. When tops reach a value of -15, precipitation begins; when they reach a value of -25, the chance of thunder is considerable. Some towering tops (in red) rise above a more widespread cloud mass. These are embedded cumulonimbus towers.

MAX

MAX is the tool to identify the strongest cells in a multicell system. This is essential e.g. for aviation briefing. High dBZ values at higher altitudes (e.g. +30 dBZ at 5 kilometres) almost certainly relate to wet graupel or hail, and indicate strong updrafts.

These two images are taken at the same time as the cross sections below, east and north from the radar site (located at the centre of the image). Note the additional information about the vertical structure of the cells in the MAX image, provided in the side panels.

Range Height Indicator (RHI)

RHI or cross section is not the primary tool, but once you have identified a possible MCS, a good cross section helps you to study the vertical structure and to identify possibly hail-generating cells. The image above shows a cross section through several Cumulonimbus cells, one of which is stronger than the others. Note that the weaker intensities in the thinner parts of the anvil (below -10 dBZ) can only be detected close to the radar (left part of the image). The bright band (maximum reflectivity layer due to melting snow near 0 degree isotherm) can be detected even in convective rain, even though, due to strong vertical movements, it is not always as clear as in stratiform rain.

Accumulated precipitation for n hours (RAINN product)

A typical question asked after an MCS passed over an area is "How many millimetres of rain?" Gauge measurements are quite accurate (despite wind-induced error!) but the synoptic network is rather sparse - and often you have to wait until 06.00 UTC or 18.00 UTC to get the readings. Radar products for Accumulated precipitation are less accurate, but they have outstanding resolution both in space and time. They are generated from CAPPIs, so all error sources for CAPPIs are present here. Remember that far from the radar source the measurement volume is quite large and well above the ground! For a radar RAINN product, it is often safer (in press interviews etc.) to:

• Point out the areas of maximum precipitation
• Give the millimetres assuming 50% accuracy (so "about one", "less than five", "20 - 30 mm" are quite acceptable phrases).

A small but intense shower gave flash flooding over a limited area - impossible to detect by any other instrument than radar! However, the absolute maximum values (over 60 millimetres in six hours) must be suspect here, as hail was present. The striped structure of the precipitation area is caused by movement of Cb cells between the individual CAPPIs which have been used as raw material in this composite.

Doppler winds

A doppler radar can be used to determine wind speed from echo-giving droplets. A classical Doppler wind PPI is difficult to interpret without experience. A well-formed mesocyclone can be detected by a Doppler radar as a couplet in the velocity data, which indicates a circulation (motion away and towards the radar). The couplet is most typical for tornadic supercells and other intense twisters. Nowadays the data are often displayed as more sophisticated wind products. Doppler speed as measured (wind component parallel to radar beam). Blue and green is away from radar, red and orange towards radar. Ambigous speed scale is too small so the colours have to be interpreted carefully (e.g. white is either zero or eight or fifteen). Above, the same information, interpreted to standard wind arrows. As each arrow is averaged from a relatively large slice, small phenomena like tornadoes are likely to be filtered out.

Time - Height cross section of Volume Velocity Processing wind profiles (THVVP - time series of wind profiles)

The tool to study changes in wind shear. This is the best mesoscale tool for it, given enough time resolution (typically a new sounding every 15 or 30 minuntes). Remember that radar winds always represent an average of a larger volume, e.g.in this case a cylinder 40 kilometres in radius, 200 metres thick. If the wind field is not linear, the average through such a volume can be erroneus. Suspect non-continous winds. Time as x-coordinate, height as y-coordinate. Coloured background is averaged reflectivity in a cylinder with 40 kilometres radius around the radar. Wind barbs averaged in the same cylinder. Note the wind shear near ground due to the Ekman spiral and warm advection, and reflectivity maximum near 2.5 kilometres due to bright band (melting layer) after 01.00 UTC. (This example is not from an MCS case, but a more stratiform precipitation, first snow, later rain.)

Vertical velocity sounds like a good idea, but you cannot distinguish the speed of falling droplets from the speed of rising or sinking air parcels, so if you study this put more emphasis on changes to this parameter than actual values. (Vertical velocity can be used instead of reflectivity in the THVVP. Another possible parameter is divergence/convergence.)