Special Investigation: Overshooting Tops

 

Introduction

An overshooting convective cloud top (OT) is a dome-like protrusion above the cumulonimbus anvil. It represents an updraft core of sufficient strength to rise above the storm's equilibrium level (the point where the surrounding air is the same temperature or even warmer) near the tropopause region and penetrate into the lower stratosphere. Because of their relatively short duration and small diameter, recognition of OTs in satellite images is strongly dependent on the spatial and temporal resolution of the satellite instruments.

 

Appearance in Satellite Data

OTs can be most easily identified in the high-resolution visible (HRV) channel imagery as a lumpy-textured area with its characteristic shadowing within the convective cloud in the mature stage. However, this only applies during the daytime. OTs are best seen during early morning or late afternoon hours, when the angle of the sun is low and the shadows cast by OTs are easily seen.

In the color-enhanced IR 10.8 µm images, available during both day and night, a small cluster of very low brightness temperatures can indicate that an OT is present.

OT as it appears in HRVIS image.
OT as it appears in color-enhanced IR 10.8 µm image.
12 September 2012/08.50 UTC - Meteosat 9 HRVIS image. The OT location is marked with a red arrow.
12 September 2012/08.50 UTC - Meteosat 9 color-enhanced IR 10.8 µm image. The OT location is marked with a black arrow.

 

Appearance in RGB composite imagery

RGB combinations of channels make it easier to identify smaller fine-scale cloud structures, such as OTs, which otherwise may be hidden or hard to identify in the satellite images by a human eye.

In the so called Severe Storm RGB (WV6.2-WV7.3, NIR3.9-IR10.8, NIR1.6-VIS0.6) images OTs usually appear as yellow pixels within a reddish convective cloud in the mature stage.

OTs can be most easily identified using an RGB composite of the high resolution visible and infrared 10.8 µm channels (HRVIS, HRVIS, IR10.8i). Because of a resolution that is three times higher than in the other RGB combinations, the 3D structure of the convective clouds is very clear here.

OT as it appears in High resolution visible RGB (HRVIS, HRVIS and IR10.8i) image.
OT as it appears in Severe Storms RGB (WV6.2-WV7.3, NIR3.9-IR10.8, NIR1.6-VIS0.6) image.
12 September 2012/08.50 UTC - Meteosat 9 High resolution visible RGB (HRVIS, HRVIS and IR10.8i) image. The OT location is marked with a red arrow.
12 September 2012/08.50 UTC - Meteosat 9 Severe Storms RGB (WV6.2-WV7.3, NIR3.9-IR10.8, NIR1.6-VIS0.6) image. The OT location is marked with a black arrow.

 

Appearance in 'sandwich' images

'Sandwich' images combine the information from one grey-scale image and one color image. Typically, the underlying image is the HRV image, overlaid with a color enhanced brightness temperature (BT) image or a standard RGB product. For detection and study of the characteristics of the OT, the most appropriate combination is HRV image overlaid with a color enhanced IR 10.8 µm image. Such a combination provides information about the 3D structure of the cloud, but also about the temperature of the cloud top.

12 September 2012/08.50 UTC - Meteosat 9 'sandwich' product (HRV image overlaid with enhanced IR 10.8 µm image ). The OT location is marked with a red arrow.

Another useful combination is produced by an overlaying a HRV image with a Severe Storm RGB. Yellow pixels in a Severe Storm RGB image indicate a largely positive NIR3.9-IR10.8 BTD (brightness temperature difference), due to high reflection of small ice particles, which indicate severe updrafts and possible OTs. It should be mentioned that not all yellow pixels indicate small ice particles, some of them are a consequence of very low BT in IR10.8 µm (app. -70 °C) and relatively low reflectance in IR3.9 µm, indicating only very cold cloud tops with usual updraft and relatively large ice particles.

12 September 2012/08.52 UTC - Meteosat 10 'sandwich' product (HRV image overlaid with Severe Storm RGB image). The OT location is marked with a red arrow.

 

The life cycle of overshooting tops

Rapid-scan satellite data reveals the fact that an OT can exist for less than 15 (even less than 5) minutes and has a maximum diameter of ~15 km.

12 September 2012/06.15 UTC - Meteosat 10 "Sandwich" product
(HRV image overlaid with enhanced IR10.8 image)
12 September 2012/06.00 UTC - Meteosat 10 "Sandwich" product
(HRV image overlaid with enhanced IR10.8 image
  Loop: 12 September 2012/06.15 UTC - 15 minutes image loop   Loop: 12 September 2012/06.00 UTC - 5 minutes image loop

An example of the life cycle of OTs can be seen in the image above. Sandwich products from 06:45 to 12:00 UTC show an intense convective storm developed in northern Italy and spread across western Slovenia. In the mature stage of the storm many OTs could be recognized at the top of the storm in the satellite images. Rapid scan SEVIRI 5 min and 2.5 min satellite data enables recognition of the OTs that last for less than 15, or even less than 5 minutes.

12 September 2012/06.00 UTC - Meteosat 10 "Sandwich" product (HRV image overlaid with enhanced IR10.8 image)
12 September 2012/06.00 UTC - Meteosat 10 "Sandwich" product (HRV image overlaid with Severe Storm RGB image)
  Loop: 12 September 2012/06.00 UTC - 2.5 minutes image loop   Loop: 12 September 2012/06.00 UTC - 2.5 minutes image loop

 

Meteorological Physical Background

Defined as a domelike protrusion above a cumulonimbus anvil, representing the intrusion of an updraft through its equilibrium level (level of neutral buoyancy), an overshooting convective cloud top (OT), sometimes also called "hot tower", is a manifestation of a very strong updraft in the convective cloud. An OT forms when a thunderstorm's updraft, due to momentum from its rapid ascent and the strength of its lift, penetrates its equilibrium level (EL, the point where the surrounding air is about the same temperature or even warmer) near the tropopause region. It also usually penetrates into the lower stratosphere, but it should be noted that overshooting does not necessarily imply penetration into the stratosphere. Sometimes it merely distorts the tropopause region. Above the EL, the air parcels are colder than the surrounding environment. Consequently, the cloud particles become negatively buoyant and sink back toward the equilibrium level, before spreading out into the anvil.

OTs often generate gravity waves which can produce significant turbulence, posing a risk to aviation. Wave-like perturbations in the potential temperature often occur above the OTs and some of these waves may become unstable and broken over the cloud top. The wave breaking process is an irreversible, non-adiabatic mass transfer from the troposphere to the stratosphere. The breaking of the wave produces turbulence but also various storm top plume formations such as anvil-sheet cirrus plume or overshooting cirrus plume, also called "jumping cirrus" due to moisture being injected into the stratosphere from the cloud body (Wang, 2007).

Penetrating convective storms affect the transport of various chemical species, and especially that of water vapor from the troposphere into the stratosphere. Ice particles from OTs can reach a height of up to 18.8 km (Corti et al, 2008). Bearing in mind the importance of water vapor as the absorber of infrared emissions in the atmosphere, it is clear that its distribution in the stratosphere may have an important impact on the global climate (e.g. Liou, 2002).

Typical storm-top satellite signatures, such as cold-ring or cold U/V shapes, are a consequence of very strong convective activity, represented with OT. Exceptionally tall and persistent OTs are the result of a nearly continuous stream of updrafts in a thunderstorm. OTs are more frequently detected over land than over sea. Over sea, OTs often appear close to the coastline. The largest number of OTs generally occur during the afternoon and early evening between app. 15 and 16 UTC. Between 06 and 10 UTC, OTs are rather rarely seen.

 

For Key Parameters see the relevant chapters in MCS

 

Typical Appearance In Vertical Cross Sections

In the case of severe thunderstorm with very strong updraft the overshooting top penetrate through the tropopause into the lower stratosphere. The main features of the penetrating convection are plumes and jumping cirrus which are produced by wave breaking in the environment of high instability. The potential temperature field is characterized by very large gradients in the OT region. During the penetration some moisture is ejected into the lower stratosphere.

Vertical distribution of (equi)potential temperature.
Vertical distribution of humidity.
  Loop: Simulated storm with the overshooting top. Relative humidity with respect to ice (color) overlaid with the potential temperature isotherms (black).

The example below shows a radar cross section of a storm which occured above the Czech Republic on 25 June 2006. The highest OT penetrated through the tropopause and into the lower stratosphere, up to app. 17 km.

25 June 2006 - Radar cross-section of the convective storm with OT from 13:00 to 13:20 UTC. Tropopause is estimated from the Prague 12:00 UTC sounding (marked with red line). Red circle indicates position of the OT. (Adapted from Setvak et al., 2010)

 

Detection and Monitoring

 

Satellite-based OT detection methods

One of the most commonly used methods for detecting the OTs in satellite data is based on the brightness temperature difference (BTD) between the 6.2 µm and 10.8 µm (WV-IR) channels. This technique could be used for day/night OT detection. BTDs greater than 0 K are related to convective clouds with high vertical extension. Positive BTDs appear when deep convective clouds penetrate through the tropopause, moistening the stratosphere. The brightness temperature in the WV channel can be larger than the one in the IR channel by as much as 6 to 8 K. In such cases the brightness temperature in WV channel is greater than the one in the IR channel because of the presence of the water vapor above the cloud top. The contribution of the water vapor is included in the emission from the water vapor band. Because the OT often protrudes into the lower stratosphere, the area where temperature increases with height, the water vapor at that height has a warmer temperature than the cloud top, which makes the BTD positive.

However, in some cases the WV-IR BTD technique shows false alarms (see more information in e.g. Setvák et al., 2007; Setvák et al., 2008; Putsay et al., 2011) which are caused by water vapor anomalies, especially in the anvil region. It should be noted that the positive values of WV-IR BTD can also be caused by the moisture existing in a layer above the cold cloud tops but not being connected to or produced by the storm itself. Moreover, the case studies showed that the enhanced BTD can also occur downwind of the OTs, rather than always above the coldest tops.

06 July 2010/17.05 UTC - Meteosat 8 'sandwich' product (HRV image overlaid with enhanced IR 10.8 µm image)
06 July 2010/17.15 UTC - Meteosat 9 WV-IR BTD

BTD of the ozone channel (9.7 µm) and the IR 10.8 µm channel also shows a positive signature for the cloud tops above 11 km. The signal in this BTD is even more significant than the one in WV-IR BTD, suggesting that it could be a better indicator for deep convective activity.

BTD of carbon dioxide (13.4 µm) and the IR10.8 µm channel can also be used for determining the height of the opaque clouds. The reason is that with higher cloud tops the absorption effect of CO2 gets smaller, bringing the BTD of the CO2 and IR channels close to 0. In the case of very deep convective clouds the BTD becomes positive.

06 July 2010/17.05 UTC - Meteosat 8 'sandwich' product (HRV image overlaid with enhanced IR 10.8 µm image)
06 July 2010/17.15 UTC - Meteosat 9 CO2-IR BTD
06 July 2010/17.15 UTC - Meteosat 9 O3-IR BTD

A combination of WV-IR and O3-IR BTD can be used in order to avoid a significant number of false alarms occurring in WV-IR BTD images, but also to overcome the seasonal variation of the O3-IR BTD, which is caused by the seasonal variation of ozone concentration above the mid latitudes (Mikuš and Strelec Mahović, 2012).

06 July 2010/17.05 UTC - Meteosat 8 'sandwich' product (HRV image overlaid with enhanced IR 10.8 µm image)
06 July 2010/17.15 UTC - Meteosat 9 WV-IR BTD

OTs are usually observed during very strong and severe convective development. For monitoring convective development and the strength of the developed Cbs, in-situ observations and the following remote sensing data are used:

  • Satellite images
  • Radio soundings
  • Radar images
  • Lightning reports
  • Weather reports

For more information about the different detection and observation methods of convective cells, see Manual chapter Convective weather features:

For more information about the key parameters typical for convective systems, see the Basic chapter Numerical parameters for small scale convective cloud systems:

  • Convection and Instability
  • CAPE
  • Stability Indices

CAPE and stability indices provide information about the potential for convection at a certain location, but don't imply the formation of OTs at the top of a developed convective storm.

 

Weather Events

According to investigations, deep convective storms with OTs are significantly correlated with severe weather conditions such as heavy rainfall, damaging winds, large hail, and tornadoes (e.g. Mikuš and Strelec Mahović, 2012; Bedka et al., 2010; Bedka, 2010). Thunderstorms with OTs are also often associated with strong horizontal and vertical wind shear. Cloud to ground lightning and turbulence occur frequently near the OT area, posing a risk to aviation.

The OT severe weather correlation is strong for large hail (diameter > 2cm; 53%) and severe wind (wind speed > 25 m/s; 52%). These events are usually accompanied with relative humidity increase and a temperature drop. The results of the comparison between OTs and weather elements measured on the automatic stations showed that the best correspondence is found for precipitation. Very good correlation (70%) is found between wind gusts and OT occurrence. Although OTs are a transient feature and usually short-lived, the presence of an OT is highly indicative of very intense updrafts and potentially severe weather.

Type of automatic station data Correlation between detected OTs
and measured parameter (%)
wind gust 70
precipitation 80
relative humidity increase 57
temperature drop 64
Table: Correlation between the OTs and each measured parameter on the automatic stations. The analysis has been performed for the periods of May-September 2009 and 2010, i.e. the warm part of the year when convective activity is at its strongest. 1380 cases were recorded during the analyzed period.

 

For a more complete description of the Weather Events see the relevant chapter in MCS.

 

References

  • Bedka, K. M., 2010: Overshooting cloud top detections using MSG SEVIRI Infrared brightness temperatures and their relationship to severe weather over Europe. Atmos. Res., doi:10.1016/j.atmosres.2010.10.001.
  • Bedka, K. M., Brunner, J., Dworak, R., Feltz, W., Otkin, J., Greenwald, T., 2010: Objective Satellite-Based Detection of Overshooting Tops Using Infrared Window Channel Brightness Temperature Gradients. J. Appl. Meteor. Climatol., 49, 181 - 202.
  • Corti, T., B.P. Luo, M. de Reus, D. Brunner, F. Cairo, M.J. Mahoney, G. Martucci, R. Matthey, V. Mitev, F.H. dos Santos, C. Schiller, G. Shur, N.M. Sitnikov, N. Spelten, H.J. Vössing, S. Borrmann and T. Peter (2008), Unprecedented evidence for deep convection hydrating the tropical stratosphere, Geophys. Res. Lett. 35, doi:10.1029/2008GL033641.
  • Liou, K.-N., 2002: An Introduction to Atmospheric Radiation, 2nd edition. Academic Press., 582 pp.
  • Putsay, M., Setvák, M., Simon, A., Kerkmann, J., 2011: Simultaneous BTD (WV6.2-IR10.8) anomaly and above-anvil ice-plume observed above the storm of 06 July 2010, North Italy. 6th European Conference on Severe Storms (ECSS 2011), 3-7 October 2011, Palma de Mallorca, Balearic Islands, Spain.
  • Setvák, M., Rabin, R. M., Wang, P. K., 2007: Contribution of the MODIS instrument to observations of deep convective storms and stratospheric moisture detection in GOES and MSG imagery. Atmos. Res., 83, 505-518.
  • Setvák, M., Lindsey, D.T., Rabin, R.M., Wang, P.K., Demeterová, A., 2008: Indication of water vapor transport into the lower stratosphere above midlatitude convective storms: Meteosat Second Generation satellite observations and radiative transfer model simulations. Atmos. Res., 89, 170-180.
  • Mikuš, P., Strelec Mahović, N., 2012: Satellite-based overshooting top detection methods and the analysis of correlated weather conditions. Atmos. Res., 10.1016/j.atmosres.2012.09.001,http://dx.doi.org/10.1016/j.atmosres.2012.09.001
  • Wang, P. K., 2007: The thermodynamic structure atop a penetrating convective thunderstorm. Atmos. Res., 83, 254-262.