Special Investigation: 'Cold Ring' and 'Cold U/V' Shaped storms

 

Introduction

'Cold ring' and 'Cold U/V' are pairs of cold and warm areas on top of severe convective storms. The original name of these features was 'enhanced-V' (a V-shaped structure on top of convective clouds) because it can be seen only in enhanced infrared satellite images. According to recent studies, there is a significant correspondence between these cloud-top patterns and convective storms' severity (Irsic Zibert et al., 2010; Bedka et al., 2011).

 

Appearance in Satellite Data

Cold rings and cold U/V shapes are features of thermal anomalies on the tops of severe convective storms, visible in enhanced infrared images.

Figure 1: Cold ring as it appears in color-enhanced IR 10.8 µm image
Figure 2: Meteosat 9 color-enhanced infrared (EN IR) 10.8 µm of cold ring-shaped storm from 19 September 2014.
Figure 3: High resolution visible (HRV) satellite image of cold ring-shaped storm from 19 September 2014.

Figure 4: Cold U/V as it appears in color-enhanced IR 10.8 µm image
Figure 5: Meteosat 9 cold-U/V-shaped storm in color-enhanced infrared 10.8 µm satellite image from 19 September 2014.
Figure 6: Meteosat 9 cold-U/V-shaped storm in high resolution visible (HRV) satellite image from 19 September 2014.

 

Video 1: 19 September 2014 - Meteosat 9 color-enhanced infrared 10.8 µm satellite image.

These Meteosat 9 color-enhanced infrared 10.8 µm satellite images from 02:00 to 13:15 UTC show an intense cold U/V storm developing in southeastern France and a cold ring shaped storm over northern Italy.


Video 2: 19 September 2014 UTC - Meteosat 9 'sandwich' product (HRV image overlaid with enhanced IR 10.8 µm image ).

A sandwich product is composed of a HRV satellite image overlaid with a color-enhanced 10.8 µm satellite image. This product shows the 3D structure and temperature distribution of the tops of storm clouds, which are useful in monitoring the development of intense convective storms. Sandwich products from 05:50 to 13:15 UTC show an intense cold U/V storm developing in southeastern France and a cold ring-shaped storm over northern Italy.

 

Meteorological Physical Background

Cold rings and cold U/V features can be differentiated by their shapes. A cold ring is situated around the central warm spot (CWS). In a cold U/V cloud top there is a U-shaped close-in warm area (CWA) and a distant warm area (DWA) farther away from the overshooting tops. The mechanisms of cold-U/Vs with CWA and cold rings with CWSs are still not fully understood. According to the preliminary results of previous investigations (Setvák et al., 2010) suggest that both types of storm result from similar mechanisms, while environmental features determine which of those storm top features will develop. According to model simulations the crucial factors for cold ring or cold U/V shape formation are upper level wind shear and lower stratospheric inversion.

a)
b)
c)
Figure 7: Idealized soundings used in simulations and RAMS (Regional Atmospheric Modeling System) model simulations which provided simulated 10.35 µm satellite images at four different times. A) No upper level wind shear and no tropopause inversion, B) tropopause inversion and no upper level wind shear, C) tropopause inversion and upper level wind shear. Adapted from Setvák et al., 2010.

Figure 7 shows the simulated brightness temperatures of stormcloud tops in three different environments. In the case without upper level wind shear or tropopause inversion, the cold ring and cold U/V shapes have not developed (Fig. A). In the second simulation (Fig. B) a tropopause inversion is present, which results in warmer brightness temperatures: a cold ring shape has developed with a significant temperature difference (~27 K) between the CWS and the cold ring.

In situations where upper level wind shear is present, the cold U/V shape develops (Fig. C). According to previous investigations cold rings typically appear in environments with weak wind shear, while strong wind shear results in the formation of cold U/V shapes.

Satellite observations show that above-anvil plumes develop often in the case of cold U/V-shaped storms. Those plumes are warmer than the anvil, masking the cold anvil top underneath. The warm plumes might resemble CWA areas at first sight; a typical CWA, however, is often larger and of a different shape than an above-anvil plume.

 

Weather Events

Previous studies (Irsic Zibert et al., 2010; Bedka et al., 2011) based on METEOSAT and GOES imagery suggest that the cold ring and cold U/V feature are accompanied by severe weather conditions, such as large hail, severe winds or tornadoes. Severe weather conditions have been observed when a cold ring or cold U/V pattern persists at least 30-40 minutes. Analysis of cold ring and cold U/V events (Bedka et al., 2011) show that in 74% of cases some kind of severe weather conditions were detected (> 2cm hail, > 25m/s wind or tornado).

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

 

References

  • Bedka, K., Brunner, J., Dworak, R., Feltz, W., 2011: Objective satellite-based overshooting top and enhanced-V/cold ring detection: Validation and relationship with severe. 6th European Conference on Severe Storms (ECSS 2011), 3-7 October 2011, Palma de Mallorca, Balearic Islands, Spain.
  • Irsic Zibert, M., Strajnar, B., Zibert, J., 2010. Cold-ring pattern on satellite images as indication of severe weather. Proc. 2010 EUMETSAT Meteorological Satellite Conf. EUMETSAT, Cordoba, Spain.
  • Setvák, M., Lindsey, D.T., Novák, P., Wang, P.K., Radová, M., Kerkmann, J., Grasso, L., Su, S.-H., Rabin, R.M., Stástka, J., Charvát, Z., 2010. Satellite observed cold-ring-shaped features atop deep convective clouds. Atmos. Res. 97, 80-96.