1Czech Hydrometeorological Institute - Czech Republic
2National Severe Storms Laboratory, NOAA - U.S.A.
Past studies of the NOAA-AVHRR/2 and GOES I-M imagery (e.g., Setvák et al., 1996) have shown that tops of deep convective storms occasionally exhibit an increase of cloud top reflectivity when observed in the 3.7 or 3.9 µm spectral bands. Although the research did not provide a conclusive explanation of the observed phenomena, it is generally believed that reflectivity in these bands depends on microphysical characteristics of the frozen cloud tops. The ambient conditions and atmospheric processes leading to increased 3.7/3.9 µm reflectivity remain to be understood. In this study, observations of storm tops from wavelengths of 1.6 to 6.7 µm will be explored to learn more about the characteristics of these storm tops.
The basic difference between the 3.7/3.9 µm and 1.6 µm bands is the absence of a thermal component for the latter one, thus providing a more unambiguous interpretation of imagery in the 1.6 µm band. Otherwise, sensitivity of the imagery to cloud top microphysics can be similar for all of these bands. Since the future MSG (Meteosat Second Generation) satellite will host both, the 1.6 and 3.9 µm bands, it will be possible to monitor European storm tops in these spectral bands simultaneously at 15 minutes intervals.
Before MSG launch, data from the NOAA-KLM AVHRR/3 instrument can be used for first studies of storm tops appearance in the 1.6 µm band. Since the NOAA-KLM satellites are designed to use the 3.7 µm band at night and the 1.6 µm band during daylight, it is not possible to examine storm tops in both of these spectral bands simultaneously. Therefore, for selected cases of deep convective storms from the U.S. region, a cross-comparison of storm top appearance in NOAA-KLM 1.6 µm band and GOES I-M 3.9 µm band imagery will be presented.
Observations from METEOSAT (e.g., Schmetz et al., 1997) have found the brightness temperature in the water vapor band to be several degrees warmer than in the window band above cold top convective clouds. This observation was explained by stratospheric water vapor above active convection. Their results suggested that the magnitude of the difference in brightness temperature depends on the lapse rate above the tropopause and the height of the cloud top. The current study will examine this difference in brightness temperature using data from GOES-8 and 12 in relation to the occurrence of enhanced reflectivity at 1.6 and 3.7/3.9 µm. The GOES-12 is not yet operational, but high frequency (1-minute) data was analyzed as part of a science test in October, 2001. The GOES-12 imager has identical resolution in the window and water vapor bands (4 km) as compared to GOES-8 which has reduced resolution in the water vapor band (8 km).
Schmetz, J., S. A. Tjemkes, M. Gube, L. van de Berg, 1997:
Monitoring deep convection and convective overshooting with METEOSAT. Adv. Space Res., Vol.
19, No. 3, pp 433-441.
Setvák M., R. M:Rabin, V. Levizzani and C. A. Doswell, 1996:
Relations between increased 3.7/3.9 µm reflectivity above U. S. Great Plains thunderstorms
from NOAA/AVHRR and GOES-8 and internal storm structure from NEXRAD radar. Proc.
1996 Meteorological Satellite Data Users’ Conference, Vienna, Austria, EUM P 19, ISSN 1011-3932, 143-150.