3.1 IOP 11/LOW 30

This large cyclone originated over the east of North America and developed while travelling eastwards. The system underwent two distinct growth stages, including a stage of explosive development. However, although significant, its intensity did not reach an outstanding level. It was accompanied by a well-developed cloud-head. Among all FASTEX cyclones, Low 30 can be considered as a rather "typical" cyclone that could retain the forecasters' attention. The maximum amplification of low-level vorticity was 3.8 in 24 hours.

In terms of observational coverage, Low 30 was the last FASTEX cyclone to undergo a systematic survey, which involved the three instrumented aircrafts of the campaign. Consequently, IOP 11 provides a rare opportunity for intercomparing model outputs with various observational data, such as dropsonde and Doppler radar measurements. Furthermore, it is interesting to recall that the available soundings originate from the C130, and the G-IV aircrafts, as well as from the Ægir ship. Some soundings were also carried out upstream to the simulation area, although the ships were not at their best location. Therefore, the reanalysis data provided at the western boundary of the simulation domain will have a mean quality.

Another important feature of this case is that the observations sampling took place at the very moment when the system was splitting apart, leading to the generation of a secondary wave which developed its own cloud system.

The following figures give a brief summary of IOP 11:

  

Figure 2: IOP 11: METEOSAT infrared images (left) and ARPEGE MSLP and 2 meter-T fields (right) for 970204 at 00hUTC (top), 970205 at 00hUTC (middle), and 970206 at 00hUTC (bottom).

  

Figure 3: IOP 11: Available FASTEX observations.

The following pictures display cloud related fields, either simulated by the operational model ARPEGE, or retrieved from satellites observations, on 970206 at 00hUTC. In particular, SSM/I data provide very valuable information on the global distribution of liquid water and water vapor throughout the atmosphere, but only over the ocean.

  

Figure 4: IOP 11: Outgoing longwave radiation at the top of the atmosphere (W/m²), from the ARPEGE operational 24-hour forecast, on 970206 at 00hUTC.

  

Figure 5: IOP 11: SSM/I composite liquid water content (g/m²) on 970206 around 00hUTC (from NASA/MSFC DAAC).

  

Figure 6: IOP 11: SSM/I composite water vapor content (kg/m²) on 970206 around 00hUTC (from NASA/MSFC DAAC).