Summary

Flight 1, September 29 : Mission 51, intercomparison flight between Merlin and ARAT. Sounding close to Genoa and Flux measurements over Ligurian.

Flight 2, September 30 : Mission 52, Ligurian sea, flight in prefrontal conditions.
 

Mission Description

During the mission, the ABL top height was observed by lidar to be decreasing towards the south to reach about 0.4 km (see Figure 2). Some shallow cumulus were observed over the Ligurian sea in the southern part of the track, more developed over the Appenins.

The thermal structure of the lower atmosphere observed during the sounding made at 1215 UTC, 20 nm south of Genoa, is given in Figure 3. It shows the existence of several temperature inversions, the lower ones at the top of the ABL and at 1600m. The flow observed is the synoptic flow from the west in the upper layers and turns to the south in the lower layers as it is deflected by the Alps.
 

Figure 1: Trajectory of the ARAT during the intercomparison flight (mission 51).

Figure 2: Lidar reflectivity as retrieved by LEANDRE 2 south of Milano while the aircraft was flying at 12000 feet ASL.

 
 

  The two aircraft were not authorized to fly IFR at the same level. The first leg of the Merlin at upper level was performed at 13 000 feet. System operation was nominal.
 

Mission Description

During the ARAT ascent, upward lidar measurements only showed the presence of this stratocumulus layer. An important cloud cover was observed over land while the sea was relatively cloud-free. However, the northern part of the Ligurian sea was rather moist and hazy, but only a small amount of cumulus were observed, as seen in Figure 5 showing the lidar reflectivity over the Ligurian sea, between Pisa and Nice.

The front was seen to be composed of low and mid level clouds mostly with stratiform clouds. Some cumulus were developing but convection did not appear to be important. This appears to be a similar structure as the the frontal system observed during IPO3 in mission 50.
 

Figure 4: ARAT trajectory during Mission 52 superimposed on AVHRR visible image.

Figure 5: Lidar reflectivity cross-section along the Pise-Nice track at 14000 ft ASL.

Nadir lidar measurements were made between Parma and Pisa. The leg over land was cloudy, but the part over the Ligurian sea revealed the lower troposphere to be fairly cloud free (some cumulus (Cu) clouds - top altitude at about 0.5 km as seen in Figure 5) but very hazy.

As the ARAT moved closer to Nice Sc clouds as well as convective towers were encountered.  Close to Nice cumulus congestus were observed (see Figure 4). The top of the higher ones was approximately 3.5 km. No cirrus were observed during the mission.

The transition from clear to cloudy atmosphere could not be documented as the aircraft was obliged to stay about 40 nm from Nice due to trafic. During the ascent over Ligurian sea, south of Genoa, a sounding was made up to 4.2 km.Vertical profiles of temperature, moisture and horizontal wind are shown in Figures 6 and 7 for the two soundings performed close to Nice, in the vicinity of the front, and in the middle of the Nice-Pise track in clear air.

The thermal structure reveals two inversions, the first corresponding to the boundary layer top, and the second at about at 2500 m corresponding to the Sc cloud top. The temperature profile close to the front shows a smaller stability increase at the top of the ABL.
 
 

Figure 6  : ARAT sounding close to Nice.

Figure 7: ARAT sounding south of Genoa.

At 500 ft the wind direction was observed to turn from about 200 to 160 degrees as the ARAT moved from Nice to Pisa (Figure 8). The wind direction measured shows a convergence in the flow at 9 E, most probably due to the surrounding moutains including those of Corsica. The wind strength was much stronger closer to the front, with wind speed exceeding 20 m/s. It is to be noticed that at this longitude the ABL height is increasing up to 1 km.

The sounding in D (southernmost point of the track in Figure 4) showed the existence of two inversion heights, one at 700 m corresponding to the top of the marine boundary layer and a second inversion at 2600 m (see Figure 9).

At  the italian coast, the ABL height is comparable to te one in point Ligure at the same longitude. Orographic forcing lead to a strong convective development of cumulus with top heights rapidly reaching 4.2 km (see figure 9). This development is more important than the one observed in mission 50 of IOP 3 (26 September). On the track from D to Milano, the aircraft was above the Sc.
 

Figure 8: ARAT wind measurements south of Genoa at 500 ft.

Figure 9: South-North Lidar cross-section from Ligure to 30 nm north of Genoa.

 

  Operation of all instrument was nominal during the flight.