G4 Flight in support of FASTEX IOP 11A and IOP 12
Low 33 and Low 34
(report prepared by Jean-Pierre Cammas)

Date: February 08, 1997
Take-off Time Shannon (approx): 1000Z
Landing Time Saint Johns (approx): 1530Z
Flight duration: 5 hr 30 mn

Date: February 09, 1997
Take-off Time Saint Johns (approx): 0921Z
Landing Time Saint Johns (approx): 1320Z
Flight duration: 6 hr
This mission was aborted before the first way point when 
encountering malfunction of deicing equipment. 

 
Scientists: Jean-Pierre Cammas (LA/CNRS),
            Melvin Shapiro (NOAA), 
            Ron Gelaro (NRL), 
            Patrick Mascart (LA/CNRS),
            Diana Baertels (NOAA),
 

Purpose:

This flight from Shannon to St Johns was designed to target the 
far upstream sensible areas for the forecast of Low 34A. This low
is a FASTEX-like cyclone that forms in a broad trough south of 
New-Foundland and that is expected to explosively deepen as it 
crosses the upper-level jet on the western edge of the MSA the 
day after. Far upstream targets were shown to be south-east of 
St Johns according to adjoint and singular vectors computations 
both from Arpege (Meteo-France) and NOGAPS (NRL). 

On route to Saint Johns we flew across Low 33. This low was not 
scheduled to be flown by MSA aircraft due to the maintenance day. 
As this low was a Fastex-like case and because Fastex ships made 
valuable observations on it prior to the G4 flight, we decided 
to launch 10 dropsondes between 21W and 30W (53N) on this system. 


Description of the flight: 

The entire flight track was flow at FL260. On the overall, 
28 dropsondes were launched, two had bad pressure, 3 had missing 
winds. 

About one hour after taking-off decision was taken to launch 10 
dropsondes between 21W and 30W at 53N over Low 33. This cut 
involved the surface cold front, the dry intrusion and the bent 
back structure further west. Before to cross the surface cold front 
and right below the southerly upper-level winds of the jet core, 
a nice low level jet was observed (about 30 ms-1). Skew-T plots 
showed up interesting structures of the upper-level frontal zone. 
The mid-tropospheric humidity revealed a first dry intrusion below 
the upper-level jet core and a second one west of the bent back 
structure. 

The second part of the flight (1315Z-1512Z) was dedicated to the 
targetting pattern for Low 34. On a subjective point of view, 
the track pattern encompasses different interesting structures. 
The first north to south leg (5 dropsondes along 52N-48N at 42W) 
crosses the exit region of a zonal upper-level jet streak northeast 
of the surface trough where was embedded the incipient low 34A. 
A dry intrusion and winds in excess of 60 ms-1 (350 hPa) were 
recorded when crossing the jet core. Going southeastward we flew 
southward of and parallel the jet streak on its anticyclonic 
shear-side. This leg encompassed the maximum 500 hPa ascent forcing 
eastward of the surface trough. A very stable layer was observed 
all along this leg at 850 hPa. 


Overall Assessment of the Flight:

First, a successful cut across Low 33 which nicely complemented 
the ship observations of this low. 

Second, a successful targetting mission for Low 34A according to 
ARPEGE (METEO-FRANCE) and NOGAPS (NRL) targets. 
Interesting subjective targets were also sampled in this second 
part of the flight.