Mission Plan :

NOAA42 will conduct a research mission in the East Pacific for the HFP-IFEX Genesis Stage Experiment. The initial plan (Fig. 1) calls for a survey pattern immediately off the coast of Nicaragua and Costa Rica to sample; (1) the structure of the potential Papagayo Gap flow (with the easternmost southeast to northwest transect); and (2) the expected surface trough to the west of that gap flow (with the westernmost northwest to southeast transect). Dropsondes will be released at each of the points shown in Fig. 1. The pattern will be flown at 14 kft pressure to accommodate the IWRAP data collection (required to be below 15 kft). We will attempt to do TDR analyses, although there will be limited coverage due to the pattern design, as well as the possibility of having very little coverage of precipitation on station.

Figure 2. Visible satellite image of pre-EP96 at 1545 UTC 26 September

At the start of the mission, there is no invest designated in the region and NHC did not indicate any potential for genesis in their Tropical Weather Outlook, perhaps due to the fact that the dynamic models have backed off on the degree of genesis, and that there is still little convective organization (Fig. 2). It did appear that there is potentially a broad surface circulation or trough, likely caused by the interaction of the easterlies to the north (coming from the gap) and the monsoon westerlies to the south. We should sample that with the planned pattern.

Mission Summary :

Figure 3. Actual flight track for mission 20180926H1. Flight-level winds (barbs) and wind speeds (shaded, kt) indicated.

Figure 4. Time series of flight-level wind speed (green, kt), SFMR surface wind speed (blue, kt), and SFMR rain rate (red, mm/hr) for mission 20180926H1, as well as the extrapolated surface pressure (mb) (bottom, teal, mb) and altitude of the aircraft (bottom, black, m).

The actual track of the mission is shown in Fig. 3, with wind information at flight level (FL), extrapolated sea level pressure (SLP), and SFMR surface wind speed and rain rate shown in Fig. 4. The transit to the IP was about 30 min. We reached WP #1 (IP) at 1623 UTC and began the four sonde transect of the Papagayo Gap flow region off the coast of Nicaragua. The sonde at WP #2 was a fast fall, so it was backed up with another sonde. The first sonde at WP #2 and the one at WP #4 were released in combo with an AXBT (AXBT Combos #1, #2). Although the winds closer to flight level were southeasterly (not consistent with any gap flow) (Fig. 3), the dropsondes along this leg did indicate easterly winds of about 30 kt below 1.5 km, including 10-15 kt east-northeasterly winds at the surface (Fig. 5). This, instead, could be an indication of the gap flow in that layer. An example sounding from this leg is shown in Fig. 6 (sonde at WP #3). This sonde indicates that easterlies exist below about 700 mb, with the wind speed peaking around 850-900 mb.

Figure 5: Surface winds from each dropsonde during 20180926H1

Figure 6. Skew-t diagram of the 1651 UTC sonde at WP #3

We arrived at the surface wave trough sonde transect (WP #6 - #11) at 1753 UTC. Up until this time there was little convection of any great depth anywhere in the pattern, but enough so to get returns from the TDR to produce an analysis around the survey (Figs. 7 and 8). We ended up crossing a feature at flight level around 10°N, given the wind shift from northeasterly to southwesterly (Fig. 3). We then subsequently observed another wind shift near the surface a bit farther south (Figs. 5, 7 left panel). This could be the indication of an elongated surface trough. IR sondes were released at WPs #6, #7, #8, #9, and #10, with WP #9 being an AXBT combo drop (AXBT Combo #3). Southeasterlies were mostly observed in the low-levels on the return leg from WP #11, and these sondes were also the driest over the entire sonde of any dropped in the pattern (example in Fig. 9 at WP #12). This sonde in particular was indicative of strong subsidence in the midtroposphere, with an inversion near 850 mb (Fig. 9).

Figure 7: Composite reflectivity and winds at (left) 2 km, as well as (right) windspeed at 2 km (shaded) and streamlines at 2 (black) and 5 km (grey)

Overall, a broad circulation was observed near flight level and the midtroposphere (~5 km), as indicated by the TDR analyses (Figs. 7 and 8, right panels). Near the surface, the aircraft observations suggest a, more or less, east-west elongated trough axis along 9°N; which confirms what was seen in the visible satellite loops, as well as an ASCAT [scatterometer] pass (Fig. 10).

Figure 8: Composite winds at (left) 0.5 and (right) 4 km

Mission Evaluation / Problems :

Overall the mission was successful in sampling the structure across the Papagayo Gap flow, which could be involved in the increase of vorticity / circulation in the potentially developing invest, as well sampling a broad circulation in the midlevels and at the surface (characteristically more of an elongated trough). There does appear to be drier air within the region, particularly to the south) which could have been inhibiting the development of organized convection, as well as limiting the depth of any convection that does develop. NHC during the mission did indicate in their Tropical Weather Outlook at 0/40% chance of development.

Figure 9. Skew-t diagram of the 1930 UTC sonde at WP #12

Figure 10. ASCAT pass at 1630 UTC 26 September, showing surface winds

As for issues, the primary one that arose was that the operational, wing pod SFMR stopped working. It was deemed that it would not be operational for the r emainder of the flight sequence. The 2nd downlooking SFMR in the belly was used in place of the readout from the operational SFMR. There were also 2 fast fall dropsondes, each backed up with another sonde.

Jon Zawislak
Oct. 4,2018

Flight track

Temperature and Moisture

Wind and Atlitude

Flight track

Altitude, Pressure, Rain Rate, and Wind

Flight Director's log | Flight Director's manifest | NetCDF data | serial data | 1 second data
LPS log | Radar log | Drop log

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