AFLUX 2019

A third aircraft campaign (AFLUX), based at the Station Nord (Greenland), with one of the two AWI aircrafts is planned for 2019. The objectives and instrumentation are similar to ACLOUD, although somewhat reduced because probably only one aircraft will be available

Strategy

The main motivation for this project is the pressing need to improve the understanding of the physical processes in Arctic clouds, their interaction with the sea ice cover, and their contribution to the current changes in the polar environment. Especially the interaction with the ice surface is not well represented in models, which is due to the small scales involved.

Current studies have furthermore shown that depending on their properties clouds may act as a warming or cooling factor in the arctic environment. However, parameterizations and general assumptions about clouds lead to uncertainties of the representation of these cloud effects in regional and global weather prediction and climate models. Airborne observations are a valuable method to verify and to improve those assumptions and to reduce the model uncertainties. However, airborne measurements are limited in time. Therefore, one aim of AFLUX is also to combine the airborne observations with ground-based long term observations at Ny-Ålesund.

Objectives

Our research target will be cloud fields (in particular, mixed-phase, ice and liquid water stratiform clouds in the boundary layer), which can appear anywhere within the range of our aircraft.  As a reference, also flights in clear-sky condition over the same area are envisioned. The maximum distance from Longyearbyen is 750 km, or 400 nautical miles. The exact flight tracks will depend on the current weather situation, in particular on the occurrence of cloud fields, the sea ice cover, and wind direction since the ideal flight pattern will be performed parallel to the mean wind.

Location

The Polar 5 aircraft will perform multiple science flights from Longyearbyen airport, Svalbard (Svalbard Lufthavn, LYR). Permission is requested to operate the aircraft in a possible sector defined by four points (A, B, C, D) with coordinates

A          85° N / 28° E
B          75° N / 28° E
C         75° N / 10° W
D         85° N / 30° W.

Individual flights will be located within this sector but in a smaller sub-region depending on the aircraft’s range (payload), on the sea ice conditions and on weather. In the last decade sea ice cover was highly variable in the highlighted region shown in Figure 1. The variability concerns especially the position of the pack ice edge North and West of Svalbard and the sea ice cover on Storfjorden (SouthEast of Spitzbergen).

During each flight we will try to reach the ice edge with a reasonable reserve for flight patterns also beyond the sea ice edge over the sea ice covered part.

The airborne observations will be closely coordinated with surface-based observations at the AWIPEV station (Ny-Ålesund).

We will perform airborne measurements with the Polar 5 owned by Alfred Wegener Institute Helmholtz Centre for Polar und Marine Research, Germany, and operated by Kenn Borek Air, Canada. In general, the aircraft will not fly at low altitudes until it is safely away from the coast. No landing of the aircraft outside Longyearbyen will take place during each mission. The aircraft will avoid bird/wildlife sanctuaries. The crew on board the aircraft will look for marine mammals on the sea ice, and if encountered during low altitude flight, the aircraft will change course to avoid them.

The target of the research flights is the sea ice covered area. Flights will be performed there in cloud covered, but also in cloud free conditions. The latter need to be investigated in order to compare the turbulent and radiative fluxes with those measured in the cloudy areas. Part of the flight strategy will include profiling the atmosphere, again in both cloud covered and cloud free regions.

To characterize the cloudy boundary layer, we aim to probe low-level stratiform clouds over sea ice and open water (typically cloud altitude between 1000–6000 ft / 300–2000 m). The scientific objective is to observe those clouds from above and to probe the atmosphere below and inside the clouds. To this purpose, two kinds of flight strategies are required, one for “remote sensing” (passive solar, microwave, active radar, lidar) and one for “in-situ probing” of the turbulent fluxes and individual cloud particles (concentration, size, phase, shape). These different strategies can be realized by Polar 5 either by combining flight sections for remote sensing with in situ sections during individual flights or by realizing only one strategy, e.g. remote sensing, during some flights. In situ and remote sensing flight section should be collocated on the same flight tracks (following the same track but in different heights). This is essential for our research strategy during most of the flights to reach our research goals.

Possible flight patterns are explained below in more detail. There, we distinguish between the ferry flights to/from the locations of main measurements and the patterns at these locations/regions

Ferry 1: Polar 5 will perform remote-sensing observations (see Figure 3) typically at a constant altitude above the cloud top (typically near the maximum aircraft altitude; 10,000 ft / 3,000 m). These legs are straight lines and are sometimes coordinated with satellite overpasses. Some drop sondes will be launched at 10,000 ft (3 km) to determine the vertical structure of the atmosphere but only over uninhabited areas.

Ferry 2: If the endurance allows, ferries back home can be used to repeatedly probe the boundary layer by consecutive saw tooth pattern (climbing-descending, see Pattern B) in low altitude.

Pattern A:

The area of intense observation (AIO) will be approximately over sea ice. Polar 5 will arrive at the pack ice edge and will then go down with a fast descend rate of 1000 ft/ min to reach 200 ft above the surface. A horizontal distance of 80 Nm is flown parallel to the mean wind at this height. A fast ascend will follow to 10.000 ft. At this level the aircraft flies back while releasing a series of drop sondes. This basic pattern can be varied with respect to heights and lengths of the tracks.

Pattern B:

This patterns consist basically of a saw tooth pattern over the ice and cloud covered region. Ascend and descend rates as well as the number of saw teeth can be varied. But a typical climbing rate should be 200 ft /min. This allows to measure turbulent and radiative fluxes as well as cloud particles sufficiently accurate.

Pattern C:

This pattern serves as the basic pattern to measure vertical profiles of fluxes and particles. A large number of horizontal flight legs (e.g., 10) should be flown to achieve a high vertical resolution. If flight time allows, the pattern should be repeated at different positions preferably on the downwind and upwind side of leads or in a region with a section characterized by very homogeneous sea ice cover. Lengths of legs and heights must be varied according to the cloud and sea ice conditions. Such a pattern is also helpful when no clouds are present.

Pattern D (Alternative – Satellite Overpass):

Coordination with satellite observations. This pattern will be flown on a straight leg along the satellite overpass. Polar 5 will perform mostly remote sensing at 10 000 ft altitude with a few sections in the cloud if icing conditions will allow this. The total length of the leg may range about 60 nm, about 15 min before and 15 min after the satellite overpass.