HOP Helicopter Observation Platform @ Duke University

Spatial variability of the Earth’s surface has a considerable impact on the atmosphere at all scales, and understanding the mechanisms involved in land-atmosphere, and ocean-atmosphere interactions in this highly heterogeneous environment is hindered by the lack of appropriate observations. Observing the physical and chemical properties of the atmosphere near the Earth’s surface, both over land and water, remains a great challenge. This is particularly true for the turbulent fluxes of heat, trace gases and aerosols.

Towers and Aircraft Observations - Recognizing the Limitations

Tower-based observations are the best available technique to record long time series over the land. However, they only provide a very limited number of points in the lower atmosphere, and even using a high-density network of towers (which is practical only at the microscale), deciphering the footprints of spatial variability in the atmospheric variables collected with them has had only very limited success. Combining towers and remote sensing techniques (from space and/or the ground) helps mitigate the obvious deficiency of point observations, yet many of the processes linking the two methods are empirical in nature and the fundamental mechanisms needed to use such an approach more efficiently and more accurately remain to be elucidated.

Many different types and sizes of aircraft have been used to make spatio-temporal observations of the atmosphere. As aircraft have a limited flight time capability and are expensive to operate, they are used only in relatively short missions, typically as part of dedicated intensive field campaigns. Yet, in spite of these obvious limitations, they fulfill a key role in our observation strategy.

In general, large airplanes have expensive costs for fuel, maintenance, and personnel, but house a full complement of scientific investigators. They have long flight durations, large payloads, and fast transit speeds. But at the airspeeds needed for large airplanes to maintain lift (at least 60-70 m/s), inlet losses make sampling “super-micron” particles very inefficient and, as explained in detail below, turbulent fluxes are measured less accurately. While they can fly low (as they do, obviously, on landing and takeoff), it is not practical and quite risky to do it outside of an airport environment. Furthermore, as explained in the side textbox, the USA Federal Aviation Regulation (FAR) §91.119 practically prohibit low-level flights with airplanes over most of the continental USA (except for the western deserts), as it would be difficult to find a long enough leg without operating “...closer than 500 feet to any person, vessel, vehicle, or structure.”

Small aircraft have lower costs, but they also have limitations on duration, speed and maximum payload. The slower speed (as compared to large airplanes) is an advantage for aerosol sampling and measuring turbulent fluxes, but it prohibits the use of small airplanes in areas more than 100 km from an airport since the transit time will often require 50% or more of the allowable flight duration. This limitation is particularly relevant for off-shore research missions. To alleviate the payload limit, the Network of Airborne Environmental Research Scientists (see www.naers.org) suggests simultaneously using well-coordinated aircraft, each one dedicated to a particular instrument. An important point is that there is no distinction in FAR §91.119 between types of airplanes, and they are all subject to the same altitude restrictions, no matter how small they are.

Advantages of Slow, Low-Altitude Flights

The main advantage of a HOP is that it combines slow sampling speed and near-surface flight capability. The importance of slow speed is maybe best illustrated with a realistic example. Assuming that a helicopter flies at an airspeed that is 1/3 that of an airplane (say, 25 m/s vs 75 m/s), it measures atmospheric variables at a spatial resolution three times better than that obtained by the airplane if both use the exact same sensors. This is crucial for measuring the high-frequency turbulent perturbations that are an important component of all turbulent fluxes in the atmospheric boundary layer (ABL).

The importance of low-altitude flight capability is illustrated in the figure at right, which shows a characteristic vertical profile of sensible heat flux in the convective boundary layer (CBL). Understandably, an airplane not allowed to fly below the altitude illustrated with the greyline would be limited to sampling the CBL at heights where the absolute value of the flux is near zero. Exacerbated by the loss of accuracy and precision associated with the loss of high-frequency turbulent motions due to high sampling airspeed, this could result in measurements that generate an error in the flux calculation that is at least of the same magnitude as the flux itself.

Given the linearity of the sensible heat flux distribution in the CBL, the entire profile could be assessed from two altitudes, yet minor absolute errors at two altitudes near the CBL top could result in large errors in derived surface fluxes. On the other hand, a sampling near the ground surface and near the top of the CBL results in a much more reliable flux profile. It is worth noting that during the Cloud and Land Surface Interaction Campaign (CLASIC) in June 2007, surface sensible heat fluxes of less than 30 W/m² and ABL heights of 200-300 m were frequently observed. Thus, airplane measurements of that variable would have been worthless given the precision and sampling frequency of even the most sophisticated, state-of-the-art sensors currently available. A similar case could be made for any turbulent flux that has a strong source or sink at the ground surface. This is even more crucial when the source/sink term is dependent on the land-cover type (as is the case for heat, momentum, moisture, CO₂ and many trace gases and aerosols), in which case it is unrealistic to expect reasonable estimates of turbulent fluxes from airplane observations.

Ease of Deployment

The helicopter platform can also enjoy an effectively longer duration at the designated sampling area, since it can land and refuel at locations inaccessible to fixed-wing aircraft, removing the waste of fuel and time that occurs in transit. Indeed, it is logistically possible to bring a fuel truck to a landing site at or near the sampling area where the helicopter could stop regularly for refueling. Perhaps the biggest advantage of all, which has never been exploited by the scientific community in spite of the need and feasibility, is the opportunity to perform marine observations far from shore using a helipad aboard a ship. Such a helipad is available, for instance, on the NOAA David Starr Jordan and could be adapted to fit other research vessels to make remote marine locations requiring a US Class I research ship accessible, with effectively all of the flight hours available on station for observations. Modern commercial cruisers are also typically equipped with helipads, and cooperation with the scientific community, as is maybe best demonstrated with the past research missions conducted on the Explorer of the Seas (see www.royalcaribbean.com) is feasible. It is therefore conceivable to deploy a properly equipped HOP for marine operation in collaboration with passenger and/or cargo ships. Unlike with even large aircraft that can remain on station for a few hours before heading back to shore, a helicopter on a ship could stay at sea for extensive periods thus providing the opportunity for long marine atmospheric campaigns. The magnitude of turbulent fluxes, aerosols and atmospheric chemistry above the oceans remain uncertain, and the HOP has the potential to revolutionize the quality and quantity of scientific information that could be gathered there.

Precedence for Helicopter Observations

Despite these advantages, helicopter platforms have been used mostly for remote sensing applications (e.g., Babin 1996), and only sporadically for in-situ atmospheric sampling. Maybe this can be attributed to the popular belief in our scientific community that atmospheric sampling on a helicopter is not feasible because of the main rotor “downwash.” But as illustrated in Leishman (2006, e.g., Figure 11.7, Page 661, among many other examples in that textbook), even at low airspeed, the wake created by the main rotor is skewed backward and has no impact on the air in front of the helicopter nose. This is why the pitot tube of many helicopters is installed at that location (including on the Jet Ranger) so that even at airspeed as low as 6-7 m/s the rotor wake has no impact on the helicopter instrument readings. Obviously, accurate flight instruments readings are essential for flight safety and measuring the rotor wake instead of the undisturbed atmosphere would be unacceptable.

A few observational studies performed onboard helicopters are, however, quite noticeable. Among them, a series of air sampling campaigns was carried out by the Tennessee Valley Authority (TVA) with a Bell 205 specifically equipped to observe various atmospheric oxidants (e.g., Imhoff et al 1995, Valente et al 1998, Luria et al 1999, among many others). Air quality monitoring was also conducted by Roeckens et al (1992), De Saeger et al (1993), and Desmet et al (1995). A helicopter-based observation system, the “helipod,” samples atmospheric properties, including fluxes, using a pod towed on a long cable underneath the main cabin (e.g., Muschinski and Wode 1998, Roth et al 1999a, Roth et al 1999b, and van den Kroonenberg and Bange 2007).

While the helipod benefits from some of the advantages of a helicopter platform (e.g., time on station), it is in fact a glider that needs to be flown at a relatively high speed to remain stable, restricting some of the maneuverability of the towing helicopter (e.g., flight very near the ground surface). Motivated by the need for near-surface observations and the increased accuracy of measurement obtained at low speed, and building upon the success of previous research missions (especially those of the TVA), we developed the HOP described here.

References

• Babin, S. M., 1996. Surface duct height distributions for Wallops Island, Virginia, 1985 to 1994. J. Appl. Meteorol., 35, 86-93.
• De Saeger, E., G. Dumont, E. Roekens, D. Tielemans and G. Verduyn, 1993. Study of the Photochemical Pollution in Belgium, Guest Contribution. Proceedings of EUROTRAC Symposium '92, 93-97.
• Desmet, G., G. Dumont, D. Tielemans, R. De Lathouwer, and E.J. Roekens, 1995. Technical Note: Measurements of Atmospheric Pollutants Using Helicopters: Evaluation of the Possible Contamination of the Sample Air by Turbine Exhausts. Atmos. Environ., 29(18), 2547-2552.
• Imhoff, R.E., R. J. Valente, J.F. Meagher, and M. Luria. 1995. The production of O3 in an urban plume: Airborne sampling of the Atlanta urban plume. Atmos. Environ., 29, 2349-2358
• Leishman, J.G., 2006 Principles of Helicopter Aerodynamics. Cambridge Aerospace Series.W. Shyy and M.J. Rycroft, Editors, Second Edition, 826 pages.
• Luria, M., R.J. Valente, N.V. Gillani, R.L. Tanner, R.E. Imhoff, and J.F. Meagher, 1999. The evolution of photochemical smog in a power plant plume. Atmos. Environ.,31, 3023-3036.
• Muschinski, A., and C. Wode, 1998. First in situ evidence for coexisting submeter temperature and humidity sheets in the lower free troposphere. J. Atmos. Sci., 55(18),2893-2908.
• Roekens, E.J., G.F. Dumont, D.M Tielmans, G.E. Verduyn, and E.G. De Saeger, 1992. Ozone Levels in Belgium. Air Pollution Control: papers from the 9th World Clean Air Congress, Montreal, Quebec, Canada, September 4, 1992.
• Roth, R., M. Hofmann, and C. Wode, 1999a. Geostrophic wind, gradient wind, thermal wind and the vertical wind profile - A sample analysis within a planetaryboundarylayer over Arctic sea-ice. Bound.-Layer Meteorol., 92, 327-339.
• Roth, R., M. Hofmann, and C. Wode, 1999b. The determination of the geostrophic windspeed on the basis of pressure measurements with the HELIPOD-system. Meteorologische Zeitschrift, 8(1), 36-39.
• Valente, R.J., R.E. Imhoff, R.L. Tanner, J.F. Meagher, P.H. Daum, R.M. Hardesty, R.M. Banta, R.J. Alvarez, R.T. McNider, and N.V. Gillani, 1998. Ozone production during an urban air stagnation episode over Nashville, Tennessee. J. Geophys. Res., 103, 22,555-22,568.
• vanden Kroonenberg, A., and J. Bange, 2007. Turbulent flux calculation in the polar stable boundary layer: Multiresolution flux decomposition and wavelet analysis. J. Geophys. Res., 112(D6)D06112.