Airborne true airspeed and fast temperature measurements with an ultra sonic anemometer thermometer

A fast response thermometer for the measurement of static air-temperature on board a research aircraft is required for numerous purposes including the study of temperature spectra, turbulent heat flux and calculations of true airspeed.

An airborne thermometer-anemometer is designed and built using the well-known sonic anemometer thermometer technique. The shape of this new airborne sonic sensor, without the conventional style probes, is a cylinder with its axis colinear to the aircraft's longitudinal axis. The true airspeed and the air temperature are measured in the same volume.

Between 1996 and 1999, this Airborne Ultra Sonic Anemometer Thermometer (AUSAT) was mounted on the Météo-France's Merlin-IV aircraft to be compared with other airborne thermometers and has flown more than 200 hours. First, it was mounted on a pole above the fuselage and, at the begining of 1997, it was mounted on one of the standard PMS pylon under the aircraft's nose.

The AUSAT was in test for four international field experiments in Livourne (Italy, 1996), Canary Islands (ACE 2, 1997), above the Mediterranean sea (FETCH, 1998) and France (TRAC, 1998). Data recorded during the last flights, after improvements of the sensor's shape, electronics and location on the aircraft, suggest that AUSAT is capable of measuring at high rate the true airspeed and fast air temperature fluctuations.

A simple method to compare response time of different thermometers is to obtain their response to a very sharp step of temperature. Such steps are not easy to find in clear air but are usual at the border of cumulus clouds. So, some results are obtained during constant level flight legs with penetrations in cumulus turrets during the experience TRAC 98.

The response time of the AUSAT is very fast compared to the Rosemount, at the entrance as well as at the exit of the clouds. A comparison between in-cloud and out-cloud AUSAT temperature and a Rosemount and a Météo-France’s thin wire thermometers is too developped.

On board the Merlin-IV, the true airspeed may be calculated from data measured by dynamical and thermodynamical sensors such as a five hole radome and an impact thermometer. Complete calculation of true airspeed needs data provided by several other instruments generally with a very different response time and band pass.

The direct AUSAT airspeed data, compared to the classical calculation, shows that the two determinations are very close. There is neither correlation between the AUSAT true air speed and the angles of attack and sideslip nor correlation between these two angles and the difference between the AUSAT true air speed and the radome one.

In conclusion, although the in-cloud AUSAT behaviour in high humidity area needs further investigations, its weak sensitivity to water droplets and its very fast response time, provide a noticeable improvement in airborne temperature measurements to estimate the entrainment of surrounding air into a growing cumulus and also to calculate the turbulent fluxes.