Dynamics of shallow convection over land : airborne measurements of wind, temperature, humidity and pressure in small cumulus clouds

This thesis reports the calibration of an airborne measurement system, its applications to measure properties of shallow convective clouds over land and an evaluation of a conceptual model of such clouds. In a first step the measurement system on the research aircraft is calibrated. On a Cessna Grand Caravan 208B provided by the German Aerospace Center (DLR) a meteorological sensor package is installed and certified. The system is capable of fast and accurate measurements of pressure, temperature, humidity and three-dimensional wind. The quality of the pressure and wind measurements depends on a careful parametrisation of the aerodynamical interferences caused by the flying aircraft. An optimised method is presented to correct for the pressure error of the dynamic (qc) and static pressures (ps) as measured by the system. The use of a differential global positioning system (DGPS) reduces former uncertainties in the height determination. The calibration data are calculated from the high frequency time series in order to reduce averaging errors. The height correction of the pressure values compensates residual height changes during the flight manoeuvre. The overall accuracy of the pitot-static system is estimated to be _ps < 0.25 hPa. An additional dynamic calibration process bases on a series of test flight manoeuvres performed in different heights yielding a dynamic correction for the pressure signal. A correction for the flow angles ( and ) estimation is derived. The analysis shows, that no time shifts between different data sources exist. A novel procedure for calculating the overall measurement uncertainties of the measurement system is presented. For the different parameters the results are: horizontal wind speed _ws 0.3 ms-1, vertical wind _w 0.25 m/s, temperature _ts 0.15 K, humidity mixing ratio _r 2 % (4 % below 0.5 g/kg). The second step of the thesis covers the investigation of the dynamic structures of shallow convection. In 2012 and 2013 measurement campaigns were conducted to probe individual cumulus clouds over different terrain and for different synoptic conditions. During 6 measurement flights successful cloud transects were performed and a total of 191 single cloud transects were evaluated. The main focus lay on the vertical wind distribution at the border of the cumulus clouds. Recent results of large eddy simulations suggested a thin layer of sinking air around the cloud. This layer has been called \subsiding shell" and according to our definition it is supposed to have a thickness of less than 20 % in cloud diameter. The strength of the downdraft in the shell (O1 m/s) is expected to be much stronger than subsidence. Thus, a significant portion of the downward vertical mass transport is expected to happen within the subsiding shell. From the cloud transects analysed in this thesis just 29 out of 191 showed such a subsiding shell on both sides of the cloud, which is in contradiction to the idea that the subsiding shell is an ubiquitous feature surrounding each individual cumulus cloud. Outside the cloud borders downdrafts and also updrafts occurred with a high variability of size, strength and distribution. No significant differences could be attributed to different terrain as well as activity status, cloud level or transect direction. However, the mean distribution of the vertical wind for all transects shows a distinct Minimum in vertical wind on both sides of the cloud. This leads to the conclusion, that the subsiding shell works as a valid concept for the mean cloud properties, but it is not a characteristic feature of individual cumulus clouds over land. Strong updrafts and downdrafts are rather randomly distributed in the vicinity and also inside the investigated cumulus clouds. Outside of the clouds the downdrafts occur more frequently which leads to a subsiding shell in the mean wind distribution around shallow convection. Within a distance outside the cloud of 20 % in cloud diameter half of the upward directed vertical mass flux of the cloud is compensated. Due to this effect major parts of the cloud air remain in the near surrounding of the cloud.This thesis reports the calibration of an airborne measurement system, its applications to measure properties of shallow convective clouds over land and an evaluation of a conceptual model of such clouds. In a first step the measurement system on the research aircraft is calibrated. On a Cessna Grand Caravan 208B provided by the German Aerospace Center (DLR) a meteorological sensor package is installed and certified. The system is capable of fast and accurate measurements of pressure, temperature, humidity and three-dimensional wind. The quality of the pressure and wind measurements depends on a careful parametrisation of the aerodynamical interferences caused by the flying aircraft. An optimised method is presented to correct for the pressure error of the dynamic (qc) and static pressures (ps) as measured by the system. The use of a differential global positioning system (DGPS) reduces former uncertainties in the height determination. The calibration data are calculated from the high frequency time series in order to reduce averaging errors. The height correction of the pressure values compensates residual height changes during the flight manoeuvre. The overall accuracy of the pitot-static system is estimated to be _ps < 0.25 hPa. An additional dynamic calibration process bases on a series of test flight manoeuvres performed in different heights yielding a dynamic correction for the pressure signal. A correction for the flow angles ( and ) estimation is derived. The analysis shows, that no time shifts between different data sources exist. A novel procedure for calculating the overall measurement uncertainties of the measurement system is presented. For the different parameters the results are: horizontal wind speed _ws 0.3 ms-1, vertical wind _w 0.25 m/s, temperature _ts 0.15 K,by Christian MallaunKumulative Dissertation aus drei ArtikelnUniversity Innsbruck, Dissertation, 2018OeBB(VLID)278136