Aircraft Measurements of Temperature and Liquid Water Content

in Entrainment Interface Layer of Stratocumulus Clouds.

by

Krzysztof E. Haman¹, Hermann Gerber², Wojciech Kumala¹, Szymon P. Malinowski¹ and Marta Kopeć¹

¹ University of Warsaw, Poland

² Gerber Scientific Inc. Reston, VA, USA.

Corresponding Author: Krzysztof E. Haman, e-mail: khaman@igf.fuw.edu.pl University of Warsaw, Instituteof Geophysics, Warsaw, Poland

Entrainment of dry, warm air from above the cloud and its mixing with the colder cloudy air is an important process in dynamics of an inversion topped stratocumulus (Fig. 1), leading to formation of a transition layer of complex structure - Entrainment Interface Layer (EIL). It consists of mutual filaments if cloudy and clear air of various thickness at different stages of stirring, mixing and homogenization. Borders between these filaments are often very sharp, with temperature jumps of few kelvins and liquid water content (LWC) jumps of up to 0.5 gm^-3 over distance of few centimeters, which cannot be resolved by means of standard aircraft instrumentation. This layer is an area of various specific dynamic and thermodynamic phenomena; in particular it is a source of downdrafts penetrating the cloud as the so called "cloud holes". Small scale structure of EIL has been investigated in 2001 during DYCOMS II campaign in marine stratocumulus over Eastern Pacific, by means of Ultrafast Aircraft Thermometer (UFT-F) from University of Warsaw and PVM-100A LWC-meter from Gerber Scientific, Inc. Some results of this research has been published in 2007 in Quarterly Journal of RMS. UFT-F has a thermoresistive sensing element protected against impact of cloud droplets and response time constant of order 10^-4s. PVM-100A is an optical instrument and has spatial resolution of order 10 cm. For recording a sampling rate of 1kHz has been typically applied with 10 kHz (for UFT-F only) on selected fragments of flights. Unfortunately, for some technical reasons, these two instruments, installed on the NCAR C-130 aircraft, were separated by about 6 meters what limited possibilities and precision of comparing their indications. There were also some failures during the flights due to which many potentially interesting measurements and observations have been lost.

Opportunity to get improved observations of EIL appeared in 2008 at POST (Physics of Stratocumulus Top) Project. During POST seventeen flights in inversion topped marine stratocumulus over Eastern Pacific have been made between 16 July and 15 August 2008 with CIRPAS De Havilland DHC-6 Twin Otter aircraft flying in porpoising manner through the stratocumulus top. UFT-F ( modernized version) and PVM-100A were on this aircraft separated by less than 1m.(Fig. 2). Both instruments worked more reliably than during DYCOMS II, yielding a considerable amount of valuable data. UFT-F was sampled at 20 kHz rate what under relatively low speed of DHC-6 (ca 55m/s) gave spatial resolution of temperature field below 1cm. In order to compare with PVM-100A these data were averaged down to 1 kHz rate, corresponding to temporal resolution of the latter sensor.

The modernized version of UFT-F has two sensing wires on one support close to each other with separate electronics, permitting continuation of measurements in case when one of them becomes damaged. Miniaturized electronics with first stage amplifier is located inside the vane (Fig. 3), so that path of low voltage signal is very short and hence less sensitive to electric disturbances than in the elder versions.

Due to limited time and space of this presentation only few items from a number of potentially interesting results of high resolution measurements can be shown here. We decided to concentrate on cases showing evidently inhomogeneous stage of stirring and mixing of cloudy and cloudless air. Two flights of seventeen available were selected for more detailed examination, one made during daytime the second during night. The following examples are taken from the latter one.

In a case of Cloud Top Entrainment Instability (CTEI) conditions, which prevailed during examined flights, presence of inhomogeneous stirring can be detected, even with relatively slow instruments, if LWC is found in the air with temperature higher than that of neighboring pure cloud at the same level (provided that considerable thermal effects of vertical displacement can be excluded). This follows from the so called mixing diagrams for homogeneous and inhomogeneous mixing (Fig. 4) and can happen only when coarse mixture consists of grains or fibers of pure (or nearly pure) cloudy air of size smaller than resolution of the instruments, entangled with such structures of dry air. Examples of such situation are presented in Fig. 5(slide 6) on records of temperature and LWC made with 1kHz sampling rate. For comparison, a short fragment of record made in the clear air above the inversion is presented in the upper left corner with expanded temperature scale. Time on the horizontal axis is counted from the beginning of GMT julian day. Selected fragments of temperature records extracted from there with 20 kHz sampling are presented in Figs 6 and 7 showing small scale structure of the stirring process. Fast fluctuations of ca 0.1-0.2^oC peak to peak amplitude visible on the red line are of instrumental origin; black line shows the results of filtering with 10-th order Butterworth filter with 2kHz cut-off frequency. Notice ca 10 cm thick layers in the 6537.025 - 045 interval (where in the 1kHz record positive LWC is shown) with temperature going down to 9.5 - 9.5^oC which is close to 9.4^oC corresponding to the typical temperature of the neighboring pure cloud (effect not visible in 1kHz sampling). The latter value is not achieved probably due to the inertia of the thermometer. Notice nearly 1^oC jump of temperature over ca 2 cm thick layer located in Fig. 6, 6537.040 - 045 interval. Similar low temperature layers, but thicker, can be seen in Fig. 7 (with changed time and temperature scales). Thinner and sharper structures of this sort had not been found until now, but it doesn't mean that they don't exist and will not be detected in further studies.

The presented examples are only a small fraction of results which are expected after analysis of vast amount of data, collected during the POST experiment, is completed. This work has just started and will probably take many months (if not years), but hopefully will considerably improve our understanding of complicated physical processes going on in stratocumulus clouds.