Meteorol. Atmos. Phys. 41,247-254 (1989) Meteorology, and Atmospheric Physics by Springer-Verlag 1989 551.576.1 DLR 1-Institute for Atmospheric Physics, Oberpfaffenhofen, Federal Republic of Germany Cloudphysical Parameters in Dependence on Height Above Cloud Base in Different Clouds H.-E. Hoffmann and R. Roth With 5 Figures Received February 9, 1989 Revised August 7, 1989 Summary 1. Introduction On flights with the DLR icing research aircraft the depend- ence of aircraft icing on cloudphysical parameters was de- About four years ago, the Institute for Atmos- termined; both for aircraft-referred icing and for normalized pheric Physics of the DLR began to work on re- icing, as well as for various clouds and locations in clouds. search project "Icing of aircraft", One of the ob- This is done with an improvement of icing predicitons in jects of this research is defined by Hoffmann and mind. The species of the cloud and the distance from cloud base are called here "cloud parameters"; while under "cloud- Demmel (1984) as Determination of Flight Re- physical parameters" are understood liquid water content, strictions of Aircraft Caused by Icing in Dependence temperature, particle size distribution and particle phase. Re- on Meteorological Parameters. sults from four icing flights are discussed, selected from a Meteorological parameters here are: Cloud- total of forty vertical soundings.- The results are arranged physical, synoptic and cloud parameters. First re- in four classes: Stratus/cumulus mixed, stratus; with and without precipitation at the ground. sults, concerning the dependence of the normal- 1. Stratus/cumulus with either simultaneous or earlier ized ice thickness, and by this also of the nor- (3 h) precipitation at ground: Maxima of liquid water content malized icing-degree, on the cloudphysical param- (LWC: 0.75 and 0.55 g/m3, resp.) and maxima of the median eters liquid water content, temperature, particle volume diameter (MVD: 183 and 123 gin, resp.) both located size distribution and particle phase, were pub- in lower half of clouds, 2. Stratus/cumulus without precipitation: Maximum of lished in the report mentioned above and in Hoff- L WC (0.45 g/m3) in upper third of cloud, but M VD increasing mann et al. (1986). Normalized ice thickness is the from base to top of cloud (7 to 20 pm). ice thickness in flow direction on metal cylinders 3. Stratus with precipitation: No results. with diameters of 12.5; 25.0 and 50.0mm on a 4. Stratus without precipitation: Both LWC and MVD flight path in clouds of 10 nautical miles at a flight increase from base to top of cloud; maxima directly beneath cloud top: LWC 0.50g/m3; MVD 17~tm. velocity of about 125 kt. Figure 1 was taken from Hoffmann (1988) and shows the dependence of the normalized icing-degree and the normalized icing thickness on the total water content. Total 1 DLR, formerly DFVLR = Deutsche Forschungsanstalt f/ir Luft- und Raumfahrt (German Aerospace Research Es- water content here is the water content formed by tablishment). liquid and also by solid particles. The regression 248 H.-E. Hoffmann and R. Roth 7~ Table 1. The Icing-Relevant Cloudphysical Parameters Tem- perature T, Total Water Content TWC, and Median Volume Diameter MVD .for Do 28 Aircraft Referred Icing Degrees IDeg 54 "'Trace" to "Light" and "Moderate" to "Severe" L 44 EN 23-1 T [~ T [~ T 2"1 -7.8 (-4.2; - 10.8) -3.7 (-2.0; -7.9) 14 TWC [g/m 3] : TWC [g/m 3] : 0.19 (0.04; 0.44) 0.12 (0.05; 0.31) OI f I I F l O.00 O.lO 0.20 0.30 g/m3 0.50 MVD [pro] : MVD [pm] : 14.5 (11.0; 18.5) 26.5 (15.0; 36.5) Fig. 1. The normalized icing-degree IDes and the normalized icing thickness IN on a cylinder with a diameter of 12.5 mm Do 28 aircraft referred icing Do 28 aircraft referred icing in dependence on total water content TWC. Temperature: 0 degree trace to light degree moderate to severe to -16~ particle size distribution (MVD): 13 to 340gm, phase of particles: Liquid or mixed T: Normalized icing-degree Trace; L: Normalized icing-de- gree Light the cloudpyhsical situation for the aircraft-re- ferred icing-degree moderate to severe. Reference aircraft here is the DLR icing research aircraft of type Do 28. The mean values of the left part were line of Fig. 1 represents 53 single results. These obtained from 37 single values and those of the were obtained on 53 flight sections for determining right part from 17 single values. The values in the normalized icing-degree during the icing flights parantheses are the minimum, respectively max- of the winter 1984/85. The regression line is valid imum values. When the temperature was relative for temperatures between 0 and -16~ and high- mean value - 3.7 ~ - and when there were MVD-values between 13 and 300 t,tm. From Fig. big particles in the particle spectrum - mean value 1 results: Under the test conditions of Fig. 1 the for the median volume diameter 26.5 pm - the air- normalized icing-degree is linearily growing with craft-referred icing-degree was moderate to severe. increasing total water content. The normalized ic- Significant differences of the value for the total ing-degree "trace" is attained at total water con- water content cannot be seen in both the icing- tent values of up to 0.30 g/m 3 and the normalized degree ranges of Table 1. The aircraft-referred icing-degree "light" at total water content values icing-degree moderate to severe was caused by the between 0.30 and 0.50 g/m 3. At total water content ice which was built up on the wing underside. values greater than 0.50 g/m 3 you have to expect When the liquid cloud particles are big, and the the normalized icing-degree to be moderate up to temperature is relatively high, only a part of the severe. water impinging upon the front edge of the wing Such a simple statement as for the normalized is freezing instantly. The other part is running on icing-degree cannot be made for the aircraft-re- the underside of the wing where it is freezing, see ferred icing-degrees, according to the test results also Sand etal. (1984). obtained up to now. The causes for this are that Here the results of four icing flights are reported their definitions are very unprecise, see Air at which three different stratus/cumulus clouds Weather Service (1980) and Hoffmann (1988), but and one stratus cloud could be traversed from their also the difficulties to obtain them in flight ex- tops to their bases or vice versa. At the same time, periments. The values of Table 1, too, which also the cloudphysical parameters liquid water content, was taken from Hoffmann (1988), are based on particle size distribution and temperature were the results of the icing flight sections of the winter measured. These 4 vertical soundings are a selec- 1984/85. The values for the cloudphysical param- tion from all the 40 soundings which could be eters temperature, total water content and median conducted and evaluated so far and are represen- volume diameter of the left half of Table 1 are tative for the classes in which the results, hitherto mean values and represent the cloudphysical sit- existing, are summarized: Stratus/cumulus cloud- uation for the aircraft-referred icing-degree trace iness (mixed cloudiness) and stratus cloudiness, to light. The mean values of the right half represent without and with precipitation on the ground, see Cloudphysical Parameters in Dependence on Height Above Cloud Base 249 Hoffmann etal. (1988). All results, existing and an Optical Array Probe (OAP), both manufac- of the future, shall serve for an extension of a tured by Particle Measuring Systems (PMS) at climatology of icing relevant cloudphysical pa- Boulder, were used for measuring the size and the rameters, see Roach et al. (1984). number pro unit volume of the cloud particles. FSSP is measuring the particle size up to diameters of 47 lain by forward scattering of light and OAP 2. Test Procedure, Used Instruments, up to diameters of 600 gm by 1-dimensional im- and Preparation of Measured Values aging. For the determination of the value for the For conducting icing flights from DLR a two- median volume diameter (MVD), the L WC values, propeller aircraft of type Do 28 has been equipped derived from PMS measurements were used. To as an icing research aircraft. From this, all icing eliminate additional problems, concerning the size relevant atmospheric parameters and the icing can determination of cloud particles in mixed phased be measured. The icing is measured on three metal particle populations measurements were used only cylinders and on the front edge and on the un- when the particles were all liquid. The discrimi- derside of one of the wings, see Hoffmann and nation between only liquid, mixed phased and only Demmel (1986). solid particles was done by visual observation by The values for the liquid water content (L WC), the pilots and measuring engineers and by means given in the following, were obtaind from a John- of the character of the signals of a Csiro-King hot son-Williams hot wire instrument (J.-W.). A For- wire instrument and of a backscatter probe. The ward Scattering Spectrometer Probe (FSSP) and signals of both the instruments demonstrate sig- H I 221 I0-2 jL~ [m]10001200 i" ~ ~- 10-1_~ iJ ................ 10 ~ - (5) 10- 4 :~ / LW C [g/m 3 ] 10.2 800 :, " ." n IO-3 ' -(4) ? ~ lO-4 .