Observations of precipitation elements in cumulus clouds

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<ul><li><p>551.508.765 : 551.574.1 </p><p>Observations of precipitation elements in cumulus clouds </p><p>By R. J. MURGATROYD and M. P. GAFSOD Meteorological Research Flight, Farnborough </p><p>(Manuscript received 27 July 1959) </p><p>SUMMARY </p><p>Cloud particles of diameters greater than about 100 p can be sampled from an aircraft by exposing within the cloud a thin aluminium-foil surface to the airstream and counting and sizing the indentations made on it, A series of flights has been made through cumulus clouds using this technique with the object of obtaining data on the concentrations of these larger particles in clouds of different size and temperature. A series of observations showing the measured concentrations is presented and discussed. Concentrations range from less than l/m3 in the smaller cumulus clouds up to l/litre or more in clouds about 1 km thick. </p><p>1. INTRODUCTION </p><p>Sampling of cloud droplets from an aircraft is usually carried out by exposing within the cloud some form of slide with a prepared surface, e.g., soot, magnesium oxide or oil and then counting and sizing the holes made by the droplets on impact or the number of droplets caught in the oil. The spectrum so obtained is limited at its lower end by the collection efficiency of the exposed slide. The limitation at the upper end is usually imposed by the small volume of air that can be sampled without swamping the slide by the smaller-sized droplets. The concentration of cloud droplets measured by this method is usually in the order of hundreds per cubic centimetre and the exposure time and size of the slide are such that a few tens of cubic centimetres of air are sampled. Raindrop concentrations at the surface, however, are usually found to be in the order of 0.1 to 1 per litre, and therefore to measure rain or precipitation elements in the air we need to sample tens or hundreds of litres of cloud air and the resulting record must not be spoilt by the effects of the very much larger concentrations of smaller particles. By precipi- tation elements we mean, in this context, those particles with diameters greater than about loop, a size where droplet growth by coalescence is very much greater than that by condensation, and approximating to the smallest drizzle size. An instrument has been developed which measures only particles greater than this size and in one of its applications a series of flights has been made in cumuliform clouds of different thickness and tempera- ture to obtain data, useful in cloud-physics studies, on the number of precipitation elements they contain and hence their potentiality of producing rain. </p><p>2. THE SAMPLING INSTRUMENT </p><p>The instrument used was the aluminium-foil impactor (Garrod 1957) which was based on laboratory work carried out by the Mechanical Engineering Department, Royal Aircraft Establishment, Farnborough, and is now regularly used for sampling rain from aircraft. A photograph of this instrument and typical precipitation records have also been given by Durbin (1958). In order to determine the minimum size of particle which would give a record, a calibration against several hundred samples obtained on oiled slides was carried out in an icing wind tunnel operated at aircraft speeds and with droplet sizes and water contents similar to those found in clouds. The method consisted in counting all marks made in the aluminium-foil surface over a given period and comparing the number against the overall spectrum of all the oil samples taken during the same period to find the particle size above which the total numbers were equal. No great precision is claimed for the method, but the conditions of calibration simulated flight conditions </p><p>167 </p></li><li><p>168 R. J. MURGATKOYD and M. P. GARROD </p><p>realistically and it seems very likely that, with care, cloud particles down to 100 p diameter can be detected. Larger sizes, e.g., greater than 250 p diameter can be measured easily. With experience some differentiation is also possible between the indentations made by water droplets, snow and ice crystals. Water droplets make characteristic circular marks, ice crystals leave sharper dents or sometimes lines, and snow produces a blurred impression with no distinct outline. </p><p>3. FLIGHT PROCEDURE Occasions on which sampling was possible were limited to days when individual </p><p>cumulus clouds were well separated. This ensured that a series of runs could be made in one particular cumulus cloud, and made positioning of the aircraft easier. A suitable cloud having been selected, a number of runs was made through it at different levels with 500 ft or 1,000 ft (150 to 300 m) separation and at an airspeed in the range of 160 to 200 kt (80 to 100 m sec-I). Owing to rapid changes in the cloud structure it was found necessary to limit the number of runs to about six, but on occasions the clouds dispersed after one or two runs. </p><p>The impactor was normally exposed before entering cloud, and withdrawn after leaving cloud, the time in cloud being noted. However, if the cloud was particularly large, or the drop concentration very high, the impactor was exposed for a shorter period inside the cloud. During exposure the aluminium tape was kept stationary, the sampling area on it being equal to that of the aperture, i.e., 3 cm2, so that the volume of air sampled was 300 litres per km flown. </p><p>Other observations included the height of the base and top of the cloud, the mean airspeed during the exposure of the impactor, and the temperature structure in clear air from 1,000 ft (300 m) to the cumulus tops. </p><p>4. RESULTS Appendix 1 gives details of the day's weather and synoptic conditions, with particular </p><p>reference to the Farnborough area in southern England where all the flights were made. Fig. 1. shows individual observations, with the concentrations in numbers/litre of particles greater than 100 p diameter plotted at the cloud sampling heights. Base and top tempera- tures and maximum theoretical adiabatic water content for each cloud are also given. The clouds are plotted in order of size in six arbitrarily chosen groups which are discussed below. It will be noticed that there is great variability between concentrations of precipita- tion particles even in clouds of the same group. The histograms on the right of the diagram show the total number of observations in each group in which concentrations of nil, 0.001 to 0.01, 0.01 to 0-1, etc./litre were found. </p><p>(a) Cutnulus clouds oJ small uertical extent having temperutures ubowe 0C throughout These warm clouds were all less than 2,500 ft (750 m) in vertical extent and had </p><p>adiabatic water contents of less than 2 g m-3. In general, the smallest clouds in this group did not contain any drops exceeding 1 O O p diameter, and even in the largest of these clouds, concentrations of .Ol/litre were only exceeded infrequently. No noticeable differences were observed between the small clouds in stable synoptic conditions and the small clouds accompanying larger clouds in less stable situations. No changes in charac- teristics between morning and afternoon clouds could be detected. </p><p>( b ) Cumulus clouds of moderate vertical extent having temperatures above 0C throughout The clouds in this group had thicknesses from 3,000 ft to 8,000 ft (1,000 m to 2,500 m) </p><p>and theoretical water contents of up to about 3 g m-3. Droplets greater than 100 p dia- meter were usually present and on a substantial number of occasions concentrations of </p></li><li><p>PRECIPITA'I'ION ELEMENTS IN CUMULUS 169 </p><p>O*l/litre or greater were found. In their development from the preceding group the number of precipitation elements had therefore risen to concentrations equal to those commonly found in rainfall at the surface, and some possibility exists of rainfall being produced by the condensation-coalescence process in clouds in this size range. Most of the observa- tions were made in unstable situations with showers, and sometimes thunderstorms were observed in larger clouds during these flights. </p><p>(c) Mixed cumulus and stratocumulus clouds with temperatures above 0C throughout It was noticed on some occasions that the concentrations of large droplets were very </p><p>much greater than those given in groups ( a ) and (b ) above. These cases could be associated with mixed situations, e.g., cumulus forming from a dissipating stratocumulus layer, cumulus penetrating a stratocumulus layer, or the existence for several hours, of a strato- cumulus layer with cumulus forming and continuously spreading out. Evidently these types of situations, as distinct from purely cumuliform cloud cover, are likely to produce rainfall more easily and it is often difficult to decide what is the role of the stratocumulus in the rainfall mechanism when stratocumulus and cumulus clouds are both reported. The case of 25 Sept. 1957 is of particular interest when cumulus clouds with a base at 2,200 ft (700 m) and tops at 8,100 ft (2,500 m) were growing through a stratocumulus layer base 5,000 ft (1,500 m), tops 7,000 ft (2,000 m). Precipitation was observed falling both from the cumulus and from the layer cloud. </p><p>( d ) Small cumulus clouds with temperatures above 0C at their bases uritl </p><p>These clouds had thicknesses of 2,500 ft to 5,000 ft (750 m to 1,500 m) with adiabatic water content up to about 2.0gm-3. Comparing them with group (b) it seems that in spite of smaller thicknesses and water content they are at least equally likely to have concentrations of precipitation particles greater than about O*l/litre. In none of these observations was there any report of ice crystals within the cloud. The lowest cloud top temperature in this group was - 8C. </p><p>below 0C at their tops </p><p>(e) Larger cumulus clouds with temperatures above 0C at their bases arid below 0C at their tops </p><p>The clouds in this group had thicknesses between 5,000 ft (1,500 m) and 8,000 ft (2,500 m) and theoretical water contents of up to about 4 g mT3. In some cases both super- cooled water and ice crystals were observed. The cloud of 13 Feb. 1957 is of special interest because it produced very large concentrations of precipitation particles, mainly ice crystals. The temperature of its top was - 11"C, somewhat lower than the others and it suggests that the Bergeron process was playing an important role in this case. Freezing also appeared to have started in the cloud of 2 July 1958 with a cloud-top temperature of - 6"C, although in this case the concentration of precipitation particles was not high. Discounting the case of 13 Feb. 1957 there appears to be little significant difference between the ability of clouds in groups (b), (d) and group (e) to produce precipitation. </p><p>(f) Cumulus clouds with temperatures below 0C throughout </p><p>These clouds were from 4,000 ft (1,200 m) to 7,000 ft (2,000 m) thick and had very low adiabatic water content, and 1.8 g m-3 being the highest. The low theoretical water content does not seem to be a very important factor in limiting the likelihood of producing precipitation particles as their number is very similar to that in groups ( b ) and (e). In two cases, 6 Feb. 1957 when the cloud-top temperature was - 9"C, and 14 Feb. 1957 when it was - lO"C, freezing was apparently taking place and, in the former, there </p></li><li><p>+9'</p><p>c z+</p><p>/6158 </p><p>(I 3) </p><p>I7ll</p><p>kX</p><p> (2</p><p>9 11</p><p>/6/5</p><p>8 (z</p><p>4) </p><p>1600 </p><p>1530 </p><p>1200 </p><p>0 (b</p><p>) CU</p><p>MUL</p><p>US </p><p>CLOU</p><p>DS </p><p>0" C</p><p> TH</p><p>ROUG</p><p>HOUT</p><p> OF</p><p> MOD</p><p>ERAT</p><p>E VE</p><p>RTIC</p><p>AL </p><p>F L</p><p>- -------- </p><p>&gt;lo</p><p>op</p><p>16 no </p><p>EXTE</p><p>NT (J</p><p>OO</p><p>OFT</p><p>--~O</p><p>OO</p><p>FT </p><p>THIC</p><p>K) </p><p>AND </p><p>ABOV</p><p>E </p><p>&gt; 250p &gt;</p><p> 500p </p><p>.09</p><p>0 </p><p>-16</p><p>5 </p><p>.24</p><p>8 </p><p>1.7</p><p>2 </p><p>No</p><p>/ LIT</p><p>RE</p><p>SY</p><p>MB</p><p>OL</p><p>S. </p><p>0 </p><p>RA</p><p>IN </p><p>X </p><p>ICE</p><p>&lt; ) M</p><p>AX</p><p> AD</p><p>IAB</p><p>AT</p><p>IC </p><p>WA</p><p>TER</p><p> C</p><p>ON</p><p>TE</p><p>NT</p><p>FL</p><p> FR</p><p>EE</p><p>ZIN</p><p>G L</p><p>EV</p><p>EL</p><p> T</p><p>EM</p><p>PE</p><p>RA</p><p>TU</p><p>RE</p><p>S IN </p><p>'C</p><p>(C) </p><p>MIX</p><p>ED </p><p>CUM</p><p>ULUS</p><p> AND</p><p> ST</p><p>RATO</p><p>CUM</p><p>ULUS</p><p> CL</p><p>OUDS</p><p> AB</p><p>OVE </p><p>Oo C</p><p> THR</p><p>OUGH</p><p>OUT.</p></li><li><p>- 9' 23</p><p>.bk8</p><p> (25</p><p>) 2</p><p>/7/5</p><p>8 </p><p>(32</p><p>) 13</p><p>/2/5</p><p>7 -@</p><p>3) </p><p>11/6</p><p>/58 </p><p>(J.5</p><p>) 2</p><p>/7/5</p><p>8 (36) </p><p>0 </p><p>1100</p><p> I5</p><p>00</p><p> 14</p><p>00 </p><p>I200 </p><p>I500 </p><p>'Or-</p><p>0 1. 0) </p><p>AND </p><p>TOPS</p><p> BE</p><p>LOW</p><p> 0</p><p>C </p><p>m L? O</p><p> -0</p><p>01 .0</p><p>1 I </p><p>1.0</p><p> lo</p><p> lo</p><p>o 1000 </p><p>2: </p><p>No</p><p>/LIT</p><p>RE</p><p>6/2</p><p>/57</p><p> (I </p><p>4) </p><p>6/2</p><p>/57</p><p> (1</p><p>.6) </p><p>14/2</p><p>/51 </p><p>1500 </p><p>15</p><p>00</p><p> 1</p><p>50</p><p>0 </p><p>CUMU</p><p>LUS </p><p>CLOU</p><p>DS @</p><p>OOFT</p><p>-7OO</p><p>On) </p><p>BELO</p><p>W </p><p>('9 </p><p>0" c</p><p>Figu</p><p>re 1</p><p> (a)</p><p>, (b)</p><p>, (c</p><p>), (d</p><p>), (e</p><p>), (f</p><p>). Number </p><p>per </p><p>litre</p><p> of </p><p>part</p><p>icle</p><p>s &gt;</p><p> 100 p </p><p>diam</p><p>eter</p><p>. In</p><p>divi</p><p>dual</p><p> obs</p><p>erva</p><p>tions</p><p> and</p><p> fre</p><p>quen</p><p>cies</p><p>. </p></li><li><p>172 It. J. MURGATROYD and M. P. GAIiKOD </p><p>was noticeable precipitation. In the latter, in spite of the production of some much larger particles in 500 p - 1 mm diameter range, the particle concentration was too small to have produced noticeable rainfall at the surface. </p><p>In all groups the higher portions of the cloud tended to contain a much larger number of precipitation particles than the lower portions (about 80 per cent compared to 20 per cent below the centre). This would be expected as long as the updraught is capable of support- ing the particles. Once drops fall downwards through the cloud, however, they will tend to grow by further collisions and at that stage the larger particles would be expected near the base. In most of the cases examined, the cloud at the beginning of the traverses was hard in appearance and the precipitation process had apparently not developed to this extent. </p><p>5. DISCUSSION The principal factors which determine the growth of droplets in the condensation- </p><p>coalescence stage are believed to be the population of condensation nuclei, the strength of the updraughts and lifetime of the droplets in the cloud. </p><p>Data have been accumulated regarding the larger salt particles (Durbin 1959), from which it appears that near cloud base the average concentration of salt nuclei with mass greater than 104 g, is about equal to the average concentration of precipitation particles found in the clouds above. This cannot, however, be immediately regarded as establishing a direct relationship between the larger salt nuclei and the precipitation particles and further experiments are necessary to discover whether or not this approximate equality has any physical significance. It has not been possible in this series to demonstrate whether the recent history of the air has any effect on precipitation particle content of the clouds, as in nearly all cases the air had a maritime track. This is, of course, the usual case for the United Kingdom and the type of data given above may be regarded as representative for this country. </p><p>The lifetime of a droplet in a cloud is not an independent parameter, being largely, but not entirely, controlled by updraught strength and the size of the cloud. It would be desirable in investigating possible relationships between precipitation particle concentra- tions and updraughts to be able to measure simultaneously the distribution of updraughts and precipitation particles throughout the clouds. In this investigation it was only possible to make rough estimates of the updraught strength on each occasion in terms of the positive area on the tephigram. No relationship, however, was detected in this way, probably because the sample was too small and the effect overshadowed by other factors. </p><p>It would also be useful to be able to relate numbers of prec...</p></li></ul>