1. Condensational growth of cloud droplets, COST 722 Condensational growth of cloud droplets, with reference to warm cumulus clouds and the impact on formation of precipitation sized drops. Alan Gadian, Alan Blyth, Jean-Louis Brenguier, Alison Coals, Wojtek Grabowski, John Latham + others 1 Title

2. Condensational growth of cloud droplets, COST 722 2 To examine the growth of droplets in the condensation phase. Under what circumstances does the the droplet spectra become large enough for the coalescence processes to become significant and to start precipitation. Although primarily a study of the condensation processes in cumulus clouds, the immediate extension is to processes in stable Fog clouds. Contents: Numerical Model. Details of Observations … two case studies. Discussion of droplet profiles. Implications for entrainment and mixing in warm clouds and fog. Aims

3. Condensational growth of cloud droplets, COST 722 3 In warm clouds, droplet spectra is broader than predicted Bi-modal / multi-modal spectra are observed in cumulus clouds. Limited data from fog suggests that this is not observed. Adiabatic parcel calculations, only in special cases, replicates the multi-modality in warm cumulus clouds. Size distributions need to be represented / predicted accurately. Inadequate representations of cloud droplet size distributions have a number of consequences … coalescence rates rates are sensitive to droplet sizes. This has crucial importance for precipitation development. Brenguier et al developed a simplified approach for the condensational growth equation. Model and observational data are compared: role of homogeneous and in homogeneous processes examined. Background

4. Condensational growth of cloud droplets, COST 722 4

5. 5 DRY AIR Activation and condensation Dry Air Saturated Air,  = 1 Air rises, cool and enough vapour exists to produce a saturated volume DRY AIR Activation and partial condensation Partially saturated air,  < 1 DRY AIR Activation and condensation Saturated Air,  = 1 Evaporation Partially saturated air,  < 1 Dry Air  = 0 Partial condensation Further condensation occurs in the “wet” part of the cell. No further condensation Random no. used to determine option

6. Condensational growth of cloud droplets, COST 722 6 SCMS study The Small Cumulus Microphysics Study was conducted in Florida, near Cape Canaveral, during July and August 1995. The objective of the study was to examine the initiation of warm rain in cumulus clouds. Data was from the NCAR CP-2 dual-wavelength radar, the NCAR C-130, the Meteo-France Merlin, and the Wyoming King Air. NCAR C-130 data is displayed here. The cumulus clouds examined during SCMS typically had cloud bases with 950 mb (about 500 m above mean sea level, or MSL) and temperature23C. Here model clouds on two days with maximum observed concentration of cloud drops of about 800 and 500 cm -3 on the 24th July and 10th August. The size distributions observed in clouds on the 10th August was rarely bimodal, which is unusual for the SCMS clouds. This is investigated. Radar, aircraft and visual observations of the Florida small cumulus clouds suggest that the upper parts of the clouds contained single thermals of about 1 km in size, when they initially grew, to about 4km. The initial clouds that ascended to about 4 km usually collapsed and decayed.

7. Condensational growth of cloud droplets, COST 722 7 The cloud base temperature and pressure on this day were approximately 23C and 940 mb respectively, corresponding to an altitude of about 700m. Radar scans indicated that the cumulus clouds ascended to about 4 km: this was typical of the cumulus clouds that developed in this area. A temperature and dew-point sounding made 22 km SW of the radar starting at 1838 UTC indicated that the atmosphere was conditionally unstable above a relatively well-mixed boundary layer -- typical of the Florida environment. The wind near the surface was from the west, the direction of mainland Florida. This case is from near the end of the first period of the project where the wind was generally from the west and the concentration of clouds droplets was higher. Data collected by the Particle Measuring Systems FSSP-100 during aircraft penetrations of the cloud showed a maximum cloud droplet concentration of 800 cm -3. July 24 background meteorology

8. Condensational growth of cloud droplets, COST 722 8 Time series of data gathered by aircraft. Top panel: 25 Hz vertical wind speed superimposed with wind vectors. Bottom three panels:10 Hz values of mean diameter, total concentration of cloud drops (N), and liquid water content (L) respectively, derived from the FSSP. 10 Hz drop size distributions measured by the FSSP. x-axis: diameter ranging from 0-50  m. y-axis: N(d), ranging from 0-200 cm -3. July 24 aircraft penetration through the cloud

9. Condensational growth of cloud droplets, COST 722 9 Vertical slice through simulated cloud, showing cloud droplet spectra at each grid point (resolution 95 m). N 0 =1000 cm -3., aircraft altitude 2.7km July 24 droplet spectra, vertical velocity and qc

10. Condensational growth of cloud droplets, COST 722 10 Vertical cross-sections of simulated cloud water mixing ratio at (top left) 46, (top right) 48, (bottom left) 51, and (bottom right) 53 minutes. July 24 model qc cloud values

11. Condensational growth of cloud droplets, COST 722 11 August 10 background meteorology Five vertical scans through the cloud. 14:56, 14:58:15, 15:00:30, 15:02:27, 15:06:39. Horizontal and vertical scales are 1 and 2 km. The cloud base temperature and pressure on this day was approximately 24C and 965 mb, respectively, corresponding to an altitude of about 550 m MSL. Individual clouds reached a height of 5 km. The particular cloud used in this study developed over 11 minutes, reaching a height of about 5 km before collapsing and dissipating. The main radar echo developed to 30 dBZ near cloud top as the cloud was growing. The region of high reflectivity descended as the cloud dissipated. The temperature and dew-point sounding taken at the same location as for the 24 July case at 1408 UTC (1008 local time) indicated that there was conditionally unstable atmosphere above a well-mixed boundary layer. The low-level winds were from along the shoreline. The maximum cloud droplet concentration measured by the PMS FSSP-100 for this day was 530 cm -3, which is consistent with there being a continental component to the CCN distribution.

12. Condensational growth of cloud droplets, COST 722 12 August 10 model qc cloud values Vertical cross-sections of simulated cloud water mixing ratio at (top left) 65, (top right) 68, (bottom left) 71, and (bottom right) 73 minutes.

13. Condensational growth of cloud droplets, COST 722 13 Time series of data gathered by aircraft. Top panel: 25 Hz vertical wind speed superimposed with wind vectors. Bottom three panels: 10 Hz values of mean diameter, total concentration of cloud drops (N), and liquid water content (L) respectively, derived from the FSSP. 10 Hz drop size distributions measured by the FSSP. x-axis: diameter ranging from 0-50  m. y-axis: N(d), ranging from 0-100 cm -3. August 10 aircraft penetration through the cloud Simulated drop size distributions (N 0 =500 cm -3) through cloud along flight path (altitude 2.4 km). Each plot represents a model grid point (95 m), and corresponds to approximately 1 second of flight time, from left to right. x-axis: diameter ranging from 0-40  m. y-axis: normalized distribution

14. Condensational growth of cloud droplets, COST 722 14 August 10 droplet spectra, vertical velocity and qc Vertical slice through simulated cloud, showing cloud droplet spectra at each grid point (resolution 95 m). N 0 =500 cm -3

15. Condensational growth of cloud droplets, COST 722 15 24 th July case (higher droplet concentration): model predicts the number of drops larger than 25 microns, and shows that the size of the largest drop increases with height in the diluted updraught at x= -0.6 km updraught is the centre part of the thermal circulation and contains a mixture of cloud base air and environmental air. Table 1 shows values of L/L ad ~ 0.6 and w ~ 9 m s -1 ; ideal conditions for enhanced growth due to entrainment and mixing (Baker et al, 1980). observations suggest that although the number concentration is likely constant in the updraught due to re-activation of CCN, the larger drops compete more effectively for the water vapour, and growth of these drops is favoured (Baker et al, 1980). 10 th August case (medium droplet concentration): observed and modelled spectra both indicate less bimodality and are narrower than the 24 th July case simulation spectra indicate the effects of the turbulent mixing and entrainment processes at the top, sides, and around the “holes” – in this case only observable near the top of the cloud. evidence of broadening of spectra in updraught after entrainment occurs. Sensitivity studies show bimodality decreases with increasing N 0 Summary - 1

16. Condensational growth of cloud droplets, COST 722 16 A high-resolution, 3D cloud model was to examine the evolution of the droplet size distributions during the development stages of a cumulus cloud. The air motions within the model cloud were initially consistent with a simple thermal circulation with an updraught over most of the cloud, divergence near cloud top, downdraughts at the cloud edges, and inflow at the rear of the thermal into the updraught. Turbulence destroyed this simple pattern and the symmetry of the flow, although the general pattern was always present. The DSD’s can be explained in the context of this thermal model. For example, the DSDs were narrow in the strong updraughts while the peak of the DSDs shifted to smaller sizes at the sides of the cloud and at cloud top where the cloud has been diluted due to entrainment. Also, the size distributions were bimodal in the updraught near the rear of the thermal where the liquid water content was reduced. It is likely that the peak at smaller sizes is due to activation on CCN that are either entrained or that result from evaporation of cloud drops. Ascending regions of cloud with reduced liquid water content are the right ingredients for the enhanced growth of large drops proposed by Baker et al.(1980) and Cooper (1987). The model results showed that larger drops were produced at the top of diluted updraughts in both cases. The DSD’s show bimodality and growth with turbulence mixing and entrainment. Modelling Fog requires that these processes be accounted for. Summary - 2

17. Condensational growth of cloud droplets, COST 722 17 Observations were only made at a single level ---- limited comparison only but encouraging that the model results are similar to the observed ones in terms of the breadth of the DSDs, relative locations of bimodal DSDs, and behaviour of the DSDs at cloud edge. Sensitivity studies, for different concentrations of CCN, No. Increasing No in the 24 July and the 10 August cases, produces fewer bimodal size distributions because the the main peak of the distribution was at a smaller size. However, no other significant differences were found. In partially-saturated model regions where evaporation and condensation occur, the trajectory of the air parcel into a ``cloudy'' or ``dry'’ sector is determined by a random number comparison, depending on the value of  for the cloudy grid. Future development e.g. incorporation of a turbulence-related weighting factor, could better describe physical processes of entrainment. Role of entrainment is important in broadening. Turbulent mixing is very much a three-dimensional process. The vortices tend to exist for longer in the 3D simulations. Despite the simplicity, the model results compared well with the observations. Summary - 3