Modelling and observations of droplet growth in clouds A Coals 1, A M Blyth 1, J-L Brenguier 2, A M Gadian 1 and W W Grabowski 3 Understanding the detailed processes during the formation, continued existence and dissipation of Fog needs comprehension of::- an understanding the long and short wave radiation absorption and scattering properties of clear air and saturated cloudy air, which is affected by the droplet size and concentration distributions an understanding of the local air dynamics, orographical and surface heating forcings and general large scale flow the micro-physical properties of the cloud, the role of turbulent mixing and entrainment and the relevant CCN concentrations.

The problem Despite a great deal of research into the understanding of microphysical properties of clouds over the past 40 years, an important issue remains unresolved; that of cloud droplet size distributions predicted by adiabatic parcel models being much narrower than those observed in warm cumulus clouds. In addition, simple calculations of cloud droplet growth tend to produce single-mode distributions, while bi- or multi-modal drop size distributions are commonly observed

SCMS study The Small Cumulus Microphysics Study was conducted in Florida, near Cape Canaveral, during July and August 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.

July 24 background meteorology 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.

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 cm -3. July 24 aircraft penetration through the cloud

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

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.

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 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

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

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 (to be published).

Summary 24 th July case (high 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 (Fig. 3, 48 min). 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).

Summary The DSD’s can be explained in the context of this thermal / dynamical model. 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. The DSD’s show bimodality and growth with turbulence mixing and entrainment. Modelling Fog requires that these processes be accounted for.