Aircraft observations of cloud droplet number concentration: Implications for climate studies

Droplet number concentration (N-d) is a major parameter affecting cloud physical processes and cloud optical characteristics. In most climate models, N-d is usually assumed to be constant or a function of the droplet and aerosol number concentration (N-a). Three types of cloud systems over Canada, namely Arctic clouds, maritime boundary-layer clouds, and winter storms, were studied to obtain values of N-d as a function of temperature (T). The probability density function of N-d was also calculated to show the variability of this parameter. The results show that N-d reaches a maximum at about 10degreesC (200 cm(-3)) and then decreases gradually to a minimum (similar to1-3 cm(-3)) at about -35 degreesC. A comparison of relationships between N-d and N-a indicates that estimates of N-d from N-a can have an uncertainty of about 30-50 cm(-3), resulting in up to a 42% uncertainty in cloud short-wave radiative forcing. This study concludes that the typical fixed values of N-d, which are similar to100 cm(-3) and similar to200 cm(-3) for maritime and continental clouds, respectively, and the present relationships of N-d to N-a, could result in a large uncertainty in the heat and moisture budgets of the earth's atmosphere. It is suggested that the use of relationships between N-d and T can improve climate simulations.