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 10 °C (200 cm −3 ) and then decreases gradually to a minimum (∼1–3 cm −3 ) at about −35 ° C. 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 ∼100 cm −3 and ∼200 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.