A Ten-Winter Record of Cloud-Droplet Physical and Chemical Properties at a Mountaintop Site in Colorado

• Published in: Journal of applied meteorology (1988) 1994-07, Vol.33 (7), p.797-807
• Main Authors: Hindman, Edward E., Campbell, Mechel A., Borys, Randolph D.
• Format: Article
• Language: English
• Subjects: Aerosols; Chemical composition; Chemical properties; Cloud physics; Clouds; Earth, ocean, space; Exact sciences and technology; External geophysics; Ion concentration; Liquids; Meteorology; Moisture content; Storms; Water pollution
• ISSN: 0894-8763; 1520-0450
• DOI: 10.1175/1520-0450(1994)033<0797:ATWROC>2.0.CO;2

Cloud-droplet spectra and coincident cloud water pH measurements have been made for a portion of ten consecutive winters (1983/84–1992/93) from clouds that enveloped Storm Peak Laboratory in northwestern Colorado; cloud water ion measurements were made for eight of the winters. To determine if the physical and chemical properties are related, the data were stratified into three populations: pH ≤ 3.6, 3.6 < pH < 4.6, pH ≥ 4.6. It was found that clouds with the smallest pH values (3.4) had the largest droplet concentrations (N = 329 cm−3), smallest mean droplet diameters (Dbar = 6.4 μm), and largest ion concentrations (e.g., $\mathrm{S}{\mathrm{O}}_{4}^{=}=5.7 \ \mathrm{m}\mathrm{g} \ {\mathrm{L}}^{-1}$), while clouds with the largest pH values (5.1) had the smallest N values (189 cm−3), largest Dbar values (8.0 μm), and smallest ion concentrations ($\mathrm{S}{\mathrm{O}}_{4}^{=}=3.9 \ \mathrm{m}\mathrm{g} \ {\mathrm{L}}^{-1}$). Nevertheless, all three populations had similar liquid water contents (LWC ≅ 0.070 g m−3). The equation $\mathrm{L}\mathrm{W}\mathrm{C}=\mathrm{\pi }/6{\mathrm{D}}_{\mathrm{b}\mathrm{a}\mathrm{r}}^{3}\mathrm{N}\mathrm{\rho }$, where ρ is the density of water, closely describes the relationship between LWC, Dbar, and N. The range in pH values could not be completely explained by entrainment, or variations in cloud-base height or in LWC; differences in cloud condensation nucleus composition appear to be a major factor. No significant trends in average winter N, Dbar, and pH values were found in the ten-winter record.