Characterizing CCN Spectra to investigate the Warm Rain Process

The objective of the RICO (Rain in Cumulus over the Ocean) project was to characterize shallow precipitating Caribbean cumulus clouds. These clouds are known to be ubiquitous over most of the tropical oceans and understanding them is vital to the estimation of the global heat, radiation and moisture budgets. In the broadest sense, studying the nature of the trade wind cumulus from the microphysical scale to the large scale with particular emphasis on the onset of precipitation was one of the most important objectives of RICO.

The properties of environmental aerosol are known to affect the dependence of trade-cumulus on microphysical and radiative processes. At the microphysical/cloud scale, the measurement of aerosol, CCN in particular is very critical to the evaluation of all hypotheses that explain the onset of precipitation. During the project, aerosol measurements were made by the two DRI CCN spectrometers to analyze their distribution and properties. Use of two instruments ensured redundancy and enabled in-flight calibrations. These instruments have the advantage of the extension of the traditional CCN (Aitken) range below 0.1% to Large Nuclei which is necessary because a large proportion of CCN have S < 0.1%. Large Nuclei are embryos for precipitation and also provide interface with Giant Nuclei. CCN measurements are very challenging and somewhat controversial. Hence, comparisons of two CCN spectrometers operating at different supersaturation(S) ranges (Figure 1) suggest validity of CCN measurements over the full extended S range.

These measurements include more than 180 flight hours from 19 flights over a two month period in the western Atlantic near the northeastern corner of the Antilles (Antigua) in December and January (2004-5). During 17 of these flights there were two hours of subcloud measurements at constant altitudes. Table 1 shows a partial list of averages and standard deviations of the total particle and CCN concentrations at three supersaturations(S). This shows that even in clean maritime air there is some significant day-to-day variability in concentrations, which seems to be related either to wind velocity or to cloudiness. It seems to suggest long- range transport of continental aerosol. Indeed vertical profiles (Figure 2) suggest that cloud scavenging in the middle levels, where there were usually cumulus clouds reduced CCN and CN concentrations. Measurements of the sizes of CCN and volatility measurements suggest that the maritime CCN in RICO are composed of completely soluble material, which is consistent with ammonium sulfate or NaCl (Fig.3) and with Hudson and Da(1996). Volatility measurements suggest that most CCN in the boundary layer are not NaCl. CCN are even more volatile at higher altitudes (even less NaCl).

Table 1. Concentrations measured during RICO

Date	CN	CCN 1%	CCN 0.1%	CCN 0.04%
Dec. 7	--	220± 60	45±18	12± 8
Dec. 16	385±193	234±203	49±28	15±13
Dec. 17	163± 94	45± 16	20± 8	8± 5
Dec. 19	202± 37	93± 19	56±16	29±12
Jan. 5	297±134	121±107	48±46	27±14
Jan. 7	272±269	100± 64	28±12	12± 8
Jan. 11	194± 73	86± 35	24± 8	8± 4
Jan. 12	193± 19	131± 18	71±11	28± 9
Jan. 14	221± 63	115± 21	54±11	23± 7
Jan. 16	165± 30	83± 20	38±10	15± 6
Jan. 23	281± 28	129± 20	52±12	25± 9
Jan. 24	323± 91	121± 40	36±22	13±11

                               

Average and standard deviaitions of total particle (CN) and cumulative CCN concentrations during low altitude horizontal legs of 1-hour duration.

Figure- 1: Comparison of CCN spectra from the two DRI instruments over the entire S range (.01%-2%S)

Figure 2. Total particle (CN) and cumulative CCN concentrations during a sounding

Figure 3. Sizes of CCN measured in RICO. Also plotted is the theoretical versus critical supersaturation (Sc) relationship