3.1.1. A Case Study

[26] The linkage between ingested subcloud aerosols and cloud microphysics is best illustrated by a case study on the afternoon of 28 February 2006. A cold front had passed through the area the previous night and a postfrontal cold air mass moved from the west southwest over all of Central California by the following afternoon. Postfrontal instability caused convective clouds over the ocean, and triggered convective clouds over the coastal hills and over the Sierra Nevada. Although the instability decreased gradually during the day, rain showers from shallow clouds were still occurring over the ocean and the coastal ranges at 00Z on 1 March 2006. Figure 5 shows the Oakland radiosonde at that time.

Figure 5 Open in figure viewer PowerPoint The Oakland radiosonde of 1 March 2006 at 00Z, which is near the time that the aircraft flew near Oakland.

[27] A coordinated mission of the Cloud and Aerosol airplanes originated from the Sacramento Executive Airport to document the gradient in aerosols and cloud properties by doing cross sections from the Sierra Nevada to and from the Pacific Ocean. The aircraft departed Sacramento at 23:05Z and flew due east to the foothills and measured the convection generated there by the mountains. The next destination was the clouds that formed over the hills bounding the Central Valley to its west, about 60 km to the NE of Monterey. Next the aircraft sampled the clouds forming over the hills just at the Pacific coast at Big Sur. There the aircraft continued 35 km westward over the ocean and then turned north to measure convective clouds that were triggered by the ocean shoreline of San Francisco. Then the aircraft turned east over the north part of San Francisco Bay and measured a cloud just inland over Richmond, and then another cloud over Sacramento before finally landing. The tracks of the two aircraft and the locations of the measured clouds are provided in Figure 6.

Figure 6 Open in figure viewer PowerPoint The tracks of the Cloud (black) and Aerosol (colored) airplanes. The time marks every 5 min are posted on the aerosol aircraft trajectories, and labeled every 10 min. The CCN concentrations adjusted to supersaturation of 0.9% are shown in the color scale. The relative height of the aerosol aircraft above sea level is shown by the vertical displacement of the track. The measured clouds by the cloud physics aircraft are marked with green circles and numbered sequentially.

[28] The aerosol aircraft measurements are summarized in Figure 6. Because the supersaturation (or the temperature difference between the plates, dT) in the Cloud Condensation Nuclei Counter cycles every ∼7 min, there was a need to correct the CCN data measured at low supersaturations to a common SS. Without correction or adjustment there would be too few data points measured at the same SS. In order to do this, it was necessary to find the relation between dT (instead of SS) and the CCN concentration for each flight separately, because this relation might be affected by the chemical composition of the aerosols, their sizes and their concentrations. After determining and applying the correction, the CCN concentrations were plotted for an entire flight to a common 0.85% SS for measurements in the boundary layer. On average, the ratio of CCN counts at super saturations of 0.85% and 0.5% was 1.89 with a standard deviation of 0.4.

[29] The aircraft aerosol measurements show CCN concentrations varying between 300 and 800 cm−3 over the first section to the SE at the western slopes of the Sierra Nevada. The CCN concentrations fell to about 100 cm−3 over the hills 60 km NE of Monterey, and continued falling to less than 40 cm−3 over Monterey Bay and likely also over Big Sur. The CCN increased again gradually to the north along the coastline and reached about 70 cm−3 there. They kept rising to about 100 cm−3 over the peninsula of San Francisco airport, and jumped locally to 800 cm−3 just to the north of the airport, but recovered back to less than 80 cm−3 to the north of the Golden Gate Bridge. The aircraft turned to the east and experienced a sharp increase of the CCN to more than 700 cm−3 over Richmond. The condensation nuclei (CN) then shot up > 10,000 cm−3. This suggests an ample source of fresh small aerosols. The CCN remained generally above 500 cm−3 within the boundary layer all the way to landing in Sacramento.

[30] The cloud‐ and precipitation particle size distributions are given in Figures 7-Figures 7–11. Cloud 1 was sampled stepping upward from base through its upshear towers, whereas its more mature portions glaciated and precipitated. Because of air traffic control limitations it was necessary to use different clouds in the same area for the lower and upper portions of the cross sections. The modal liquid water cloud drop diameter (DL, defined as the drop diameter having the greatest LWC) increased with height above cloud base. It reached 21 μm at the altitude of 3635 m, which is about 1900 m above cloud base. The temperature there was −8°C. This size is below the DL threshold for the development of warm rain that was documented elsewhere as 24 μm [Andreae et al., 2004]. In agreement with that, the DL did not expand to drizzle size. Large precipitation particles occurred as graupel and formed a well separated distribution at the 1‐mm size range.

Figure 7 Open in figure viewer PowerPoint −3. Panel A shows the Cloud Droplet Probe (CDP) measured LWC distribution. Each line represents the gross cloud drop size distribution of a whole cloud pass. The legend of the lines is composed of the pass height [m] to the left of the decimal point, and the pass starting GMT time [hhmmss] to the right of the point. The passes are ordered in altitude ascending order. Note the increase in cloud drop volume modal size with increasing cloud depth. Panel B shows the combined distributions of the CDP and the cloud imaging probe (CIP). According to the figure the large precipitation particles were well separated from the cloud drop size distribution, indicating lack of appreciable coalescence. Plot of cloud droplet diameters as a function of liquid water content (LWC) for Cloud 1 over the western slopes of the Sierra Nevada (see location in Figure 6 ). The modal liquid water drop diameter occurs at the droplet size having the greatest water content. Cloud 1 developed in an air mass that had 300–800 CCN cm. Panel A shows the Cloud Droplet Probe (CDP) measured LWC distribution. Each line represents the gross cloud drop size distribution of a whole cloud pass. The legend of the lines is composed of the pass height [m] to the left of the decimal point, and the pass starting GMT time [hhmmss] to the right of the point. The passes are ordered in altitude ascending order. Note the increase in cloud drop volume modal size with increasing cloud depth. Panel B shows the combined distributions of the CDP and the cloud imaging probe (CIP). According to the figure the large precipitation particles were well separated from the cloud drop size distribution, indicating lack of appreciable coalescence.

Figure 8 Open in figure viewer PowerPoint −3. The cloud drops are quite large and the distribution continues smoothly into the raindrop sizes. This indicates active warm rain processes. Same as Figure 7 , but for Cloud 2 over the hills 60 km NE of Monterey (see location in Figure 6 ). It developed in an air mass that had 100 CCN cm. The cloud drops are quite large and the distribution continues smoothly into the raindrop sizes. This indicates active warm rain processes.

Figure 9 Open in figure viewer PowerPoint −3. The cloud drops are very large and the distribution continues smoothly into the raindrop sizes. This indicates very active warm rain processes. Same as Figure 7 , but for Cloud 3 over the hills near Big Sur (see location in Figure 6 ). It developed in an air mass that had about 40 CCN cm. The cloud drops are very large and the distribution continues smoothly into the raindrop sizes. This indicates very active warm rain processes.

Figure 10 Open in figure viewer PowerPoint −3. The drops become markedly smaller with increasing CCN concentrations. Warm rain ceases at cloud 3 where 300 CCN cm−3 were present. Same as Figure 7 , but for single heights in clouds 4–8 in a cross‐section from the Pacific Ocean to Sacramento, marked by C4, C5, C6, C7, and C8 respectively. The respective approximated CCN concentrations from the measurements made by the aerosol aircraft are denoted by the circles and are located under the peaks of the DL plots having the same color. The CCN values are to be read from the right ordinate. The CCN concentrations are: C4: 70, C5: 100, C6: 300, C7: 600, C8: 800 cm. The drops become markedly smaller with increasing CCN concentrations. Warm rain ceases at cloud 3 where 300 CCN cmwere present.

Figure 11 Open in figure viewer PowerPoint −3. The cloud drops are very small and do not expand much with height into raindrops, again as in Cloud 1. Same as Figure 7 , but for the vertical cross section in Cloud 8 over Sacramento (see location in Figure 6 ). It developed in an air mass that had about 800 CCN cm. The cloud drops are very small and do not expand much with height into raindrops, again as in Cloud 1.

[31] From the location of Cloud 1 the aircraft was flown diagonally to the southwest and across the Central Valley. The valley was mostly cloud‐free, except for some midlevel layer clouds. The next area of clouds was triggered by the ridge that bounds the Central Valley on its west. The cloud tops had a convective appearance and were sampled at the lowest allowed altitude ‐ (2100 m, to provide safe‐ ground clearance over the highest terrain) up to the cloud tops at 2700 m. The temperature there was −3°C, but the maturing clouds were visibly turning into a diffused fibrillation texture, indicating the conversion of the cloud water to precipitation and/or ice crystals. Glaciation would be in such case produced probably by a mechanism of ice multiplication. The modal LWC drop size was 28 μm at 2100 m and reached 33 μm at the cloud top at 2700 m. This is clearly beyond the threshold (DL = 24 μm) for warm rain [Gerber, 1996; Yum and Hudson, 2002]. In agreement with that, the cloud droplet size distribution (DSD) was extended smoothly to the drizzle and small raindrop sizes, as measured by the CIP and presented in the lower panel of Figure 9. The appearance of the warm rain is consistent with the decrease of the CCN concentrations to about 100 cm−3.

[32] The aircraft continued flying to the SW to the next area of clouds (cloud 3). These were triggered by the coastal hills near Big Sur. The aircraft stepped vertically through the convective‐ looking cloud tops from the lowest safe height of 1880 m to their tops at a height of 2250 m at temperature of ‐3°C. The CCN concentrations as measured by the aerosol aircraft in Monterey Bay varied between 20 and 50 cm−3. These low CCN concentrations produced large cloud drops ranging from a modal LWC drop diameter of 30 μm at 1880 m to 43 μm at the cloud tops. The DSD extended smoothly into drizzle and small raindrops (see Figure 8). Large hydrometeors were nearly absent. The cloud drops were so large so that the solar radiation reflected from the particles near the cloud top formed a cloud bow. These clouds had clearly created active warm rain.

[33] From Big Sur the flight continued over the ocean and then turned north and flew at a constant altitude across Monterey Bay to the Golden Gate and then eastward back to Sacramento. This flight path took the aircraft along an aerosol gradient that increased from pristine over the ocean to polluted air just to the east of San Francisco Bay. Convective clouds grew along that flight path and reflected the impact of the changing CCN concentrations at that fixed altitude. Clouds 4 to 8 were penetrated along this gradient flight (Figure 9).

[34] Cloud 4 was penetrated at the coastline of the peninsula to the west of San Francisco. The CCN concentration there was about 70 cm−3 and the cloud had a DL of 31 μm and created warm rain. A faint cloud bow was barely visible. Cloud 5 was penetrated a short distance to the north, where the CCN increased to 100 cm−3. Cloud 5 still had warm rain, but to a lesser extent than Cloud 4. Shortly after passing directly over San Francisco International airport, over the Golden Gate Bridge, a short jump in the CCN occurred to about 600 cm−3 and recovered to the background of <70 cm−3.

[35] The aircraft turned east and crossed the northern arm of San Francisco Bay. The CCN concentrations increased to about 300 cm−3 shortly after crossing the coast line. Cloud 6 formed over the eastern part of Richmond. Its modal LWC DSD decreased to 17 μm, well below the warm rain threshold of 24 μm. The CIP confirmed that this cloud had no precipitation particles. This occurred less than an hour after the time of the Oakland sounding at 00Z, which represented pretty well the local conditions and showed light southwesterly winds near the surface that veered to stronger west‐southwest winds at the higher levels.

[36] Cloud 7 occurred a few km farther east of cloud 6, where the CCN concentrations increased to 600 cm−3. Its DL decreased further to 15 μm. Cloud 8 developed farther east over Sacramento, where the CCN concentration varied between 600 and 1000 cm−3. The cloud had a similar microphysics to cloud 7. A vertical stepping through cloud 8 showed little widening of the DSD with height (Figure 11), which serves as an additional indication of the scarcity of coalescence in that cloud.

[37] A satellite analysis (Figure 12) shows that the satellite retrieved microphysics of the cloud field is in agreement with the in situ measurements that suggest suppression of precipitation in Area 1, which includes Cloud 1, while showing ample warm rain in Area 8, which includes Cloud 3.

Figure 12 Open in figure viewer PowerPoint Rosenfeld and Lensky [1998] μm solar reflectance component. The green is brighter for smaller cloud particles. Therefore the polluted clouds with small drops appear yellow (see Areas 1, 5, and 6); whereas the ice clouds appear red (see areas 3 and 7). Pristine water clouds appear magenta (see Area 8), because they have low green (large water drops) and high blue (warm temperature). The line graphs provide the relations between the satellite indicated cloud top temperatures and the cloud top particle effective radii. At the foothills in Areas 1 and 5 the cloud top effective radius is much smaller than the precipitation threshold of 14 μm [ Rosenfeld and Gutman, 1994 μm in Area 8 is much larger than the precipitation threshold. Aqua MODIS image of the clouds in central California on 2006 02 28 at 21:00Z. The color scale is a composite followingwhere the red is modulated by the visible solar reflectance, blue modulated by the thermal temperature, and green modulated by the 3.7m solar reflectance component. The green is brighter for smaller cloud particles. Therefore the polluted clouds with small drops appear yellow (see Areas 1, 5, and 6); whereas the ice clouds appear red (see areas 3 and 7). Pristine water clouds appear magenta (see Area 8), because they have low green (large water drops) and high blue (warm temperature). The line graphs provide the relations between the satellite indicated cloud top temperatures and the cloud top particle effective radii. At the foothills in Areas 1 and 5 the cloud top effective radius is much smaller than the precipitation threshold of 14m [] whereas the effective radius of 18m in Area 8 is much larger than the precipitation threshold.

[38] In summary, a detailed analysis of a single flight of SUPRECIP 2 showed a clear relationship between CCN concentrations, cloud microphysics and precipitation forming processes. The distribution of the CCN showed an unambiguous urban source, at least in the San Francisco Bay area. The role of the anthropogenic aerosols is demonstrated by the contrast between Cloud 2 some 50 km inland in a relatively sparsely populated area, compared with clouds 6 and 7 only several km inland over the heavily populated and industrialized Bay area. While Cloud 2 was quite pristine and produced ample coalescence and warm rain, coalescence in cloud 7 was highly suppressed and it produced no precipitation.

[39] The differences in the anthropogenic CCN likely explain the observed differences. Cloud base temperature over the coast (San Francisco) was warmer by about 2°C than the cloud base inland (Sacramento). This cannot explain the observed differences in the clouds microstructure for the same height above cloud base, because it incurs a difference of less than 10% in the amount of adiabatic water for the same height above cloud base for the heights of interest. The fastest growth of DL in near the coast line cannot be explained by the probable greater abundance of sea‐spray generated giant CCN, because they would act to enlarge the tail of the cloud DSD and not its mode. Furthermore, both cloud base temperature and sea salt CCN should change at the same rate with distance from the coast over the urban and rural areas. Differences in land use would, if anything, contribute to the opposite effects with respect to the actually observed. The mountains at the coast line near Big Sur should enhance the updraft and cause smaller cloud drops and less coalescence, but in fact the largest drops and strongest warm rain were observed there. The urbanized area should have provided more sensible heat for greater updrafts, but this should play a minimal role with the weak winter solar heating. Therefore there is no probable mechanisms that can explain the observed differences in the cloud microstructure and precipitation properties to which the authors are aware of, except for the differences in the anthropogenic CCN.

[40] The satellite image (Figure 12), taken 3 to 4 h before the flight, supports the aircraft observations and shows that an even greater source than the urban San Francisco Bay area for aerosols occurred in the central and southern Central Valley. A flight earlier in the day measured CN concentrations exceeding 20,000 cm−3 and CCN concentrations reaching 1000 cm−3 over the southern Central Valley, including the location of Area 5 in Figure 12.

[41] The pristine clouds with large drops and warm rain processes produced a continuum of drop sizes from the cloud drops through the drizzle sizes to the small raindrops. In contrast, clouds with suppressed coalescence due to large CCN concentrations that grew to heights with cold temperatures still produced mixed phase precipitation mainly in the form of graupel. They produced distinctly different size distribution of the hydrometeors, which was separated from the cloud drop DSD. It is known from theoretical considerations and simulation studies that the decreased cloud drop sizes reduce also the mixed phase precipitation [Khain et al., 2001; Rosenfeld and Ulbrich, 2003], but the extent of this possible effect from the cloud physics measurements remains to be documented.

[42] Similar response of clouds and precipitation forming processes to aerosols is apparent also in all the other research flights of SUPRECIP‐2 as shown in the next subsection. The continued analyses and evaluation of the aircraft measurements provides compelling evidence for the detrimental role of anthropogenic aerosols on orographic precipitation in California, and explains how a climatological trend of increased CCN aerosols would cause the climatologically observed trends of the reduction in the orographic precipitation component in the southern and central Sierra Nevada.