**1.1. Clouds in the Inter-Tropical Convergence Zone (ITCZ)**

The ITCZ is one of the most important weather systems in the Tropics. The area shows a decisive influence on the characterization of different clime and weather conditions in tropical region. The ITCZ is characterized by several interactions between the ocean and the atmosphere, identified as:


Therefore, the ITCZ is represented as a line of clouds of deep convection extending across the Atlantic and Pacific oceans, located between the 5° and 10° N (Holton, 1992). The band moves depending on the season, always matching in areas with high solar intensity or where the sea surface has higher temperatures. The clouds movement is towards the Southern hemisphere between September and February, and in the opposite direction in the next few months until the end of the summer in the Northern Hemisphere. However, just at the north of the Ecuador, the ITCZ movements are lower (Wallace and Hobbs, 1977). In those areas, the rain intensifies with solar heating. An exception occurs with El Niño-Southern Oscillation (ENSO), and the ITCZ is deflected towards where the ocean surface increases its temperature. A special feature of the region is the presence of a warm water pool, nearby the coast of Mexico, located in the ITCZ between 12° and 8° N from September to November. The presence of high temperatures at the ocean surface promotes a greater amount of moisture and increases convection effects, which result in larger vertical clouds.

## **1.2. Development of convective clouds**

A convective cloud is formed when a mass of moist air acquires buoyancy due to the increase in surrounding temperature; with this process, the presence of atmospheric instability helps to lift the air masses.

Once the air mass reaches the saturation point, water vapor condenses on the CCN. The change of physical state releases latent heat that is absorbed by the air increasing the buoyancy forces. From now on, the cloud experiences a violent vertical development reaching the maximum height generated by the strong temperature gradient between the cloud core and the environment surrounds it. However, the growing process will be affected by dry air entrainment on the cloud's side walls, inhibiting the vertical development of convective cloud (Squires, 1958; Emanuel, 1982).

The air entrainment dilutes and evaporates droplets releasing aerosol particles, which served as CCN, back into the atmosphere and cooling the air surrounding. The process generates downward movements in the cloud and promotes a high turbulence that mixes the air masses.

## **1.3. Interaction of atmospheric particles with solar radiation**

Atmospheric particles play a very important role in the climate system. Their effects on the direct radiative forcing scattering or absorbing sunlight, and facilitating indirectly the formation of clouds are a relevant object of study since there is no adequate knowledge of their significance on clouds creation. Particles radiative forcing is globally comparable to greenhouse gases, but in the opposite direction because it causes a cooling climate (Charlson et al, 1992). Coakley and Grams (1976) consider that particles between the range 0.05 < r < 1 μm may cause a cooling surface. Research has shown that the radiative forcing of atmospheric particles depends on their composition, size and altitude (Hansen et al, 1980; Pollack et al, 1981). So, it may be considered that a change in one or several properties on the atmospheric particles might affect the local forcing.

The indirect radiative forcing occurs when the aerosol particles are used as CCN creating cloud droplets. The effects are classified in two types:


## **1.4. EPIC 2001**

222 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

**1.1. Clouds in the Inter-Tropical Convergence Zone (ITCZ)** 

Area of confluence of trade winds from the Northeast and Southeast.

Area that has the band of maximum coverage of convective clouds.

radiation and the presence of convective phenomena.

Area of maximum temperature on the sea surface

Maximum mass convergence zone

inhibiting the rain process (Rosenfeld, 1999).

Tropical Convergence Zone (ITCZ).

atmosphere, identified as:

larger vertical clouds.

**1.2. Development of convective clouds** 

instability helps to lift the air masses.

in an area influenced by polluted sources modify the local cloud albedo and its life time,

So, there are many processes that change the properties of atmospheric particles and their interactions with clouds. This chapter is focused on identifying and assessing the main processes involved with particles in the vicinity of maritime convective clouds at the Inter-

The ITCZ is one of the most important weather systems in the Tropics. The area shows a decisive influence on the characterization of different clime and weather conditions in tropical region. The ITCZ is characterized by several interactions between the ocean and the

Area where the Equatorial depression is located due to an increased incidence of solar

Therefore, the ITCZ is represented as a line of clouds of deep convection extending across the Atlantic and Pacific oceans, located between the 5° and 10° N (Holton, 1992). The band moves depending on the season, always matching in areas with high solar intensity or where the sea surface has higher temperatures. The clouds movement is towards the Southern hemisphere between September and February, and in the opposite direction in the next few months until the end of the summer in the Northern Hemisphere. However, just at the north of the Ecuador, the ITCZ movements are lower (Wallace and Hobbs, 1977). In those areas, the rain intensifies with solar heating. An exception occurs with El Niño-Southern Oscillation (ENSO), and the ITCZ is deflected towards where the ocean surface increases its temperature. A special feature of the region is the presence of a warm water pool, nearby the coast of Mexico, located in the ITCZ between 12° and 8° N from September to November. The presence of high temperatures at the ocean surface promotes a greater amount of moisture and increases convection effects, which result in

A convective cloud is formed when a mass of moist air acquires buoyancy due to the increase in surrounding temperature; with this process, the presence of atmospheric

Once the air mass reaches the saturation point, water vapor condenses on the CCN. The change of physical state releases latent heat that is absorbed by the air increasing the East Pacific Investigation of Climate Processes in the Coupled Ocean-Atmosphere System 2001 (EPIC 2001) was sponsored by The U.S. Climate Variability and Predictability Research Program (CLIVAR), which has the goal of providing the observational basis needed to improve the representation of certain key physical processes in coupled ocean atmosphere models. In addition to physical processes, EPIC 2001 research was directed toward a better understanding and simulation of the effects of short-term variability in the east Pacific on climate. This variability is particularly important in the region because conditions in the ITCZ are highly variable on daily to intra-seasonal time scales. The effects of such variability rectify strongly onto climate time scales in this region.

Interaction Between Aerosol Particles and Maritime Convective Clouds:

**Table 1.** Aerosol and cloud particle instrumentation on the C130 aircraft

Instrument type Parameter Range Accuracy

Coefficient 10-7 to 10-2 m-l ±5%

Coefficient 1.0x10-7 to 10x10-3 m-l ±5%

<sup>100</sup> Size spectra of aerosols 0.3 to 20 mm (30 channels) ± 20% (Diameter)

0.01 to > 3 µm 0 to 2x10<sup>4</sup>

cm-3

Size spectra of aerosols 0.12 to 3.0 mm (15 channels) ± 20% (Diameter)

and cloud droplets 2 to 47 mm (30 channels) ± 20% (Diameter)

CCN 0.2 to 1.0 % supersaturation 10% at 1% supersaturation

Light-Absorption

Number concentration of

Size spectra of aerosols

Number concentration of

PCASP-100X Light-scattering

aerosol

more detailed description and analysis of this instrument operation.

Strapp et al (1992).

CN counter (TSI Model 3760)

PMS Model FSSP-

PMS Model FSSP-

300

CCN Counter

*2.1.2. Instruments location* 

The CCN is a subset of the total concentration of particles, which can form droplets in an environments of over-saturation, as in a cloud (101 - 110% SS). The CCN counter model 100, determines the concentrations of these atmospheric particles. The operating principle is based on measuring the variation within a thermal gradient diffusion chamber, to create an environment of over-saturation. An electrical system controls the temperature of two plates that create the conditions of over saturation. A beam of laser light passes through the chamber and the instrument measures the amount of light scattered and estimates the concentration of CCN per volume. Delene et al (1998) and Delene and Deshler (2000) have a

The PCASP, FSSP300, and FSSP100 provide information on particle concentrations in size defined ranges. The instruments pass a beam of light with specific wavelength and intensity, through the air sample. The particles in the sample scatter the light beam with an intensity that depends on their size, shape and composition. The instrument measures the amount of light scattered with sensors. By knowing the intensity of scattered light and particles composition (water for FSSP100 and FSSP300) or sodium chloride (for PCASP), it is possible to infer the size range of particles to which they belong. A more detailed description of the operation and measurement uncertainty associated with the PCASP can be obtained on

The external instruments are installed in a pod on the wing and internal within the fuselage in the cabin. The location characterizes the way that each machine takes the air sample to analyze. The external instruments are in direct contact with the air all the time. Thus, the air sampling is instantaneous. But the internal instruments take the air sample by an air inlet and a section of hose that transports it to the device. In this case, the instrument uses a

Measurements in ITCZ During the EPIC 2001 Project 225

Varies with concentration, about 6% at 3300 cm-3.

±16% (Concentration)

±16% (Concentration)

±16% (Concentration)

EPIC 2001 was conceived as an intensive process study along and near 95ºW during September and October 2001. This longitude was chosen to coincide with the Tropical Atmosphere Ocean project (TAO) mooring array in order to provide an overlap between the process study and long-term monitoring.

In addition to the TAO moorings, two aircrafts, the National Center for Atmospheric Research's (NCAR) C-130 and NOAA's P-3, plus two ships, NOAA's R/V Ron H. Brown and the National Science Foundation's (NSF's) R/V New Horizon, and Galapagos-based soundings, were used to make measurements of the atmosphere and ocean in this region. The aircraft were based from 1 September to 10 October 2001 in Huatulco, Mexico. The ships spent approximately 3 weeks in the vicinity of 10º N, 95º W, and then traversed the 95º W line to the equator. After a short stop in the Galapagos Islands, the Ron H. Brown then proceeded south along 95º W and then to the Woods Hole Oceanographic Institute Improved Meteorological Recorder (IMET) mooring at 20º S, 85º W. Meanwhile the New Horizon reversed its track along 95º W and then returned to port.
