**3. Data products and methodology**

Satellite observations have advantages over the ground‐based measurements, in that, they provide information over the larger spatial domain [21]. The MODIS was designed with aerosol and cloud remote sensing in mind [22]. The MODerate Resolution Imaging Spectror‐ adiometer (MODIS) aboard the Terra (launched in 1999) and Aqua (launched in 2002) monitors the earth–atmosphere system twice daily over a given location. It is sun‐synchronous and near polar orbiting satellite with a circular orbit of 705 km above the surface. MODIS has 36 bands ranging from 0.4 to 14.4 µm wavelengths with three different spatial resolutions (250, 500, and 1000 m) and views the Earth with a swath of 2330 km, thereby providing near‐global coverage on daily basis, with equatorial crossing local time of 10:30 am and 1:30 pm for Terra and Aqua, respectively (http://modis.gsfc.nasa.gov/).

Among the hundreds of products derived from MODIS‐measured radiances are a suite of aerosol products [23] and another set of cloud products [24], including aerosol optical depth (AOD), cloud top pressure, and cloud fraction. Often, the AOD is used as a proxy for the cloud condensation nucleus (CCN) concentration. The reliability of this proxy depends on the uniformity of the aerosol size, composition, vertical distribution, but may in many cases be used as a first approximation. The MODIS data are available at different processing levels, level 1.0 (geolocated radiance and brightness temperature), level 2.0 (retrieved geophysical data products) and level 3.0 (gridded points) [24]. MODIS uses infrared bands to determine the physical properties of cloud in relation to cloud top pressure and temperature, and visible and near‐infrared bands to determine optical and microphysical cloud properties [23, 25]. For water vapor, the retrieval the near‐infrared region is adopted.

For this study, simultaneously retrieved datasets from MODIS (Terra) for the period 2000–2010 were used. Terra MODIS level\_3 (C\_005) monthly data products of aerosol optical depth (AOD), Ångström exponent (AE), cloud fraction (CF, day), water vapor (WV, above cloud), cloud effective radius (CER, liquid), liquid water path (LWP), and cloud optical depth (COD, liquid) were retrieved over the study region. As shown in **Figure 1**, the study regions were divided into 1° × 1° grid box centered at (1) DSL (32°16′ N; 76°23′E), (2) MND (31°43′ N; 76°58′ E), (3) SML (31°06′ N; 77°13′ E), (4) LDN (30°55′ N; 75°54′E), (5) PTL (30°20′ N; 76°25′ E), (6) MZR (29°28′ N; 77°44′E), (7) PUN (18°31′ N; 73°55′ E), (8) STR, (17°42′ N; 74°02′ E), and (9) KPR(16°42′ N; 74°16′ E).

For the estimation of aerosol indirect effect (AIE) on the basis of the observed AODs, we grouped the selected stations into different categories viz., CAT‐H (heavy aerosol loading), CAT‐M (moderate aerosol loading), and CAT‐L (low aerosol loading). The MODIS (Terra) level\_3 (C\_005) daily data products of AOD, LWP, and CER were retrieved for this estimation, and analysis was performed over each category and as well as over each station keeping the fixed LWP constraint. The retrieved LWP and CER were divided into 14 different bin sizes.

**Figure 1.** Study regions selected based on the dominant aerosol sources over India.

interplay of topography with WDs determines orographic precipitation over the Himalayan

Satellite observations have advantages over the ground‐based measurements, in that, they provide information over the larger spatial domain [21]. The MODIS was designed with aerosol and cloud remote sensing in mind [22]. The MODerate Resolution Imaging Spectror‐ adiometer (MODIS) aboard the Terra (launched in 1999) and Aqua (launched in 2002) monitors the earth–atmosphere system twice daily over a given location. It is sun‐synchronous and near polar orbiting satellite with a circular orbit of 705 km above the surface. MODIS has 36 bands ranging from 0.4 to 14.4 µm wavelengths with three different spatial resolutions (250, 500, and 1000 m) and views the Earth with a swath of 2330 km, thereby providing near‐global coverage on daily basis, with equatorial crossing local time of 10:30 am and 1:30 pm for Terra and Aqua,

Among the hundreds of products derived from MODIS‐measured radiances are a suite of aerosol products [23] and another set of cloud products [24], including aerosol optical depth (AOD), cloud top pressure, and cloud fraction. Often, the AOD is used as a proxy for the cloud condensation nucleus (CCN) concentration. The reliability of this proxy depends on the uniformity of the aerosol size, composition, vertical distribution, but may in many cases be used as a first approximation. The MODIS data are available at different processing levels, level 1.0 (geolocated radiance and brightness temperature), level 2.0 (retrieved geophysical data products) and level 3.0 (gridded points) [24]. MODIS uses infrared bands to determine the physical properties of cloud in relation to cloud top pressure and temperature, and visible and near‐infrared bands to determine optical and microphysical cloud properties [23, 25]. For

For this study, simultaneously retrieved datasets from MODIS (Terra) for the period 2000–2010 were used. Terra MODIS level\_3 (C\_005) monthly data products of aerosol optical depth (AOD), Ångström exponent (AE), cloud fraction (CF, day), water vapor (WV, above cloud), cloud effective radius (CER, liquid), liquid water path (LWP), and cloud optical depth (COD, liquid) were retrieved over the study region. As shown in **Figure 1**, the study regions were divided into 1° × 1° grid box centered at (1) DSL (32°16′ N; 76°23′E), (2) MND (31°43′ N; 76°58′ E), (3) SML (31°06′ N; 77°13′ E), (4) LDN (30°55′ N; 75°54′E), (5) PTL (30°20′ N; 76°25′ E), (6) MZR (29°28′ N; 77°44′E), (7) PUN (18°31′ N; 73°55′ E), (8) STR, (17°42′ N; 74°02′ E), and (9)

For the estimation of aerosol indirect effect (AIE) on the basis of the observed AODs, we grouped the selected stations into different categories viz., CAT‐H (heavy aerosol loading), CAT‐M (moderate aerosol loading), and CAT‐L (low aerosol loading). The MODIS (Terra) level\_3 (C\_005) daily data products of AOD, LWP, and CER were retrieved for this estimation, and analysis was performed over each category and as well as over each station keeping the fixed LWP constraint. The retrieved LWP and CER were divided into 14 different bin sizes.

region.

172 Aerosols - Science and Case Studies

**3. Data products and methodology**

respectively (http://modis.gsfc.nasa.gov/).

KPR(16°42′ N; 74°16′ E).

water vapor, the retrieval the near‐infrared region is adopted.

AIE was estimated for different seasons and for the entire study period 2000–2010 by evolving a linear least square fit to the plot between CER and AOD at fixed LWP and using the following equation [26],

$$\text{AIE} = -\frac{d\ln r\_e}{d\ln \tau\_a} \tag{1}$$

Here, re is the cloud effective radius (CER) for fixed LWP and τa is the AOD. The degree of significance of AIE and correlation coefficients of linear regression fit has been also determined over the selected stations using two‐tailed t‐tests at 90 and 95% of confidence level. The correlation coefficients between AOD and other parameters (AE, CF, COD, CER, LWP, and WV) for 11 years data at each station are given in **Table 2**, and seasonal correlations of these parameters are given in **Table 3**. In these tables, the doubly underlined correlation coefficients are significant at 0.05 level (95% confidence level) while singly underlined correlation coefficients are significant at 0.1 level (90% confidence level) and the rest are less significant.
