**10. Mean latitude variation of the altitude structure of** τ**P over the Indian longitude sector**

The latitudinal structure of the annual pattern of τp in the tropical UTLS region over the Indian longitude Sector is examined using the altitude profiles of particulate extinction at 525 nm obtained from SAGE-II data archive in the latitude region 30°S to 30°N for the period 1998-2005. The profiles are grouped in different latitude bands each having a width of 5°. Contours presenting the annual variation of τp in different latitudes bands in the UTLS region are presented in Figure16. The mean annual variation of the altitude of the lapse rate tropopause for each of these latitude bands is superposed on the respective contours.

For the latitudinal region between 0-15°N, the tropopause is cooler (higher) during the December-May period and warmer (lower) during July-October period, in accordance with that reported for tropical locations by prior investigators [67,68]. For the latitudinal sector 15-20°N, the tropopause is higher during April-June and lower during the July-August period. At latitudes > 20°N, a pronounced maximum in tropopause altitude can be observed during the boreal summer and minimum during boreal winter. Though the tropopause altitude varies with latitude, time of the day and day of the year, on an average, this altitude mostly lies in the range 16-18 km.

In general, αp is relatively large (>10-7 m-1) in the UT region for the region north of 20°S. In both the hemispheres αp shows two peaks. The summer peak (in the respective hemispheres) is more prominent, compared to the winter peak. South of 20°S, αp in the UT region is relatively small and does not show any pronounced annual variation. The summer-winter contrast in the UT region is almost insignificant beyond 25°S. The winter peak in τp becomes relatively weak in the region north of 15°N and becomes almost insignificant beyond 25°N. The summerwinter contrast in αp is well pronounced beyond 20°N. In contrast to the UT region, the values of αp in the LS region are relatively small (<10-7 m-1), at least by a factor of two. The annual variation of extinction shows a weak summer winter contrast in the LS region for all the latitudinal sectors between 0-30°N. Due to the influence of STCs, which occur at random in different altitudes, the standard error of αp (expressed as percentages of mean αp) is generally very large in the UT region. The mean standard error in the attitude region 10-15 km ranges from 20% to 60%. This error decreases progressively with increase in altitude. In the LS region, the error is very small and mostly confined to values in the range 5-15%. In the transition region 15-20 km, the mean error is of the order of 30% which is less than that in the UT region.

Figure 16 shows that the annual variation of αp in the UT region over the southern hemisphere is distinctly different from that over the northern hemisphere. In this hemisphere high values of αp (in the UT region) remains fairly confined to latitudes north of 20°S. This difference can

mainly be attributed to the corresponding difference in the pattern of deep convection in these two hemispheres. While the southward migration of ITCZ (during the boreal winter) is fairly confined to latitudes north of 20°S, northward migration (during the boreal summer) extends beyond ~20°N. More over, the convection over the northern hemispheric land mass is much stronger than that over the southern hemispheric oceanic regions. Because of these features, the increase in αp in the UT region is also confined to the geographical region north of 20°S. The value of αp in the LS region is relatively small and show similar variations in both the hemispheres. This indicates that the features of aerosol transport in the equatorial (±15°latitude) LS region are fairly symmetric with respect to equator.

Distribution of Particulates in the Tropical UTLS over

the Asian Summer Monsoon Region and Its Association with Atmospheric Dynamics 135

thunderstorm days (TSD) for the Indian region was studied [63] using the data for the period 1970-1980. The monthly mean values of CAPE at different latitude sectors for the period 1998-2003 over this region is estimated from the respective daily values in each month obtained from the website of IMD (www.weather.uwyo.edu /upperair/ sounding.html). The annual variation of these two parameters for different latitude bands are shown in Figure 17. On an average, the annual pattern of TSD matches well with that of CAPE in all these latitude belts except for the fact that the secondary peak in October is less prominent in CAPE for latitudes north of 15°N. Both these parameters show a significant positive correlation with coefficient exceeding 0.7. Two prominent peaks observed during April-May and October in the near equatorial region merges to become a broad peak in the latitude region 20-25°N (off-equatorial) and subsequently becomes a well defined sharp peak around June-July in the latitude region 25- 30°N. Examining the annual variation of α<sup>p</sup> in the light of the annual variation of CAPE/TDS, it can be seen that the occurrence of the high values of αp (during May and October in Figure 16 h,i, j and those during June, July and August in Figure 16k,l), coincides with the peak in thunderstorm activity (or CAPE) in the respective latitudinal belt. During high convective activity while high values of αp are observed very close to the tropopause in the latitude belt 0-15°N, significant high values of extinction are observed just above the tropopause in the latitude region between 15-30°N. Intense thunderstorm activity along with deep convection during boreal summer is highly favorable for the formation of dense STCs very close to tropopause leading to an increase in αp in the UT region. Penetration of particles from these STCs to higher altitudes increases α<sup>p</sup>

**Figure 17.** Annual variation of CAPE at different latitude bands over the Indian region along with the mean climatology of thunderstorm days [63]. The vertical bars represent the standard error associated with CAPE. The correlation coefficient (ℜ) between CAPE and thunderstorm days and its level of

in the LS1 region, as is observed in Figure 16.

significance (p) are also shown in respective panels.

**Figure 16.** Contour plots showing the mean annual variation of αp in the UTLS region obtained from SAGE-II in six latitude bands of width 5°from 30°S to 30°N averaged for the longitude region 70°E to 90°E for the period 1998-2005. Month-to-month variation of the mean tropopause altitude for each sector, with vertical bars representing its standard error, is superimposed on the respective contours. For the northern hemisphere the tropopause altitudes are derived from the altitude profiles of temperature obtained from the daily Radiosonde measurements carried out by the India Meteorology Department (IMD) at different locations in each band. For the latitudes south of 5°N, where no Radiosonde measurements are available, the tropopause altitude is obtained from the NCEP data provided along with SAGE-II version 6.2-data archive.

## **11. Tropospheric convection over the Indian region**

The strength of tropospheric convection which can be indexed based on the CAPE could in turn also be related to the thunderstorm activity. The climatology of monthly mean thunderstorm days (TSD) for the Indian region was studied [63] using the data for the period 1970-1980. The monthly mean values of CAPE at different latitude sectors for the period 1998-2003 over this region is estimated from the respective daily values in each month obtained from the website of IMD (www.weather.uwyo.edu /upperair/ sounding.html). The annual variation of these two parameters for different latitude bands are shown in Figure 17. On an average, the annual pattern of TSD matches well with that of CAPE in all these latitude belts except for the fact that the secondary peak in October is less prominent in CAPE for latitudes north of 15°N. Both these parameters show a significant positive correlation with coefficient exceeding 0.7. Two prominent peaks observed during April-May and October in the near equatorial region merges to become a broad peak in the latitude region 20-25°N (off-equatorial) and subsequently becomes a well defined sharp peak around June-July in the latitude region 25- 30°N. Examining the annual variation of α<sup>p</sup> in the light of the annual variation of CAPE/TDS, it can be seen that the occurrence of the high values of αp (during May and October in Figure 16 h,i, j and those during June, July and August in Figure 16k,l), coincides with the peak in thunderstorm activity (or CAPE) in the respective latitudinal belt. During high convective activity while high values of αp are observed very close to the tropopause in the latitude belt 0-15°N, significant high values of extinction are observed just above the tropopause in the latitude region between 15-30°N. Intense thunderstorm activity along with deep convection during boreal summer is highly favorable for the formation of dense STCs very close to tropopause leading to an increase in αp in the UT region. Penetration of particles from these STCs to higher altitudes increases α<sup>p</sup> in the LS1 region, as is observed in Figure 16.

134 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

(±15°latitude) LS region are fairly symmetric with respect to equator.

mainly be attributed to the corresponding difference in the pattern of deep convection in these two hemispheres. While the southward migration of ITCZ (during the boreal winter) is fairly confined to latitudes north of 20°S, northward migration (during the boreal summer) extends beyond ~20°N. More over, the convection over the northern hemispheric land mass is much stronger than that over the southern hemispheric oceanic regions. Because of these features, the increase in αp in the UT region is also confined to the geographical region north of 20°S. The value of αp in the LS region is relatively small and show similar variations in both the hemispheres. This indicates that the features of aerosol transport in the equatorial

**Figure 16.** Contour plots showing the mean annual variation of αp in the UTLS region obtained from SAGE-II in six latitude bands of width 5°from 30°S to 30°N averaged for the longitude region 70°E to 90°E for the period 1998-2005. Month-to-month variation of the mean tropopause altitude for each sector, with vertical bars representing its standard error, is superimposed on the respective contours. For the northern hemisphere the tropopause altitudes are derived from the altitude profiles of temperature obtained from the daily Radiosonde measurements carried out by the India Meteorology Department (IMD) at different locations in each band. For the latitudes south of 5°N, where no Radiosonde measurements are available, the tropopause altitude is obtained from the NCEP data

The strength of tropospheric convection which can be indexed based on the CAPE could in turn also be related to the thunderstorm activity. The climatology of monthly mean

provided along with SAGE-II version 6.2-data archive.

**11. Tropospheric convection over the Indian region** 

**Figure 17.** Annual variation of CAPE at different latitude bands over the Indian region along with the mean climatology of thunderstorm days [63]. The vertical bars represent the standard error associated with CAPE. The correlation coefficient (ℜ) between CAPE and thunderstorm days and its level of significance (p) are also shown in respective panels.
