**9. Contribution of STC to particulate scattering in the UTLS region**

Fine ice crystals of STCs originating either through the outflow from deep convective anvils or through freeze drying of moist air lifted up to the tropopause by normal convection contribute significantly for scattering in the UTLS region. In addition, other aerosol particles originating from the surface (mainly through bulk to particle conversion and those of vegetative origin) as well as those formed in the upper troposphere through gas- to-particle conversion (mainly of various industrial gases) also contribute for the particulate loading in the UTLS region. It would be worth examining the relative contributions of the two components in the UTLS region to make a quantitative assessment of the contribution of STCs to the scattering properties in the UTLS region. Figure 8a shows the contour plots of month-to-month variation of mean βp derived from lidar data during the period 1998–2003 (including both STC-contaminated and STC-free). Note that, as this study period was mostly devoid of major volcanic eruptions [24] the stratospheric aerosol loading can be considered to be in its background level.

While βp in the UT is a maximum during the May –September period and minimum during October–November, in the lower stratosphere it is minimum during summer (July and August) and maximum during winter. However, the winter high in the LS region was attributed [7] to the transport of tropospheric air (containing aerosols and precursor gases) in conjunction with the tropical upwelling and B− D circulation while the observed high during May–June is due to the upward influx of particles (including ice crystals of STCs) from the UT region. As the uncertainty associated with lidar-derived βp is small compared to αp [42], for the lidar based study the altitude profile of βp and ΙβP for a desired altitude region (layer integrated) is used for studying the annual pattern of particulate scattering in the UTLS region. Figure 14 shows the contour plots depicting the annual variation of βp at different altitudes for the STC-contaminated case and the STC-free case separately.

Distribution of Particulates in the Tropical UTLS over

the Asian Summer Monsoon Region and Its Association with Atmospheric Dynamics 131

the STC-top while UT1 is below the STC-base. The month-to-month variation of mean ΙβP for the three cases; viz, (i) including all profiles (ii) considering only STC-contaminated profiles and (iii) Considering only STC-free profiles, are presented in Figure 15. In the absence of STC the ΙβP in the UT2 region is significantly small. The STC contribution to ΙβP in this region works out to be around 93±5%. The ΙβP shows an annual variation with relatively high values during the winter/dry months and low during the summer monsoon period for the STC-free case. When STCs prevail, the ΙβP in the UT2 region increases significantly especially during the summer monsoon period. Even though the enhancement in ΙβP due to the presence of STC is seen during the winter/dry months also, its magnitude is relatively less. It would be worth in this context to note that, during the monsoon period, the UT2 region is dominantly influenced by dense STCs originating from the outflow of convective anvils [78]. The particle associated with these STCs will be relatively large and highly non-spherical

[8,45] and hence their contribution to βp and δ will be significantly large [7].

**Figure 15.** Mean annual variation of layer integrated particulate backscatter coefficient (ΙβP) in LS2 region (a), LS1 region (b), UT2 region (c) and UT1 region (d), for the three cases (including all profiles, only STC-contaminated profiles and only STC-free profiles) from lidar data at Gadanki, for the period

1998–2003.

**Figure 14.** Mean annual variation of particulate backscatter coefficient (βp) from lidar data at Gadanki for the STC-contaminated (a) and STC-free (b) cases during the period 1998–2003.

The cloud cover being quite large during the summer monsoon period no lidar data were available without STC during the July and August months, which lead to a data-gap for the STC-free case presented in Figure 14b. As can be seen, the variations of βp in Figure 14a is very similar to those in Figure 8a generated by considering both the STC-contaminated and STC-free profiles even though the absolute magnitude in the UT region is slightly small in the latter case especially during the October–November period. High values of βp are observed in the UT region for the STC-contaminated case during summer monsoon period. Figure 14b shows that, βp in the UT region is very small for the STC-free case compared to that of STC-contaminated case for the same period. The values of βp in the UT region are high during winter and spring and low during summer and autumn for the STC-free case. This annual pattern of βp is significantly different from that of STC-contaminated case. From this, it is quite reasonable to infer that the prominent peak of βp in the UT region observed during the summer monsoon period (in Figures 8a and 14a) is due to the influence of STC. However, the enhancement in βp in the UT region due to STC-contamination is relatively small during the winter months

As seen from Figure 5 the occurrence of STC is mostly confined to the uppermost part of the troposphere, above ~10 km. Because of their presence these STCs directly contribute to the particulate scattering in this region. Over and above, these prevailing STCs in the upper troposphere can also modify the scattering property of particulates (aerosols) above the cloud-top as well as below the cloud-base. These effects in the UT and LS regions can be inferred by examining the ΙβP at four different altitude regions, 8–10 km (UT1), 12–16 km (UT2), 18–21 km (LS1) and 21–25 km (LS2). Among these, the LS1 and LS2 regions are above the STC-top while UT1 is below the STC-base. The month-to-month variation of mean ΙβP for the three cases; viz, (i) including all profiles (ii) considering only STC-contaminated profiles and (iii) Considering only STC-free profiles, are presented in Figure 15. In the absence of STC the ΙβP in the UT2 region is significantly small. The STC contribution to ΙβP in this region works out to be around 93±5%. The ΙβP shows an annual variation with relatively high values during the winter/dry months and low during the summer monsoon period for the STC-free case. When STCs prevail, the ΙβP in the UT2 region increases significantly especially during the summer monsoon period. Even though the enhancement in ΙβP due to the presence of STC is seen during the winter/dry months also, its magnitude is relatively less. It would be worth in this context to note that, during the monsoon period, the UT2 region is dominantly influenced by dense STCs originating from the outflow of convective anvils [78]. The particle associated with these STCs will be relatively large and highly non-spherical [8,45] and hence their contribution to βp and δ will be significantly large [7].

130 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

region (layer integrated) is used for studying the annual pattern of particulate scattering in the UTLS region. Figure 14 shows the contour plots depicting the annual variation of βp at

**Figure 14.** Mean annual variation of particulate backscatter coefficient (βp) from lidar data at Gadanki

The cloud cover being quite large during the summer monsoon period no lidar data were available without STC during the July and August months, which lead to a data-gap for the STC-free case presented in Figure 14b. As can be seen, the variations of βp in Figure 14a is very similar to those in Figure 8a generated by considering both the STC-contaminated and STC-free profiles even though the absolute magnitude in the UT region is slightly small in the latter case especially during the October–November period. High values of βp are observed in the UT region for the STC-contaminated case during summer monsoon period. Figure 14b shows that, βp in the UT region is very small for the STC-free case compared to that of STC-contaminated case for the same period. The values of βp in the UT region are high during winter and spring and low during summer and autumn for the STC-free case. This annual pattern of βp is significantly different from that of STC-contaminated case. From this, it is quite reasonable to infer that the prominent peak of βp in the UT region observed during the summer monsoon period (in Figures 8a and 14a) is due to the influence of STC. However, the enhancement in βp in the UT region due to STC-contamination is relatively

As seen from Figure 5 the occurrence of STC is mostly confined to the uppermost part of the troposphere, above ~10 km. Because of their presence these STCs directly contribute to the particulate scattering in this region. Over and above, these prevailing STCs in the upper troposphere can also modify the scattering property of particulates (aerosols) above the cloud-top as well as below the cloud-base. These effects in the UT and LS regions can be inferred by examining the ΙβP at four different altitude regions, 8–10 km (UT1), 12–16 km (UT2), 18–21 km (LS1) and 21–25 km (LS2). Among these, the LS1 and LS2 regions are above

for the STC-contaminated (a) and STC-free (b) cases during the period 1998–2003.

small during the winter months

different altitudes for the STC-contaminated case and the STC-free case separately.

**Figure 15.** Mean annual variation of layer integrated particulate backscatter coefficient (ΙβP) in LS2 region (a), LS1 region (b), UT2 region (c) and UT1 region (d), for the three cases (including all profiles, only STC-contaminated profiles and only STC-free profiles) from lidar data at Gadanki, for the period 1998–2003.

It is quite interesting to note that the presence of STC in the UT2 region significantly enhances the ΙβP in the lower stratosphere also especially during the May–October period. In the LS1 and LS2 regions ΙβP shows a double peak structure with prominent maxima during May–June and October–January for the STC-contaminated cases (in the UT region), while it is very small from May to October for the STC-free case. A significant amount from the abundant ice crystals (of STC) present in the UT region during the summer monsoon period will be lifted up along with air-mass across the tropopause. Consequently, ΙβP in the lower stratosphere also increases. However, unlike the case during winter, this increase in βp (or ΙβP) is rather confined to a small region just above the tropopause (the LS1 region). All the three curves in Figure 15 for the LS1 and LS2 regions almost overlap each other during the December to April period. This indicates that the STCs in the UT2 region during this period did not contribute significantly to ΙβP in the stratosphere. In general, the influence of STC to the mean backscattering coefficient (ΙβP /Δh, where Δh is the slab thickness) in LS1 region is relatively more than that in LS2, particularly during the summer monsoon period. Thus, presence of dense STC in the UT2 region (during the summer monsoon period) enhances the particulate scattering in the lower stratosphere, even though this region is practically free from STC.

Distribution of Particulates in the Tropical UTLS over

the Asian Summer Monsoon Region and Its Association with Atmospheric Dynamics 133

A study on the influence of STC in the particulate extinction in UT and LS region using SAGE-II data over the Indian longitude sector (70-90°E) from 30°S to 30°N also revealed similar results. However,, as the occurrence of STC is less frequent in the southern hemispheric off-equatorial region, the increase in τp in the UT and LS regions due to the

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

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

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

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

tropopause for each of these latitude bands is superposed on the respective contours.

influence of STC is relatively small compared to that in the equatorial region [80].

**longitude sector** 

mostly lies in the range 16-18 km.

As lidar data yields reliable βp values only from the region above the altitude of 'full beam overlap' the effect of STCs below the cloud-base can be examined by studying the scattering property of the medium over a narrow altitude region 8–10 km (UT1), which is mostly free from STCs (Figure 6b). The mean annual variation of ΙβP in the UT1 region for the three cases, (i) when the UT2 region is STC-free, (ii) the UT2 region is STC-contaminated and (iii) with both these cases combined, is presented in Figure 15d. Except for the STC-free case, though the ΙβP in this region also shows an annual variation similar to that for the UT2 region, the amplitude of this variation is significantly small. Notwithstanding the fact that the values of ΙβP during the November to April period for the three cases are comparable, for the STC-free condition it shows a decrease during the May–October period. This shows that the presence of dense STCs in the UT2 region enhances the particulate scattering in the region below the STC-base and the magnitude of this enhancement is much smaller than that in the UT2 region (where the ΙβP increases by a factor >20). But when these STCs are thin (optically as well as geometrically), especially during the November to April period, this contribution is almost negligible. Thus, the prevailing STCs in the UT2 region during the summer monsoon period significantly enhance the scattering from the region above the cloud-top as well as below the cloud-base. The enhancement in particulate scattering in the LS region above the cloud-top could be mainly due to the lofting of STC-particles across the tropopause, which joining with the prevailing LS aerosols modify the volume scattering properties in this region. Though the Brewer–Dobson circulation is weak during summer the troposphere–stratosphere exchange during this period could be aided by an increase in wave activity depending on the prevailing atmospheric condition. Strong convection prevailing over the Indian landmass during the summer–monsoon period, which can transport abundant moisture to the upper troposphere inducing cirrus formation, is also a major source for gravity waves [79].

A study on the influence of STC in the particulate extinction in UT and LS region using SAGE-II data over the Indian longitude sector (70-90°E) from 30°S to 30°N also revealed similar results. However,, as the occurrence of STC is less frequent in the southern hemispheric off-equatorial region, the increase in τp in the UT and LS regions due to the influence of STC is relatively small compared to that in the equatorial region [80].

132 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

from STC.

major source for gravity waves [79].

It is quite interesting to note that the presence of STC in the UT2 region significantly enhances the ΙβP in the lower stratosphere also especially during the May–October period. In the LS1 and LS2 regions ΙβP shows a double peak structure with prominent maxima during May–June and October–January for the STC-contaminated cases (in the UT region), while it is very small from May to October for the STC-free case. A significant amount from the abundant ice crystals (of STC) present in the UT region during the summer monsoon period will be lifted up along with air-mass across the tropopause. Consequently, ΙβP in the lower stratosphere also increases. However, unlike the case during winter, this increase in βp (or ΙβP) is rather confined to a small region just above the tropopause (the LS1 region). All the three curves in Figure 15 for the LS1 and LS2 regions almost overlap each other during the December to April period. This indicates that the STCs in the UT2 region during this period did not contribute significantly to ΙβP in the stratosphere. In general, the influence of STC to the mean backscattering coefficient (ΙβP /Δh, where Δh is the slab thickness) in LS1 region is relatively more than that in LS2, particularly during the summer monsoon period. Thus, presence of dense STC in the UT2 region (during the summer monsoon period) enhances the particulate scattering in the lower stratosphere, even though this region is practically free

As lidar data yields reliable βp values only from the region above the altitude of 'full beam overlap' the effect of STCs below the cloud-base can be examined by studying the scattering property of the medium over a narrow altitude region 8–10 km (UT1), which is mostly free from STCs (Figure 6b). The mean annual variation of ΙβP in the UT1 region for the three cases, (i) when the UT2 region is STC-free, (ii) the UT2 region is STC-contaminated and (iii) with both these cases combined, is presented in Figure 15d. Except for the STC-free case, though the ΙβP in this region also shows an annual variation similar to that for the UT2 region, the amplitude of this variation is significantly small. Notwithstanding the fact that the values of ΙβP during the November to April period for the three cases are comparable, for the STC-free condition it shows a decrease during the May–October period. This shows that the presence of dense STCs in the UT2 region enhances the particulate scattering in the region below the STC-base and the magnitude of this enhancement is much smaller than that in the UT2 region (where the ΙβP increases by a factor >20). But when these STCs are thin (optically as well as geometrically), especially during the November to April period, this contribution is almost negligible. Thus, the prevailing STCs in the UT2 region during the summer monsoon period significantly enhance the scattering from the region above the cloud-top as well as below the cloud-base. The enhancement in particulate scattering in the LS region above the cloud-top could be mainly due to the lofting of STC-particles across the tropopause, which joining with the prevailing LS aerosols modify the volume scattering properties in this region. Though the Brewer–Dobson circulation is weak during summer the troposphere–stratosphere exchange during this period could be aided by an increase in wave activity depending on the prevailing atmospheric condition. Strong convection prevailing over the Indian landmass during the summer–monsoon period, which can transport abundant moisture to the upper troposphere inducing cirrus formation, is also a
