Weather, climate and the air we breathe - WMO

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Altitude (km) Altitude (km) 30 25 20 15 10 5 0 30 25 20 15 10 5 0 55.28 76.79 55.44 86.13 49.31 74.02 49.47 83.34 43.29 71.75 43.46 81.06 remarkably similar to the characteristic circulation pattern associated with the elevated heat pump effect (see Figure 4). The circulation pattern at 850 hPa (Figure 9(b)) also resembles the elevated heat pump pattern, indicating a partial strengthening of the monsoon flow over north-western India and central India and increased moisture in the upper troposphere (600-300 hPa). A further signature of the elevated heat pump effect can be seen in the northsouth cross-section of meridional flow and temperature anomalies from the Tibetan Plateau to southern India (75-85°E). Above-normal warming is found over the Tibetan Plateau and cooling near the surface and the lower troposphere in the lowlands of the Indo-Gangetic Plain and central India. Enhanced rising motion is found over 28 | WMO Bulletin 58 (1) - January 2009 37.25 69.82 37.42 79.12 31.18 68.11 31.35 77.41 Figure 7 — Calipso backscatter showing depth and relative concentration of the aerosol layer along a meridional cross-section over Pakistan and the Arabian Sea (above) the Indo-Gangetic Plain and the Himalayas (below). Colour key: red = high; yellow = medium; green = low concentration; grey = clouds. Numbers on abscissa represent North-latitude and East-longitude. 25.10 66.56 25.27 75.85 the southern slopes of the Tibetan Plateau and return sinking motions over southern India (Figure 9(c)). The meridional motion shows a bifurcation in the lower troposphere near 15-20°N, featuring sinking motion presumably associated with aerosolinduced cooling and rising motion, which merges in the middle and upper troposphere with the ascending motion over the foothills of the Himalayas. The lower-level inflow brings increased moisture to the southern slopes of the Himalayas, increases the monsoon low-level westerlies over central India and upper level easterlies over the southern Tibetan Plateau (Figure 9(d)). Here, the meridional circulation is likely to be forced by convection initiated by atmospheric heating by dust and amplified by positive feedback from low-level moisture convergence and ascending air in 19.00 65.12 19.17 74.41 12.89 63.74 13.06 73.03 the dust layer. While the above are not definitive confirmation of impacts of absorbing aerosols, the large-scale circulation features are consistent with the elevated heat pump effect, including the amplified warming of the upper troposphere over the Tibetan Plateau, cooling near the surface and an increase in monsoon flow with increased rainfall over northern India. Conclusions 6.83 62.42 6.99 71.71 The results shown here suggest that aerosol and precipitation in the monsoon area and adjacent deserts are closely linked to the large-scale circulation and intertwined with the complex monsoon diabatic heating and dynamical processes during premonsoon and monsoon periods. The deserts provide not only the large-
(a) April (c) June 40N 40N 35N 30N 25N 20N 15N 10N 5N 5N 35E 40E 45E 50E 55E 60E 65E 70E 75E 80E 85E 90E 35E 40E 45E 50E 55E 60E 65E 70E 75E 80E 85E 90E (b) May (d) July 40N 40N 35N 30N 25N 20N 15N 10N 5N 35E 40E 45E 50E 55E 60E 65E 70E 75E 80E 85E 90E 35N 30N 25N 20N 15N 10N 35N 30N 25N 20N 15N 10N 5N 35E 40E 45E 50E 55E 60E 65E 70E 75E 80E 85E 90E Figure 8 — Seven-day back trajectories showing possible sources and transport routes from adjacent deserts for air mass observed at 850 hPa over Kanpur for 11 days, starting from (a) 15 April, (b) 15 May, (c) 15 June and (d) 15 July 2008. Height (in hPa) of tracer is shown in colour. (a) T1000-300 June 2008 (c) T & v;w June 2008 (75E-85E) 40N 300 35N 30N 25N 20N 15N 10N 5N 40E (b) q600-300 40N 35N 30N 25N 20N 400 500 600 700 800 900 1000 50E 60E 70E 80E 90E 100E 5N -4 -3 -2 -1 0 1 2 3 4 (d) q & u 300 15N 700 10N 800 900 5N 40E 50E 60E 70E 80E 90E 1000 100E 5N 10N 15N 20N 25N 30N 35N 40N -4 -3 -2 -1 0 1 2 3 4 400 500 600 200 250 300 400 500 600 700 850 900 950 950 10N 15N 20N 25N 30N 35N 40N -4 -3 -2 -1 0 1 2 3 4 -4 -3 -2 -1 0 1 2 3 4 Figure 9 — Observed spatial distributions of June 2008 anomalies for (a) mean tropospheric temperature (°C) and 300 hPa winds (m/s); (b) mean 600-300 hPa specific humidity, 850 hPa winds and meridional vertical cross-sections over northern India and the Himalayas (75-85°E); (c) meridional-vertical streamline and temperature; and (d) zonal winds (contour) and specific humidity (shading) scale radiative forcing but also dust particles that are transported into monsoon regions, interfering with, and possibly altering, the evolution of monsoon circulation and rainfall. Because coupled atmosphereocean-land dynamical processes are the primary driver of the Asian monsoon, extreme care must be exercised in identifying aerosolrainfall relationships that are truly due to aerosol physics and do not arise because both aerosol and rainfall are driven by the same large-scale dynamics. The 2008 Indian monsoon appears to have the tell-tale signs of impacts by absorbing aerosols but further studies must be conducted to determine the details of the aerosol forcing and response of the monsoon water cycle and relative roles compared to forcing from coupled atmosphere-ocean-land processes. Acknowledgements This work is supported by the NASA Interdisciplinary Investigation Program. References Bollasina M, S. niGaM and K.M. lau, 2008: Absorbing aerosols and summer monsoon evolution over South Asia: An observational portrayal. J. Climate., 21, 3221-3239, DOI: 10.1175/2007JCLI2094.1 chenG, Y., U. lohMann, J. zhanG, Y. luo, Z. liu and G. lesins, 2005: Contribution of changes in sea surface temperature and aerosol loading to the decreasing precipitation trend in southern China. J. Climate, 18, 1381-1390. collier, J.C. and G.J. zhanG, 2008: Aerosol direct forcing of the summer Indian monsoon as simulated by the NCAR CAM3. Clim. Dyn. (in press). Devara, P.C.S., P.E. raJ, G. PanDithurai, K.K. Dani and R.S. MahesKuMar, 2003: Relationship between lidar-based observations of aerosol content and monsoon precipitation over a tropical station, Pune, India. Meteorol. Appl. 10, 253-262. GeorGe, J.P., L. harenDuPraKash, and M. Mohan, 2008: Multi-year changes of aerosol optical depth in the monsoon region of the Indian Ocean since 1986 as seen in the AVHRR and TOMS data. Ann. Geophys., 26, 7-11. WMO Bulletin 58 (1) - January 2009 | 29
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Page 5 and 6: Just as with climate, air pollutant
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