The most important source of systematic errors in all weather and climate prediction model is related with inaccuracy in formulation (or parameterization) of clouds. The parameterization of convection in weather and climate models depends on our understanding of how small scale clouds interact with the large scale environments and how aerosol interact with the clouds. In order to get data on cloud microphysics under different large range of meteorological conditions and distributions, it is important to continue these measurements for a reasonably long time. On the other hand, if we could set up a cloud physics and aerosol measurements observatory at a high altitude station where cloud bases touch the ground, one could make cloud-microphysics measurements together with aerosol and meteorological measurements for many years and could thereby collect data spanning different conditions. Therefore, it is proposed to accelerate the experimental infrastructure by introducing uninterrupted observations and monitoring of critical atmospheric parameters and cloud parameters regularly at a high altitude station in Maharashtra viz., Mahabaleshwar along with supporting infrastructure at the IITM campus in Pune.
The High Altitude Cloud Physics Laboratory (HACPL) originated from such a unique requirement, where clouds could be continuously monitored at a single location, where cloud base touches the ground. Observations on regular basis at the HACPL will provide continuous data for the study of cloud microphysics and interaction between clouds and aerosol and the process of precipitation and related dynamics. The impact of orography of Western Ghat on the precipitation dynamics will be addressed with continuous observations with radar at this site. The continuous and simultaneous observations at Mahabaleshwar will provide unique opportunity to study detailed interaction of the dynamics and microphysics over the region and explore the differences in their spatial characteristics. The observations will also be used along with other cloud physics observations in the physical parameterization development, testing etc and to establish cloud and precipitation climatology of the region.
Fig. : Time-of-Flight Aerosol Chemical Speciation Monitor.
Investigation of aerosol indirect effects on monsoon clouds using ground-based measurements over a high-altitude site in Western Ghats
The effect of aerosols on cloud droplet number concentration and droplet effective radius is investigated from ground-based measurements over a high-altitude site where clouds pass over the surface. First aerosol indirect effect (AIE) estimates were made using (i) relative changes in cloud droplet number concentration (AIEn) and (ii) relative changes in droplet effective radius (AIEs) with relative changes in aerosol for different cloud liquid water contents (LWCs). AIE estimates from two different methods reveal that there is systematic overestimation in AIEn as compared to that of AIEs . Aerosol indirect effects (AIEn and AIEs) and dispersion effect (DE) at different LWC regimes ranging from 0.05 to 0.50 g m−3 were estimated. The analysis demonstrates that there is overestimation of AIEn as compared to AIEs , which is mainly due to DE. Aerosol effects on spectral dispersion in droplet size distribution play an important role in altering Twomey’s cooling effect and thereby changes in climate. This study shows that the higher DE in the medium LWC regime offsets the AIE by 30 %.[Anil Kumar V., Pandithurai G., Leena P. P., Dani K. K., Murugavel P., Sonbawne S. M., Patil R.D., Maheskumar R.S., Investigation of aerosol indirect effects on monsoon clouds using ground-based measurements over a high-altitude site in Western Ghats, Atmospheric Chemistry and Physics, 16, July 2016, DOI: 10.5194/acp-16-8423-2016, 8423-8430]
Fig. 1: Variation of cloud microphysical parameters (droplet number concentration and effective diameter) with CCN number concentration.
Mesoscale kinematics derived from X-band Doppler radar observations of convective versus stratiform precipitation and comparison with GPS radiosonde profilesSingle Doppler analysis techniques known as Velocity Azimuth Display (VAD) and Volume Velocity Processing (VVP) are used to analyse kinematics of mesoscale flow such as horizontal wind and divergence using X-band Doppler weather radar observations, for selected cases of convective, stratiform and shallow cloud systems near tropical Indian sites: Pune (18.58°N, 73.92°E, 560 m asl) and Mandhardev (18.51°N, 73.85°E, 1297 m asl). The vertical profiles of horizontal wind estimated from radar VVP/VAD methods agree well with GPS radiosonde profiles, with the Low Level Jet (LLJ) at about 1.5 km during monsoon season well depicted in both. The vertical structure and temporal variability of divergence and reflectivity profiles are indicative of the dynamical and microphysical characteristics of shallow convective, deep convective and stratiform cloud systems. In shallow convective systems, vertical development of reflectivity profiles is limited below 5 km. In deep convective systems, reflectivity values as large as 55 dBZ were observed above freezing level. The stratiform system shows the presence of a reflectivity bright band (~35 dBZ) near the melting level. The diagnosed vertical profiles of divergence in convective and stratiform systems are distinct. In shallow convective conditions, convergence was seen below 4 km with divergence above. Low-level convergence and upper level-divergence is observed in deep convective profiles, while stratiform precipitation has mid-level convergence present between lower-level and upper-level divergence. The divergence profiles in stratiform precipitation exhibit intense shallow layers of ‘melting convergence’ at 0°C level, near 4.5 km altitude, with a steep gradient on both the sides of the peak. The level of non-divergence (LND) in stratiform situations is lower than that in convective situations. These observed vertical structures of divergence are largely indicative of latent heating profiles in the atmosphere, an important ingredient of monsoon dynamics. [Deshpande S.M., Dhangar N., Das Subroto Kumar, Kalapureddy M.C.R., Chakravarty K., Sonbawne S., Konwar M., Mesoscale kinematics derived from X-band Doppler radar observations of convective versus stratiform precipitation and comparison with GPS radiosonde profiles, Journal of Geophysical Research, 120, November 2015, DOI:10.1002/2014JD022595, 11536-11551]
Fig. 2: X-band radar observations of divergence (10-4 s-1) and reflectivity (dBZ) profiles in different cloud types such as stratiform, shallow and deep convective clouds of 29 July 2012, 27 September 2012 and 11 October 2011 respectively.
Temporal and structural evolution of a tropical monsoon cloud system using X-band radarThis is a case study focusing on the horizontal and vertical structure of monsoon precipitating clouds and its temporal evolution as observed by the X-band radar. The radar reflectivity factor is used as a proxy for measure of intensity of cloud system. Result shows that the radar reflectivity has a strong temporal variation in the vertical, with a local peak occurring in the afternoon hours. Relatively shallow structure during the late night and early morning hours is noticed. The observed cloud tops reached up to 8 km heights with reflectivity maxima of about 35 dBZ at ~5 km. The spatial and vertical evolution of radar reflectivity is consistent with the large-scale monsoon circulation. The variations in the outgoing longwave radiation (OLR) from the Kalpana-1 satellite and vertical velocity and cloud-mixing ratio from the MERRA reanalysis data were also analysed. [Das S.K., Deshpande S.M., Das S.S., Konwar M., Chakravarty K., Kalapureddy M.C.R., Temporal and structural evolution of a tropical monsoon cloud system: A case study using X-band radar observations, Journal of Atmospheric and Solar Terrestrial Physics, 133, October 2015, DOI:10.1016/j.jastp.2015.08.009, 157-168]
Seasonal variability of aerosol and CCN over HACPL
Aerosol-CCN variability and their relationship were studied for the first time at Mahabaleshwar, a high altitude (1348 m AMSL) site in the Western Ghats by using observations of one year (June 2012 to May 2013). The study was done in two sections in which first temporal variability (diurnal and seasonal) of aerosol and CCN was analysed. Later, CCN to aerosol ratio and other microphysical properties were investigated along with detailed discussion on possible sources of aerosol. Diurnal variation in aerosol and CCN concentration has shown relatively higher values during early morning hours in monsoon season, whereas in winter and pre-monsoon, it was higher in the evening hours (Fig. 1). Seasonal mean variation in aerosol and CCN (SS above 0.6%) has shown higher (less) in monsoon (winter) season. Temporal variation reveals dominance of fine-mode aerosol during monsoon season over the study region. The temporal variation of activation ratio, k-value (exponent of CCN Super-saturation spectra) and geometric mean aerosol diameter were also analysed. Variation of activation ratio was higher in monsoon, especially for SS 0.6-1%. The analysis also showed high k-value during monsoon season as compared to other seasons (Pre-monsoon and winter) which may be due to dominance of hygroscopic aerosols in the maritime air masses from the Arabian Sea and biogenic aerosol emissions from the wet forest. Analysed mean aerosol diameter is much smaller during monsoon season with less variability compared to other seasons. Overall analysis showed that aerosol and CCN concentration was higher over this high altitude site despite of dominant sink processes such as cloud scavenging and washout mechanisms indicating local emissions and biogenic Volatile Organic Compounds (BVOC) emissions from wet forest as major sources. [Leena P. P., Pandithurai G., Anilkumar V., Murugavel P., Sonbawne S.M., Dani K.K., Seasonal variability in aerosol, CCN and their relationship observed at a high altitude site in Western Ghats, Meteorology and Atmospheric Physics, Online, September 2015, DOI:10.1007/s00703-015-0406-0]
Fig. 1: Diurnal variation in CCN concentration (a, c, e) for different super saturations and total aerosol concentration 40-540 nm (b, d, f) for the period June 2012 to May 2013.
Project: Physics and Dynamics of Tropical Clouds
Project Director: Dr. G. Pandithurai, Scientist-F,
Sub-project: High Altitude Cloud Physics Laboratory (HACPL)