High Altitude Cloud Physics Laboratory (HACPL):
The High Altitude Cloud Physics Laboratory (HACPL) was conceived in order to continuously monitor clouds at a single location, where the cloud base touches the ground. The idea of the HACPL was formulated to have long term cloud physics, dynamics, radiation and chemistry observations at one place as aircraft observations are in situ and cannot generate data on diurnal and seasonal cycles.
Regular observations at the HACPL can provide continuous data for the study of cloud microphysics, interaction between clouds; and, aerosols and the process of precipitation and related dynamics.
The most important source of systematic errors in all weather and climate prediction models is related to the inaccuracy in formulation (or parameterisation) of clouds. The parameterisation of convection in weather and climate models depends on our understanding of how small scale clouds interact with the large scale environment and how aerosols interact with clouds. Therefore, during the 11th Plan Period, the Indian Institute of Tropical Meteorology (IITM) focused on cloud and aerosol interaction studies as a major research area.
A national experiment called Cloud Aerosol Interaction And Precipitation Enhancement Experiment (CAIPEEX) was launched to study cloud-aerosol interactions. Instrumented aircraft were used but, being an expensive proposition, the operations could not be continued for long. Mahabaleshwar located at a height of 1353 m in Western Ghats provides an ideal location for continuous monitoring and study of monsoon clouds and heavy rainfall events.
However, in order to obtain data on cloud microphysics over a wide range of meteorological conditions and distribution, it was important to continue taking observation for a reasonably long period. Setting up a cloud physics and aerosol measurements observatory at a high altitude station, would facilitate cloud-microphysics measurements along with aerosol and meteorological measurements over many years, and thereby help collect data spanning different conditions. An experimental infrastructure for uninterrupted observations and monitoring of critical atmospheric parameters at the high altitude station of Mahabaleshwar, Maharashtra with supporting infrastructure at the IITM, Pune was thus proposed. Both Mahabaleshwar and Pune were to have state-of-the-art instrumentation facilities for the purpose.
Objectives of HACPL
- To study the interaction of clouds with other environmental parameters through continuous observations of the clouds at high altitude using a complete range of observational systems;
- To study the interactions of clouds with aerosols using simultaneous airborne and high altitude surface measurements;
- To measure the water uptake (hygroscopicity) of aerosols and estimate the hygroscopic growth factor of aerosols which act as cloud condensation nuclei;
- To conduct remote sounding of cloud microphysics and vertical velocity (portable W-band cloud radar for cloud microphysics and vertical velocity from Doppler shift). Clouds are never near ground-level, save during the monsoons. Hence, they need to be probed through remote sensing;
- To document the chemical composition of aerosols and the mixing state for better characterisation of aerosols in numerical models; and,
- To share the observational data with other institutions and universities.
The cloud microphysical and dynamical processes within monsoon clouds and the role of aerosols in directly modifying these cloud characteristics through the scattering and absorption of radiation and indirectly by modifying the cloud macrophysical, microphysical and rainfall processes are little understood. Cloud processes introduce sub-grid scale processes that need to be parameterised. Various observations such as in-situ and remote sensing observations of clouds and their associated interactions with the environment are invaluable in designing parameterisation schemes or improving the representation of processes in numerical models. Aerosol and cloud processes are closely linked with atmospheric dynamics at various scales. The convective mixing between the lower and middle troposphere explains the major variance in climate sensitivity in several climate models.
The controls on cloud formation, organisation, and decay in monsoon convection is a multi-scale problem that needs a multi-faceted approach using satellite, ground-based, in-situ airborne observational efforts and laboratory and numerical studies to understand the multi-scale physics and dynamics of clouds. Integrated ground-based observations over an extended period of time at the HACPL can help understand the direct and indirect effect of aerosols on weather and climate, the impact of aerosols on the hydrological cycle, cloud micro scale and macro scale characteristics, the thermodynamic state of the atmosphere, among other issues.
Mahabaleshwar and HACPL
The rainfall over the Western Ghats is formed by forced lifting of moist air from the Arabian Sea. This forms raindrops at a low level, while the blocking effect imposed by the Western Ghats restricts the low level monsoon jet to transport moisture inland. A well known hill-station and pilgrim centre, Mahabaleshwar in the Western Ghats makes a unique location for HACPL, especially since it gets very heavy rainfall amounting to more than 561.7 cm, even as Mandhardevi, only 30 km away, gets hardly any.
This anomaly can help improve skills in predicting extreme rain events and droughts. The HACPL at Mahabaleshwar will be equipped with the best state-of-the-art equipment. A meteorological radar is also being set up at Mandhardevi to continuously monitor clouds and rain over the Mandhardevi-Mahabaleshwar area, to help understand the impact of orography on precipitation dynamics. The observations will also be used along with other cloud physics observations to establish the cloud and precipitation climatology of the region.
The Way Ahead
The HACPL in Mahabaleshwar is the first step in a process that needs to be followed through in other locations in India. There is an urgent requirement to have a high altitude site in southern India, at the corridor of the southwest monsoon. Munnar in Kerala, at nearly 2 km above mean sea level is one such location where a laboratory could be established. The northeast, which receives the heaviest rainfall in the world, also requires a similar extended cloud and precipitation studies facility, which can help better preparedness for nowcasts for that region.
The Western Himalayan region is another significant location for a HACPL. Greatly influenced by dust and biomass aerosols, the deep valleys here show significant stagnation, with cloud bases typically at 3-4 km due to significant pre-monsoon drying over this region. During monsoon, the clouds also contain haze particles. The propensity towards high rainfall events and floods and lightning over these regions need to be explored and a HACPL could provide detailed information to understand these processes.
The aerosol-cloud interaction and relationship with rainfall process in each of the regions mentioned are different and there are myriad types and sources of aerosols that play a role in each. It would be an important step in improving models for nowcasting, which , given our limited knowledge on the physics and dynamics of clouds, is presently fraught with a lot of uncertainty.