Year 2005 beheld unexpected very high rainfall in Bombay (07/26/2005, rainfall since past 03GMT:94 cm - world record), Chennai and Bangalore in India on a particular day of season. It doesn't mean past didn't see extremities in rainfall or Cherrapunji and other stations in India regularly receives 20-35 cm rainfall on a particular day in each Monsoon and it occurs several times in the season. 2006 has witnessed high rainfall of 64 cm or more on a day in Ratnagiri, Mahabaleshwar and Bombay. These are increased rainfall and are now frequent in industrialized region of India where moisture content of the atmosphere during Monsoon is copious.
An analysis, approbated by past works and concerns as well as meteorology, justifies changes in the atmospheric composition, modification of surface characteristics, etc. which has consequences of alteration, frequent deviation, extremities in pattern of surface temperature, precipitation and other climate elements. Anthropogenic activities in last 20-30 years have destroyed the natural pattern of the climate and introduced varied aspects of meteorology like Urban Heat Islands(UHI), Mixing Height (MH), abnormal temperature inversion, convergence of particulate matters and concentration of aerosol especially over cities, and above all green house effects and global warming.
Pollutants/particulate matters, commonly known as “aerosol”, are cloud condensation nuclei (CCN) and their presence gives substrate to supersaturated water vapors where formation of water droplets takes place. Since the nature also modifies the disturbances induced into it, on an average aerosol are removed from the atmosphere at about the same rate as they enter there. On the global scale, precipitation process account for about 80-90% of the mass of aerosol removed from the atmosphere. Increased rainfall or increasing trend in precipitation is just one new meteorological behavior which is direct upshot of unwanted emission and concentration of aerosols (particulate matters).
In nucleation event, sufficient nuclei trigger precipitation by creating ice crystals in supercooled clouds. Induced ice crystals are presumed to grow at the expense of water droplets and fall as precipitation. Warm tropical clouds, for example stratus, is thermodynamically stable where droplets of varying sizes exist together, their different rates of movement may lead to collision and coalescence. Large droplets may divide upon collision, creating several drops which set off a chain reaction of rain drop growth. Claims of changes in precipitation by cloud-seeding vary widely but precipitation enhancement on the order of 10 to 20 percent has been reported while in natural events, process is assumed to be quite accurate and efficient with much better results for increased precipitation.
Here, we can talk of potential precipitation (PP) on a particular day of a place, similar to potential evaporation (PE). This PP depends on concentration of aerosol in the atmosphere while PE depends on thermal efficiency of the place. Given the amount of water vapor, a place will have its actual precipitation which on a day can be equal to potential precipitation on a day. So, Bombay, Chennai and Bangalore unlike many other dry days or days of low rainfall, received potential precipitation which most probably be revised and new, on a particular day.
Similar excessive downpour we expect in Calcutta in nearby future but chances are less notwithstanding because atmospheric particulate matters are regularly washed out in each season. But in contrast, New Delhi is categorically stands off from meeting such extremities since it seriously lacks presence of sufficient amount of water vapor or denied of high moisture content and its accumulation due to New Delhi's geographical features. Aerosol are not wiped out exactly as opposed to their entry in the atmosphere of the city.As droplet size cannot grow in the absence of sufficient moisture content, comparatively high relative humidity during Monsoon especially in the atmosphere allows rapid oxidation and conversion of urban pollutants to acids. Mechanism for acid rain has been set in the atmosphere of the city and gradually chances of acid precipitation is increasing in coming years over an urban and industrialized city like New Delhi.
Sufficient data, much works, meteorology and a track of information precisely conceive extremities of similar kind, hazardous and mortal,over regions of industrialization and urbanization within a short period of coming years and high tech cities of rest of the world are not in exemption rather they are running fast to be first to weather mortality.
However, there are several questions unanswered here:
1) Why does such high rainfall on a day occur on a special day at a particular region of restricted area...?
: We expect UHI triggering convective activities in significant proportion which are chiefly localized events. City heat warms the surface air making it more unstable and forming convective clouds and thunderstorms. In Indian conditions, during monsoon, nucleation events result in large scale seeding and ample moisture lets growth of water droplets through coalescence and collision process in huge proportion.
2) Can we predict the amount of rainfall on a particular day with the knowledge of CCN?
: Definitely the paper has its own limitations and requires extensive work, resources, time and continuous observations to know the participation of those CCN. Current science and works are not enough to tell such story and predict accurate figure.
3)Can we predict acid rain bound to occur in many industrialized cities?
: The important aspect of the study is where, when and in what proportion... And do we expect it to set as a climatic pattern of an atmosphere...?
4)What is potential precipitation?
: This new terminology requires to be expressed with sufficient mathematics in relation with requisite parameters and conditions. The behavior of this factor is almost expected to be similar to that of PE?