A comprehensive statistical analysis of how an index of the Southern Oscillation changed from 1882 to 1995 was given by Trenberth and Hoar [1996], with a focus on the unusual nature of the 1990–1995 El Nino-Southern Oscillation (ENSO) warm event in the context of an observed trend for more El Nino and fewer La Nina events after the late 1970s. The conclusions of that study have been challenged by two studies which deal with only the part of our results pertaining to the length of runs of anomalies of one sign in the Southern Oscillation Index. They therefore neglect the essence of Trenberth and Hoar, which focussed on the magnitude of anomalies for certain periods and showed that anomalies during both the post-1976 and 1990-mid-1995 periods were highly unlikely given the previous record. With updated data through mid 1997, we have performed additional tests using a regression model with autoregressive-moving average (ARMA) errors that simultaneously estimates the appropriate ARMA model to fit the data and assesses the statistical significance of how unusual the two periods of interest are. The mean SOI for the post-1976 period is statistically different from the overall mean at <0.05% and so is the 1990-mid-1995 period. The recent evolution of ENSO, with a major new El Nino event underway in 1997, reinforces the evidence that the tendency for more El Nino and fewer La Nina events since the late 1970s is highly unusual and very unlikely to be accounted for solely by natural variability.

El Nino and climate change

Assessment of Climate Change over the Indian Region : A Report of the Ministry of Earth Sciences (MoES), Government of India

We measured carbonaceous material and water-soluble ionic species in the fine fraction (D p < 1.3 μm) of aerosol samples collected on NCAR's C-130 aircraft during the intensive field phase (February-March 1999) of the Indian Ocean Experiment (INDOEX). Polluted layers were present over most of the study region north of the equator at altitudes up to 3.2 km. The estimated aerosol mass (sum of carbonaceous and soluble ionic aerosol components) of fine-mode particles in these layers was 15.3 ± 7.9 μg m -3 . The major components were particulate organic matter (POM, 35%), SO 2- 4 (34%), black carbon (BC, 14%), and NH4 (11%). The main difference between the composition of the marine boundary layer (MBL, 0 to ∼1.2 km), and the overlying residual continental boundary layer (1.2 to ∼3.2 km) was a higher abundance of SO 2- 4 relative to POM in the MBL, probably due to a faster conversion of SO 2 into SO 2- 4 in the MBL. Our results show that carbon is a major, and sometimes dominant, contributor to the aerosol mass and that its contribution increases with altitude. Low variability was observed in the optical properties of the aerosol in the two layers. Regression analysis of the absorption coefficient at 565 nm on BC mass (BC < 4.0 μg C m -3 ) yielded a specific absorption cross section of 8.1 ± 0.7 m 2 g -1 for the whole period. The unusually high fraction of BC and the good correlation between the absorption coefficient and BC suggest that BC was responsible for the strong light absorption observed for the polluted layers during INDOEX. High correlation between BC and total carbon (TC) (r 2 = 0.86) suggest that TC is predominantly of primary origin. Good correlations were also found between the scattering coefficient at 550 nm and the estimated aerosol mass for the fine fraction. These yielded a specific scattering cross section of 4.9 ± 0.4 m 2 g -1 . The observed BC/TC, BC/OC, SO 2- 4 /BC, and K + /BC ratios were fairly constant throughout the period. These ratios suggest that between 60 and 80% of the aerosol in the polluted layers during INDOEX originated from fossil fuel and between 20 and 40% from biofuel combustion.

Carbonaceous aerosols over the Indian Ocean during the Indian Ocean Experiment (INDOEX): Chemical characterization, optical properties, and probable sources : Indian Ocean Experiment (INDOEX)

[1] Coalescence of cloud droplets is essential for the production of small raindrops at a given vertical distance above the cloud base (Dp). The rate of droplet coalescence is determined mainly by droplet size, spectrum width and concentrations. The droplet condensational growth is determined by the number of activated CCN (Na) and height above cloud base. Here we show that when the droplet mean volume radius, rv, exceeds ∼13 μm, or when droplet effective radius (re) exceeds ∼14 μm, considerable precipitation mass (>0.03 g kg−1) is likely to be present in growing convective clouds. This is because the rate of droplet coalescence is proportional to ∼rv5 which practically implies the existence of a threshold rv above which efficient warm rain formation can occur, and also because the vertical profile of rv, even in diluted clouds, nearly follows the theoretical adiabatic condensational growth curve. The small observed deviations are mainly caused by deviations from purely inhomogeneous mixing which cause partial droplet evaporation. Consequently, Dp must theoretically change nearly linearly with Na. This is confirmed here observationally, where increasing Na by 100 per milligram (≈cm3 at cloud base) of air, resulted in an increase of ∼280 m in Dpfor both Israeli and Indian deep convective clouds. This means that in highly polluted clouds or where strong cloud-base updrafts occur, clouds have to grow well above the freezing level, even in tropical atmosphere, before precipitation forms either by warm or by mixed-phase processes.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2011JD016457

Linear relation between convective cloud drop number concentration and depth for rain initiation

[1] Aircraft measurements of cloud condensation nuclei (CCN) and microphysics of clouds at various altitudes were conducted over India during CAIPEEX (Cloud Aerosol Interaction and Precipitation Enhancement Experiment) phase I and II in 2009 and 2010 respectively. As expected, greater CCN concentrations gave rise to clouds with smaller drops with greater number concentrations (Nc). The cloud drop effective radius (re) increased with distance above cloud base (D). Warm rain became detectable, i.e., rain water content >0.01 g/Kg, at the tops of growing convective clouds when re exceeded 12 μm. The re is determined by the number of activated CCN, Nad, and D. The Nad can be approximated by the maximum measured values of Nc. Higher Nc resulted in greater D for reaching the re threshold for onset of warm rain, re*, denoted as D*. In extreme cases of highly polluted and moist air that formed the monsoon clouds over the Indo-Gangetic plains, D* exceeded 6 km, well above the 0°C isotherm level. The precipitation particles were initiated there as supercooled raindrops at a temperature of −8°C. Giant CCN reduced re* and D*, by initiating raindrops closer to cloud base. This effect was found mainly in dusty air masses over the Arabian Sea. Besides, the aerosol effect on D*, D* was found to decrease with increase in cloud water path.

/pdf/aerosol-control-on-depth-of-warm-rain-in-convective-clouds-ocd8johkh3.pdf

Aerosol control on depth of warm rain in convective clouds

While the demand for enhancing rainfall through cloud seeding is strong and persistent in the country, considerable uncertainty exists on the success of such an endeavour at a given location To understand the pathways of aerosol-cloud interaction through which this might be achieved, a national experiment named Cloud Aerosol Interaction and Precipitation Enhancement EXperiment (CAIPEEX) in two phases, was carried out The rationale of CAIPEEX, the strategy for conducting the experiment, data quality and potential for path-breaking science are described in this article Pending completion of quality control and calibration of the CAIPEEX phase-II data, here we present some initial results of CAIPEEX phase-I aimed at documenting the prevailing microphysical characteristics of aerosols and clouds and associated environmental conditions over different regions of the country and under different monsoon conditions with the help of an instrumented research aircraft First-time simultaneous observations of aerosol, cloud condensation nuclei (CCN) and cloud droplet number concentration (CDNC) over the Ganges Valley during monsoon season show very high concentrations (> 1000 cm-3) of CCN at elevated layers Observations of elevated layers with high aerosol concentration over the Gangetic valley extending up to 6 km and relatively less aerosol concentration in the boundary layer are also documented We also present evidence of strong cloud- aerosol interaction in the moist environments with an increase in the cloud droplet effective radius Our observations also show that pollution increases CDNC and the warm rain depth, and delays its initiation The critical effective radius for warm rain initiation is found to be between 10 and 12 I¼m in the polluted clouds and it is between 12 and 14 I¼m in cleaner monsoon clouds

The cloud aerosol interaction and precipitation enhancement experiment (CAIPEEX): Overview and preliminary results

The timing of the dramatic 'onset' and subsequent northward progression of the Indian summer monsoon (ISM) are unique features of the monsoon annual cycle and are strongly linked to planning of agricultural activity and water management. Here, we show that the northward propagation of the first episode of intraseasonal oscillation (ISO) associated with the onset phase of ISM has slowed down significantly during the past decade compared with prior decades. The slowdown is due to systematic weakening of two regional climatic features responsible for northward progression of the tropical convergence zone (TCZ) namely, the vertical easterly wind shear and the north-south gradient of the mean low-level humidity around the equator. Eastward shift of the Walker circulation associated with increasing strength of El Ninos appears to be responsible for weakening of the vertical wind shear over Indian monsoon region. The slow northward propagation during recent decades leads to a climatological sub-seasonal oscillation representing a predictable component of the onset phase of the ISM.

On factors responsible for recent secular trend in the onset phase of monsoon intraseasonal oscillations

1. The effects of mid-latitudinal circulations on the activity of the monsoon have been studied by many workers. Malurkar (1950) showed that southward extension of extra-tropical westerly trough into northern India leads to break in the Indian monsoon. Pisharoty and Desai (1956) and Mooley (1957) concluded that passage of western disturbances (which are the remnants of midlatitude systems) in quick succession lead to break in monsoon. Chakravarthy and Basu (1957) compared the number of western disturbance days and eastern disturbance days and found that greater the number of western disturbance days lesser the number of eastern disturbance days. Changraney (1966) showed that 31 out of 52 cases of heavy rainfall in the northern river catchments occurred during the presence and movement of westerly waves. Bedi et al. (1981) observed that systems in the westerlies are more active in the weak monsoon situations. Ramaswamy (1958 and 1962) used global data and showed that during active monsoon period middle latitude systems are confined to northern middle latitudes only and in the break monsoon, they penetrate into the northern Indian latitudes and thereby replace easterlies over the north Indian plains by westerlies. Unninayar and Murakami (1978) found that during weak monsoon situation anticyclonic circulation (at 200 hPa) over Himalaya bifurcates into two cells due to penetration of the mid latitude trough into sub-tropics. At 700 hPa they found that easterlies around 20° N and 30° N over India are replaced by westerlies and north-westerlies intensified in the Arabian Sea. Sikka and Grossman (1981) found that a large amplitude trough in the regime of subtropical westerlies was present during the break period of monsoon 1979. Raman and Rao (1981) are of the views that mid latitude trough have effect of pulling the monsoon trough further north. Unninayar and Murakami (1978) studied wave number domain and found that during abnormal period strong mid latitude trough penetrates deep into the monsoon area. Bawiskar et al. (2002) showed that short waves over extra tropics are significantly active during onset and withdrawal phases and weak during established phase.

https://mausamjournal.imd.gov.in/index.php/MAUSAM/article/download/998/840

V. R. Mujumdar

Papers

The cloud aerosol interaction and precipitation enhancement experiment (CAIPEEX): Overview and preliminary results

On factors responsible for recent secular trend in the onset phase of monsoon intraseasonal oscillations

Kinetic energy of extra tropical waves and their effect on the indian monsoon rainfall