The role of low-frequency intraseasonal oscillations in the anomalous Indian summer monsoon rainfall of 2002

We analyze the dynamical features and responsible factors of the low-frequency intraseasonal time scales which influenced the nature of onset, intensity and duration of active/break phases and withdrawal of the monsoon during the anomalous Indian summer monsoon of 2002 - the most severe drought recorded in recent times. During that season, persistent warm sea surface temperature anomalies over the equatorial Indian Ocean played a significant role in modulating the strength of the monsoon Hadley circulation. This in turn affected the onset and intense break spells especially the long break during the peak monsoon month of July. Strong low-frequency intraseasonal modulations with significant impact on the onset and active/break phases occurred in 2002 which were manifested as a good association between low-frequency intraseasonal oscillations and the onset and active/break spells. Further, SST anomalies over the equatorial Indo-Pacific region on low-frequency intraseasonal time scales were found to affect the equatorial eastward and thereby off-equatorial northward propagations of enhanced convection over the Indian region. These propagations in turn modulated the active/break cycle deciding the consequent severity of the 2002 drought

Characteristics of spectral aerosol optical depths over India during ICARB

Spectral aerosol optical depth (AOD) measurements, carried out regularly from a network of observatories spread over the Indian mainland and adjoining islands in the Bay of Bengal and Arabian Sea, are used to examine the spatio-temporal and spectral variations during the period of ICARB (March to May 2006). The AODs and the derived Angstrom parameters showed considerable variations across India during the above period. While at the southern peninsular stations the AODs decreased towards May after a peak in April, in the north Indian regions they increased continuously from March to May. The Angstrom coefficients suggested enhanced coarse mode loading in the north Indian regions, compared to southern India. Nevertheless, as months progressed from March to May, the dominance of coarse mode aerosols increased in the columnar aerosol size spectrum over the entire Indian mainland, maintaining the regional distinctiveness. Compared to the above, the island stations showed considerably low AODs, so too the northeastern station Dibrugarh, indicating the prevalence of cleaner environment. Long-range transport of aerosols from the adjoining regions leads to remarkable changes in the magnitude of the AODs and their wavelength dependencies during March to May. HYSPLIT back-trajectory analysis shows that enhanced long-range transport of aerosols, particularly from the west Asia and northwest coastal India, contributed significantly to the enhancement of AOD and in the flattening of the spectra over entire regions; if it is the peninsular regions and the island Minicoy are more impacted in April, the north Indian regions including the Indo Gangetic Plain get affected the most during May, with the AODs soaring as high as 1.0 at 500 nm. Over the islands, the Angstrom exponent (α) remained significantly lower (~1) over the Arabian Sea compared to Bay of Bengal (BoB) (~1.4) as revealed by the data respectively from Minicoy and Port Blair. Occurrences of higher values of α, showing dominance of accumulation mode aerosols, over BoB are associated well with the advection, above the boundary layer, of fine particles from the east Asian region during March and April. The change in the airmass to marine in May results in a rapid decrease in α over the BoB

Aerosol microphysics over a tropical coastal station inferred from the spectral dependence of Angstrom wavelength exponent and inversion of spectral aerosol optical depths

Six years of spectral aerosol optical depth (AOD) measurements have been analyzed from a tropical coastal site, Trivandrum (8.55° N, 76.9°E, 3 m msl) to infer on the seasonal changes in the microphysical properties of columnar aerosols, by examining the derivatives of the Angstrom wavelength exponent (α) in the wavelength domain (α'λ) as well as in AOD domain (α'τ) and by retrieving the columnar size distribution by numerical inversion of the AODs. The inference of the changes in the aerosol microphysics drawn from the features of the derivatives α'λ and α'τ is consistent with the pattern revealed by the aerosol properties obtained from the columnar size distributions retrieved from the AOD spectra as well as from the surface measurements of mass-size distributions, which are supported by the back-trajectory cluster analysis and the results of chemical species analysis

Indian summer monsoon rainfall simulation by an AGCM ensemble system

Ensemble simulation of Indian summer monsoon rainfall (ISMR) and its interannual variation is investigated using Meteorological Research Institute atmospheric general circulation model. Representation of climatological seasonal variation of rainfall is found to be important for a realistic simulation of seasonal mean and interannual variation of ISMR by the ensemble mean. As a result, the ensemble mean captures the extreme ISMR events associated with pronounced large-scale equatorial sea surface temperature (SST) fluctuations. Further, the systematic bias in the simulation of mean seasonal variation of rainfall over the Asia-Pacific region is removed, based on multiple linear regression of daily rainfall from the member simulations against observation. This results in improved simulation of ISMR in the bias-removed ensemble mean which is illustrated by hindcasting the anomalous ISMR of 2002, the most severe drought recorded in recent times. The removal of bias in the mean seasonal variation of rainfall brings out the realistic precipitation response in the model to fluctuations in SST boundary forcing. As a result, the bias-removed ensemble mean captures not only the seasonal anomaly of 2002 ISMR but improves the subseasonal variation of ISMR as well

Multi-year investigations of aerosols from an island station, Port Blair, in the Bay of Bengal: climatology and source impacts

Long-term measurements of spectral aerosol optical depth (AOD) using multi-wavelength solar radiometer (MWR) for a period of seven years (from 2002 to 2008) from the island location, Port Blair (11.63° N, 92.7° E, PBR) in the Bay of Bengal (BoB), along with the concurrent measurements of the size distribution of near-surface aerosols, have been analyzed to delineate the climatological features of aerosols over eastern BoB. In order to identity the contribution of different aerosol types from distinct sources, concentration weighted trajectory (CWT) analysis has been employed. Climatologically, AODs increase from January to reach peak value of ~0.4 (at 500 nm) in March, followed by a weak decrease towards May. Over this general pattern, significant modulations of intra-seasonal time scales, caused by the changes in the relative strength of distinctively different sources, are noticed. The derivative (α') of the Angstrom wavelength exponent α in the wavelength domain, along with CWT analysis, are used to delineate the different important aerosol types that influence this remote island. Corresponding changes in the aerosol size distributions are inferred from the numerical inversion of the spectral AODs as well from (surface) measurements. The analyses revealed that advection plays a major role in modifying the aerosol properties over the remote island location, the potential sources contributing to the accumulation mode (coarse mode) aerosols over eastern BoB being the East Asia and South China regions (Indian mainland and the oceanic regions)

Simulating aerosols over Arabian Peninsula with CHIMERE : Sensitivity to soil, surface parameters and anthropogenic emission inventories

A three dimensional chemistry transport model, CHIMERE, was used to simulate the aerosol optical depths (AOD) over the Arabian Peninsula desert with an offline coupling of Weather Research and Forecasting (WRF) model. The simulations were undertaken with: (i) different horizontal and vertical configurations, (ii) new datasets derived for soil/surface properties, and (iii) EDGAR-HTAP anthropogenic emissions inventories. The model performance evaluations were assessed: (i) qualitatively using MODIS (Moderate-Resolution Imaging Spectroradiometer) deep blue (DB) AOD data for the two local dust events of August 6th and 23rd (2013), and (ii) quantitatively using AERONET (Aerosol Robotic Network) AOD observations, CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) aerosol extinction profiles, and AOD simulations from various forecast models. The model results were observed to be highly sensitive to erodibility and aerodynamic surface roughness length. The use of new datasets on soil erodibility, derived from the MODIS reflectance, and aerodynamic surface roughness length (z0), derived from the ERA-Interim datasets, significantly improved the simulation results. Simulations with the global EDGAR-HTAP anthropogenic emission inventories brought the simulated AOD values closer to the observations. Performance testing of the adapted model for the Arabian Peninsula domain with improved datasets showed good agreement between AERONET AOD measurements and CHIMERE simulations, where the correlation coefficient (R) is 0.6. Higher values of the correlation coefficients and slopes were observed for the dusty periods compared to the non-dusty periods

Impact of a mountain grassland fire on the concentration of aerosol black carbon and carbon monoxide near the surface at a remote coastal location

A case study is presented on the impact of an extensive grassland fire, at a high altitude location in Western Ghats, on the aerosol black carbon (BC) mass concentration (MB) and concentration (CO) of carbon monoxide measured at two remote, coastal, nearby locations. Observations revealed a substantial increase shortly after the outbreak of the fire at a high altitude location ( ~20 km away) in the concentrations of both species; while a two-fold increase occurred in the mass concentration of BC; the increase in CO though was less pronounced (by a factor of ~1.5). It is suggested that the sea-breeze circulation modified by the prevailing winds caused the transport of pollutants from the source region downwind to the coastal location. The impact persisted for one day after which it became insignificant. During the first maximum of these, the wind conditions were such that the pollutants emitted from the fire were transported offshore with the return flow of the sea breeze, they then sank along with the airmass and were again transported back to the coast via the sea breeze. During the second maximum (after sunset), the particles as well as the trace gases along with the airmass came directly to the regions of fire to the valley via katabatic flow and were again transported to the coast via the land breeze. This added with the nighttime confinement of pollutants near the surface caused the enhancement in nocturnal peak and it prevailed high until next day morning unlike normal days

Performance evaluation of chemistry transport models over India

Using continuous and near-real time measurements of the mass concentrations of black carbon (BC) aerosols near the surface, for a period of 1 year (from January to December 2006) from a network of eight observatories spread over different environments of India, a space-time synthesis is generated. The strong seasonal variations observed, with a winter high and summer low, are attributed to the combined effects of changes in synoptic air mass types, modulated strongly by the atmospheric boundary layer dynamics. Spatial distribution shows much higher BC concentration over the Indo-Gangetic Plain (IGP) than the peninsular Indian stations. These were examined against the simulations using two chemical transport models, GOCART (Goddard Global Ozone Chemistry Aerosol Radiation and Transport) and CHIMERE for the first time over Indian region. Both the model simulations significantly deviated from the measurements at all the stations; more so during the winter and pre-monsoon seasons and over mega cities. However, the CHIMERE model simulations show better agreement compared with the measurements. Notwithstanding this, both the models captured the temporal variations; at seasonal and subseasonal timescales and the natural variabilities (intra-seasonal oscillations) fairly well, especially at the off-equatorial stations. It is hypothesized that an improvement in the atmospheric boundary layer (ABL) parameterization scheme for tropical environment might lead to better results with GOCART

Quasi-biennial oscillations in spectral aerosol optical depth

Analysis of long-term time series of monthly mean aerosol optical depths (AOD) at four tropical stations over Asia and Africa revealed the presence of significant annual oscillations (AO) and quasi-biennial oscillations (QBO). While the AOs were the dominant features, the QBOs in AOD (QBOAOD) were also quite strong and were well associated with the QBO in stratospheric zonal wind (QBOU). At the equatorial stations, QBOAOD were out of phase with QBOU, while they were in phase at the off-equatorial stations. However, QBO in both outgoing longwave radiation (OLR) and rainfall showed an out-of-phase relationship with QBOAOD at all stations. Copyright © 2009 Royal Meteorological Society