K. Parameswaran

Wintertime spatial characteristics of boundary layer aerosols over peninsular India

During an intense field campaign for generating a spatial composite of aerosol characteristics over peninsular India, collocated measurements of the mass concentration and size distribution of near-surface aerosols were made onboard instrumented vehicles along the road network during the dry, winter season (February-March) of 2004. The study regions covered coastal, industrial, urban, village, remote, semiarid, and vegetated forestlands. The results showed (1) comparatively high aerosol (mass) concentrations (exceeding 50 μ g m(-3)), in general, along the coastal regions (east and west) and adjacent to urban locations, and (2) reduced mass concentration ( 50% of the total) of coarse-mode aerosols (>1 μ m). The spatial composite of accumulation-mode share to the total aerosol mass concentration agreed very well with the monthly mean spatial composite of aerosol fine-mode fraction for February 2004, deduced from Moderate-Resolution Imaging Spectroradiometer data for the study region, while a point by point comparison yielded a linear association with a slope of 1.09 and correlation coefficient of 0.79 for 76 independent data pairs. Pockets of enhanced aerosol concentration were observed around the industrialized and urban centers along the coast as well as inland. Aerosol size distributions were parameterized using a power law. Spatial variation of the retrieved aerosol size index shows relatively high values (>4) along the coast compared to interior continental regions except at a few locations. Urban locations showed steeper size spectra than the remote locations.

Temporal variations in surface ozone at Thumba (8.6°N, 77°E)-a tropical coastal site in India

Abstract Surface measurements of ozone and meteorological parameters are made at a tropical coastal site, Thumba (8.6°N, 77°E, 2xa0m) in India from April 1997 to March 1998. Ozone shows a diurnal variation with daytime higher levels and a sharp change in its values during evening time. The evening time change in ozone values with a secondary peak is found to be due to change in wind pattern from sea-breeze to land-breeze at this site. This secondary peak in ozone is weakest during monsoon period. A detailed study of the meteorological parameters shows that during nighttime, polluted air from land side moves to the nearby marine region relatively increasing the levels of ozone and precursor gases. Observations show that the onset time of daytime ozone increase and its rates are related with each other. If onset time of ozone increase is early, its increase rate is slower and vice versa. Maximum ozone levels are observed to be during March, probably due to intense photochemical production. However, this is different when compared to other Indian site like Ahmedabad, where maximum ozone levels are observed during late autumn and early winter. Monthly average ozone levels are observed to be very low (in the range of 13–22xa0ppbv) at Thumba.

160-Gb/s OTDM transmission using integrated all-optical MUX/DEMUX with all-channel modulation and demultiplexing

This letter provides the first report of 160-Gb/s optical time-division-multiplexed transmission with all-channel independent modulation and all-channel simultaneous demultiplexing. By using a multiplexer and a demultiplexer based on periodically poled lithium niobate and semiconductor optical amplifier hybrid integrated planar lightwave circuits, 160-km transmission is successfully demonstrated.

All-optical decrementing of a packet's time-to-live (TTL) field and subsequent dropping of a zero-TTL packet

We demonstrate an optical time-to-live (TTL) decrementing module for optical packet-switched networks. Our module acts on a standard NRZ-modulated binary TTL field within a 10 Gb/s packet and decrements it by one if the TTL is nonzero. If the TTL of the incoming packet is zero, the module signals an optical switch to drop the packet. Our technique is independent of the TTL length, does not require the use of ultrashort RZ optical pulses, requires no guard time between the end of the TTL field and the packet data, and has only a 2.4 dB power penalty at 10/sup -9/ bit-error rate.

Altitude profiles of temperature from 4 to 80 km over the tropics from MST radar and lidar

Abstract Using ground-based techniques of MST radar and Lidar, temperature profiles in the entire height range of 4 to 75xa0km are obtained for the first time at a tropical location. The temporal resolution of the profiles is ∼1 h in the lower altitudes and 12.5xa0min in the higher altitudes and altitude resolution is ∼300 m. The errors involved in the derived values are presented. Preliminary analysis of temperature variations in a night revealed fluctuations with characteristics resembling those of large-scale gravity waves.

Regional distribution of deep clouds and cloud top altitudes over the Indian subcontinent and the surrounding oceans

[1]xa0The Bay of Bengal (BoB) and the east equatorial Indian Ocean are among the most intense deep convective regions over the tropics. Long-term monthly mean distributions of deep clouds and the regional differences in the cloud top brightness temperature (CTBT) over the Indian subcontinent and the surrounding oceanic regions (the northern and eastern BoB, the southeast Arabian Sea, and the eastern equatorial Indian Ocean) are derived using National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer data for a period of 10 years and are compared with the direct observations of cloud top altitude (CTA) using Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO). The deepest clouds are found to occur over the northern BoB (NBoB) during the June–August period. The most probable CTBT in this region is ∼12 K lower than that over the other deep convective regions. CALIPSO observations also show that the frequency of occurrence of CTA peaks at an altitude of ∼16.5 km over the NBoB, which is ∼1 km higher than that over the other regions. Strong convergence of horizontal wind between the surface and ∼200 hpa level and large divergence above, combined with the most favorable sea surface temperature (>28°C), might be mainly responsible for the highest CTA being observed over the NBoB. The annual variation of the base altitude and thickness of the tropical tropopause layer (TTL) and the upper tropospheric temperature are closely associated with the corresponding variations in the deep cloud fraction. Considerable fraction of deep clouds occurs within the TTL over the BoB during the June–August period.

Temperature dependence of tropical cirrus properties and radiative effects

[1]xa0The temperature dependencies of cirrus properties are studied using a dual polarization lidar and Mesosphere Stratosphere Troposphere (MST) radar at the tropical station Gadanki (13.5°N, 79.2°E). Cirrus clouds are generally observed in the altitude region 10 to 18 km, with midcloud temperature in the range −85° to −40°C. The cloud temperature decreases with increase in cloud altitude as expected. The mean cloud thickness is generally in the range 0.7 to 1.7 km. For temperatures in the range −75° to −50°C the cloud thickness is ∼1.7 km and shows a tendency to decrease at lower temperatures. The linear depolarization ratio (LDR) within the cloud shows a small increase with decrease in temperature. The cloud extinction and optical depth increases with increase in temperature. The temperature dependence of cirrus extinction/optical depth has been parameterized using different analytical forms such as exponential, linear, and polynomial, which shows that a second-order polynomial function is well suited for describing the temperature dependence of extinction coefficient/optical depth of tropical cirrus. The climate sensitivity factor derived based on the empirical relations shows an increase with decrease in cloud optical depth. The present study, however, indicates that the cirrus becomes radiatively significant when its optical depth exceeds a threshold value of 0.03.

Lidar observations of cirrus cloud near the tropical tropopause: general features

Abstract General features of high-altitude cirrus clouds observed below the tropical tropopause are studied using lidar at Gadanki (13.5°N, 79.2°E). These clouds occur quite frequently over this tropical region. Thin clouds with optical depth 0.3. The preferred altitude of these clouds is between 14 and 16 km, with a vertical extent ranging from 0.4 to 4 km. The backscattered lidar signal from the thin cirrus clouds exhibits significant depolarisation, indicating the presence of randomly oriented nonspherical ice crystals in abundance.

Identification of tropical convective tropopause and its association with cold point tropopause

[1]xa0Using the MST radar observations at Gadanki (13.5°N, 79.2°E), a tropical station, the altitude of major convective outflow in the troposphere is identified and is considered to represent the convective tropopause. This is found to match well with the altitude of local minimum of potential temperature lapse rate obtained from simultaneous radiosonde observations. The convective tropopause altitudes are also compared with the cloud top altitudes obtained using satellite brightness temperature (BT) data and are found to match in the case of deep convection. The thickness of the tropical tropopause layer follows very closely the convective tropopause altitude and has little dependence on the cold point tropopause altitude. The thickness of the tropopause layer is found to shrink when convection reaches high altitudes. This occurs mainly during the monsoon months of July, September, and October.

Wintertime regional aerosol distribution and the influence of continental transport over the Indian Ocean

Abstract During the Asian winter monsoon period, the Indian Ocean region is affected by large scale transport of aerosols from the dry Asian continent, which can influence the radiation budget of the region. The regional distribution of aerosols over the Indian Ocean and the influence of long-range transport during the Asian dry period of November–April of 1996–1999 are studied, based on satellite derived aerosol optical depth (AOD) and the wind data obtained from the NCEP-NCAR reanalysis. AOD over the Indian Ocean region is observed to be significantly influenced by the transport of aerosols from Indian subcontinent, Southeast Asia and Arabia. AOD as well as its latitude gradient in the Northern Hemisphere generally increase from November to April. High aerosol loading observed over the Indian Ocean during February–April period of 1999 is closely associated with the changes in atmospheric circulation features and transport from the Asian continent.