Mallik Sezan Mahmud

Effect of Snow Salinity on CryoSat‐2 Arctic First‐Year Sea Ice Freeboard Measurements

The European Space Agencys CryoSat-2 satellite mission provides radar altimeter data that are used to derive estimates of sea ice thickness and volume. These data are crucial to understanding recent variability and changes in Arctic sea ice. Sea ice thickness retrievals at the CryoSat-2 frequency require accurate measurements of sea ice freeboard, assumed to be attainable when the main radar scattering horizon is at the snow/sea ice interface. Using an extensive snow thermophysical property dataset from late winter conditions in the Canadian Arctic, we examine the role of saline snow on first-year sea ice (FYI), with respect to its effect on the location of the main radar scattering horizon, its ability to decrease radar penetration depth, and its impact on FYI thickness estimates. Based on the dielectric properties of saline snow commonly found on FYI, we quantify the vertical shift in the main scattering horizon. This is found to be approximately 0.07 m. We propose a thickness-dependent snow salinity correction factor for FYI freeboard estimates. This significantly reduces CryoSat-2 FYI retrieval error. Relative error reductions of ~ 11% are found for an an ice thickness of 0.95 m and ~ 25% for 0.7 m. Our method also helps to close the uncertainty gap between SMOS and CryoSat-2 thin ice thickness retrievals. Our results indicate that snow salinity should be considered for FYI freeboard estimates.

Multi-frequency microwave backscatter indices from saline snow covers on smooth first-year sea ice

This study inter-compares observed Ku-, X- and C-band microwave backscatter from saline 14 cm, 8 cm, and 4 cm snow covers on smooth first-year sea ice. A surface-borne multi-frequency (Ku-, X- and C-bands) polarimetric microwave scatterometer system is used near-coincident with in situ snow geophysical measurements. The study investigated differences in scatterometer observations for all three frequencies, co-pol ratios, and introduced new dual-frequency ratios to discriminate dominant polarization-dependent frequencies from these snow covers. Preliminary results suggest that, thinnest 4 cm snow cover demonstrate greatest increase in microwave backscatter from all three frequencies, followed by backscatter from thicker 8 cm and 14 cm snow covers. Dual-frequency indices derived for all frequency and polarization combinations suggest greater sensitivity of Ku-band microwaves to snow grain microstructure with increasing snow thicknesses, X-band microwaves to changes in snow salinities with decreasing snow thicknesses. Our results indicate the effect of dielectric loss associated with high salinities throughout all layers of the three snow covers, as the dominant factor affecting microwave penetration and backscatter from all three frequencies.

Ku-, X- and C-Band Microwave Backscatter Indices from Saline Snow Covers on Arctic First-Year Sea Ice

In this study, we inter-compared observed Ku-, X- and C-band microwave backscatter from saline 14 cm, 8 cm, and 4 cm snow covers on smooth first-year sea ice. A Ku-, X- and C-band surface-borne polarimetric microwave scatterometer system was used to measure fully-polarimetric backscatter from the three snow covers, near-coincident with corresponding in situ snow thermophysical measurements. The study investigated differences in co-polarized backscatter observations from the scatterometer system for all three frequencies, modeled penetration depths, utilized co-pol ratios, and introduced dual-frequency ratios to discriminate dominant polarization-dependent frequencies from these snow covers. Results demonstrate that the measured co-polarized backscatter magnitude increased with decreasing snow thickness for all three frequencies, owing to stronger gradients in snow salinity within thinner snow covers. The innovative dual-frequency ratios suggest greater sensitivity of Ku-band microwaves to snow grain size as snow thickness increases and X-band microwaves to snow salinity changes as snow thickness decreases. C-band demonstrated minimal sensitivity to changes in snow salinities. Our results demonstrate the influence of salinity associated dielectric loss, throughout all layers of the three snow covers, as the governing factor affecting microwave backscatter and penetration from all three frequencies. Our “plot-scale” observations using co-polarized backscatter, co-pol ratios and dual-frequency ratios suggest the future potential to up-scale our multi-frequency approach to a “satellite-scale” approach, towards effective development of snow geophysical and thermodynamic retrieval algorithms on smooth first-year sea ice.

Encroachment of Canals of Dhaka City,Bangladesh: An Investigative Approach

Abstract Dhaka City has been suffering from many environmental problems including flooding, water logging and other related problems. Urbanization, which is occurring very fast and with larger magnitude in Dhaka, is the intrinsic reason behind these problems. High rate of urbanization causes extensive urban area expansion and as a result canals, wetland and other water bodies are quickly vanishing from the landscape. This study shows the present physical condition of the canals; identifies the processes of canal encroachment; represents the consequences of canal encroachment. 13 canals of 50 were surveyed; local people were surveyed to identify the impact and processes of encroachment. According to this study, canals of Dhaka city are under serious threat of extinction and require immediate recovery actions. Canals are being encroached in various styles and this study identifies five: unauthorized land filling, illegal construction over canal, expansion of slum, solid waste dumping, taking advantage of lack of awareness of local people as well as government agencies. However, this study also discusses the grave consequences of canal encroachment: increasing flood vulnerability, wane of ground water recharge area and ground water level, collapse of natural drainage system, loss of local ecology and biodiversity.