Analysis of Lower Tropospheric Lapse Rate Trend over Port Harcourt Coastal City, Nigeria

Abstract

Understanding the spatial distribution of temperature, especially the relationship between temperature and altitude, is essential for understanding both climatological and hydrological processes and their variabilities. This is because those processes are sensitive to air temperature, especially in sub humid tropical regions, where air temperature influences the movements of pollutants and controls exchanges of energy and water fluxes between land and atmosphere particularly within the lower troposphere. This study examined the trend of lower tropospheric lapse rate in the coastal area of Port Harcourt, Nigeria. Six years’ data (2010-2015) for temperature between 1000 mbar and 850 mbar pressure levels was retrieved from era-interim re-analysis platform for the analysis. The data was acquired at 6-hourly synoptic hours: 0000H, 0600H, 1200H and 1800H at 0.125° grid resolution. Findings from the computed environmental lapse rate (ELR) show that conditional instability with an annual lapse rate of 5.5°C/km persists at the area from January to December. It was revealed that the months of December and January constituted the highest ELR trends of 6.5°C/km and 5.9°C/km respectively. This indicates that the month of December assumes a normal tropospheric lapse rate trend. The average range of lapse rate trend in the area which is close to the moist adiabatic lapse rate (MALR) of 5.0°C/km than the dry adiabatic lapse rate (DALR) shows that the study atmospheric environment is rich in water vapour. The 6-hourly synoptic analysis of the ELR pattern shows that lapse rate range between 1°C/km - 6.4°C/km and 6.5°C/km - 10°C/km dominates throughout the year at 0000 - 0600 Hrs and 1200 - 1800 Hrs respectively. This demonstrates a higher and lower lapse rate trend during the day and night periods respectively. Relating study findings to the potential of air to disperse emissions in the area suggests that air emissions will be conveyed through far and near distances across the boundary layers due to the moderate dispersive potential of air regarding the closeness of the average ELR to the MALR. Policies that will ensure that pollutants are dispersed aloft especially emission stacks above 50 m is advocated in the city of Port Harcourt.