Assessment of WRF-CO2 simulated vertical profiles of CO2 over Delhi region using aircraft and global model data

Abstract

High-resolution regional model simulation of CO₂ may be more beneficial to reduce the uncertainty in estimation of CO₂ source and sink via inverse modeling. However, the study of atmospheric CO₂ transport with regional models is rare over India. Here, weather research and forecasting chemistry model adjusted for CO₂ (WRF-CO₂) is used for simulating vertical profile of CO₂ and its assessment is performed over Delhi, India (27.4–28.6° N and 77–96° E) by comparing aircraft observations (CONTRAIL) and a global model (ACTM) data. During August and September, the positive vertical gradient (~ 13.4 ppm) within ~ 2.5 km height is observed due to strong CO₂ uptake by newly growing vegetation. A similar pattern (~ 4 ppm) is noticed in February due to photosynthesis by newly growing winter crops. The WRF-CO₂ does not show such steep increasing slope (capture up to 5%) during August and September but same for February is estimated ~ 1.7 ppm. Generally, CO₂ is quite well mixed between ~ 2.5 and ~ 8 km height above ground which is well simulated by the WRF-CO₂ model. During stubble burning period of 2010, the highest gradient within 2.5 km height above ground was recorded in October (− 9.3 ppm), followed by November (− 7.6 ppm). The WRF-CO₂ and ACTM models partially capture these gradients (October − 3.3 and − 2.7 ppm and November − 3.8 and − 4.3 ppm respectively). A study of the seasonal variability of CO₂ indicates seasonal amplitudes decrease with increasing height (amplitude is ~ 21 ppm at the near ground and ~ 6 ppm at 6–8 km altitude bin). Correlation coefficients (CC) between the WRF-CO₂ model and observation are noted to be greater than 0.59 for all the altitude bins. In contrast to simulated fossil CO₂, the biospheric CO₂ is in phase with observed seasonality, having about 80% at the lowest level and gradually declines with height due to mixing processes, reaching around 60% at the highest level. The model simulation reveals that meteorology plays a significant role of the horizontal and vertical gradient of CO₂ over the region.