GISS Lunch Seminar

Speaker: Sarah Smith (Columbia/LDEO)
Topic: Understanding and Improving Observational Constraints of Aerosol Seasonality in the Arctic

Proper constraints on the seasonality of aerosol burdens are essential for quantifying the radiative effects of aerosols in the Arctic, where insolation varies dramatically by season. Yet, long-term (gt 10 year), year-round in situ observations of Arctic aerosols are limited to just six monitoring stations, which are not evenly distributed throughout the region and which do not show uniform seasonality. Aerosol optical depth (AOD) products, particularly from the LiDAR instrument CALIOP and from reanalysis products, may be useful for contextualizing in situ measures. However, the seasonality of reanalysis (MERRA-2 and CAMSRA) Arctic AODs is opposite to that of CALIOP, as are the seasonalities of high latitude (>60N) AODs from the passive sensors they assimilate (e.g. AVHRR, MODIS, MISR, POLDER, SeaWiFS, VIIRS). We find that seasonal differences in the bias between MODIS and CALIOP are affected by a significant and substantial dependency on the solar zenith angle, which are also mediated by spatiotemporal differences in MODIS retrieval quality. We then consider the limits of L3 passive sensor AOD datasets in this context. Next, we examine the extent to which station data is representative of region-wide seasonality, using a k-means clustering of CALIOP AODs to define four distinct subregions from 60-72N with regards to AOD seasonality. We find that most, but not all, sub-regions exhibit a peak in AOD during the spring. We also find that most monitoring stations are located in lower amplitude clusters, indicating that station data likely underrepresent the magnitude of low-Arctic aerosol seasonality.