A comparison of Nimbus 7 limb infrared monitor of the stratosphere and radiosonde temperatures in the lower stratosphere poleward of 60N

Abstract

Discrepancies have been noted in the literature between distributions of HNO3 and NO2 from Nimbus 7 limb infrared monitor of the stratosphere (LIMS) and from gas‐phase model calculations, particularly in polar winter. The inclusion of heterogeneous chemistry processes into models gives improved agreement for HNO3 but not for NO2. For this reason we considered whether there were temperature bias errors for LIMS that would have an impact on its species retrievals in polar regions. The 7 months of Nimbus 7 LIMS mapped temperatures are compared with daily, lower stratospheric radiosonde (RAOB) observations at 1200 UTC from a set of 22 stations poleward of 60N. The 7‐month average LIMS minus RAOB differences for the set are within ± 0.3 K at 30, 50, and 70 hPa; LIMS minus RAOB equals −1.2 K ± 2.2 K at 100 hPa. Springtime comparisons at 30 hPa were improved after recommended solar corrections were applied to the subset of daylight RAOB temperatures. Time series comparisons were conducted at 10 hPa for four stations in the Aleutian sector. After a correction for radiative cooling was applied to the RAOB data, the LIMS minus RAOB differences at 10 hPa were within ± 2 K, except in winter when LIMS was still significantly warmer by several degrees. Much of that bias is attributed to spatial temperature variations for LIMS/RAOB comparisons in regions of temperature extremes and to the finite vertical resolution of LIMS. All of this means that there is almost no significant error in the LIMS temperatures that could impact the accuracy of LIMS constituent retrievals throughout much of the polar lower stratosphere. The LIMS cold bias at 100 hPa is most pronounced for the 22 station set in November and December (about −1.9 K) and is statistically significant for many of the stations. LIMS tends to smooth through the vertical structure associated with elevated warm layers near 100 hPa. This negative temperature bias leads to retrieved overestimates of LIMS species concentrations below about the 70‐hPa level.