2011: Forecast Impact of Targeted Observations: Sensitivity to Observation Error and Proximity to Steep Orography

• Main Authors: E A Irvine, S L Gray, J Methven, I A Renfrew
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Greenland
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1050.8075; http://www.met.reading.ac.uk/%7Egb902035/Publications/Irvine_MWR_accepted.pdf

ABSTRACT For a targeted observations case, the dependence of the size of the forecast impact on the targeted dropsonde observation error in the data assimilation is assessed. The targeted observations were made in the lee of Greenland; the dependence of the impact on the proximity of the observations to the Greenland coast is also investigated. Experiments were conducted using the Met Office Unified Model (MetUM), over a limited-area domain at 24km gridspacing, with a four-dimensional variational (4D-Var) data assimilation scheme. Reducing the operational dropsonde observation errors by half increases the maximum forecast improvement from 5% to 7-10%, measured in terms of total energy. However, the largest impact is seen by replacing two sondes on the Greenland coast with two further from the steep orography; this increases the maximum forecast improvement from 5% to 18% for an 18-hour forecast (using operational observation errors). Forecast degradation caused by two dropsonde observations on the Greenland coast is shown to arise from spreading of data by the background errors up the steep slope of Greenland. Removing boundary-layer data from these dropsondes reduces the forecast degradation, but is only a partial solution to this problem. Although only from one case study, these results suggest that observations positioned within a correlation length-scale of steep orography may degrade the forecast through the anomalous upslope spreading of analysis increments along terrain-following model levels.