1. VALIDATION OF DUAL-MODE METOP AMVs
Ákos Horváth¹
Régis Borde²
1 Leibniz-Institute for Tropospheric Research, Leipzig, Germany
2 EUMETSAT, Darmstadt, Germany
Objectives
Conclusions
The goal of the study was to perform a preliminary validation of EUMETSAT’s soon-to-be-operational dual-mode Metop AMV product. This novel wind retrieval technique takes advantage of the swath overlap between the Metop-A/Metop-B tandem, which flies in the same orbital plane but with a half orbit separation. Dual-mode winds are extracted from a pair of Metop-A and Metop-B 10.8-μm images obtained ~50 minutes apart, allowing global wind retrievals from AVHRR for the first time. The increased coverage of dual-mode Metop winds helps filling the 50°-70° AMV data gap between the coverage areas of geostationary and polar sensors.
We evaluated ~3 months (20/10/2013 – 31/01/2014) of dual-mode Metop AMVs against GOES-15/13, METEOSAT-10/7, and MTSAT-2 geostationary AMVs as well as Terra MODIS and MISR polar AMVs. All geostationary winds were from the CIMSS Tropical Cyclones archive and used the CIMSS retrieval algorithm, including height assignment. The MISR stereo winds were from the monthly CMV product.
Winds were collocated within 150 (50) km and 90 (30) minutes. For each METOP wind a comparison wind that meets these spatial and temporal collocation criteria could be selected in a variety of ways: max QI, min vector difference, min Δpressure, min Δdistance, min Δtime, min Δdirection, etc. Validation results were also stratified according to Metop height assignment technique: equivalent blackbody temperature or IASI CO2-slicing. Below we present a few typical comparison plots.
Results are sensitive to the choice of comparison wind (max QI, min vector difference, etc.).
Best agreement for low-level winds, worst agreement (rmsd and correlation) for mid-level winds.
Good agreement with MISR for low-level winds, larger discrepancies for mid- and high-level winds mainly due to the tracking of different clouds (MISR visible stereo winds strongly favor the lower levels).
Best overall agreement between AMVs in the tropics, in contrast model – observation differences are largest in the tropics.
Good agreement between AMV heights for METOP IASI CO2-slicing height assignment method.
Larger AMV height differences for EBBT method in certain areas due to differences between cloud masks and inversion correction schemes.
Mean statistics and vertical variation
Scatter plots and geographic variation
all levels
low level
mid level
Fig. 3. METOP vs. GOES-15/13, METEOSAT-10/7, MTSAT-2, and MODIS-Terra wind speeds and heights. The logarithmic color scale indicates number frequency.
high level
Fig. 1. Mean METOP - sensor comparison statistics for the globe, northern hemisphere (25°N-90°N), southern hemisphere (25°S-90°S), and tropics (25°S-25°N), separately for all, low-level (>700 mb), mid-level ( mb), and high-level (<400 mb) winds. In this and all subsequent figures each METOP wind is matched with a comparison wind within 150 km and 90 minutes that minimizes the vector difference.
Fig. 4. Geographic distribution of METOP - sensor wind speed bias and rmsd averaged over all levels. Polar winds are from MODIS-Terra.
Fig. 2. Vertical variation of global-mean wind comparison statistics, binned according to Metop height.