The impact of melt ponds on summertime microwave brightness temperatures and sea-ice concentrations

Stefan Kern, Anja Rösel, Leif Toudal Pedersen, Natalia Ivanova, Roberto Saldo, Rasmus Tage Tonboe
2016 The Cryosphere  
<p><strong>Abstract.</strong> Sea-ice concentrations derived from satellite microwave brightness temperatures are less accurate during summer. In the Arctic Ocean the lack of accuracy is primarily caused by melt ponds, but also by changes in the properties of snow and the sea-ice surface itself. We investigate the sensitivity of eight sea-ice concentration retrieval algorithms to melt ponds by comparing sea-ice concentration with the melt-pond fraction. We derive gridded daily sea-ice
more » ... ions from microwave brightness temperatures of summer 2009. We derive the daily fraction of melt ponds, open water between ice floes, and the ice-surface fraction from contemporary Moderate Resolution Spectroradiometer (MODIS) reflectance data. We only use grid cells where the MODIS sea-ice concentration, which is the melt-pond fraction plus the ice-surface fraction, exceeds 90<span class="thinspace"></span>%. For one group of algorithms, e.g., Bristol and Comiso bootstrap frequency mode (Bootstrap_f), sea-ice concentrations are linearly related to the MODIS melt-pond fraction quite clearly after June. For other algorithms, e.g., Near90GHz and Comiso bootstrap polarization mode (Bootstrap_p), this relationship is weaker and develops later in summer. We attribute the variation of the sensitivity to the melt-pond fraction across the algorithms to a different sensitivity of the brightness temperatures to snow-property variations. We find an <i>underestimation</i> of the sea-ice concentration by between 14<span class="thinspace"></span>% (Bootstrap_f) and 26<span class="thinspace"></span>% (Bootstrap_p) for 100<span class="thinspace"></span>% sea ice with a melt-pond fraction of 40<span class="thinspace"></span>%. The underestimation reduces to 0<span class="thinspace"></span>% for a melt-pond fraction of 20<span class="thinspace"></span>%. In presence of real open water between ice floes, the sea-ice concentration is <i>overestimated</i> by between 26<span class="thinspace"></span>% (Bootstrap_f) and 14<span class="thinspace"></span>% (Bootstrap_p) at 60<span class="thinspace"></span>% sea-ice concentration and by 20<span class="thinspace"></span>% across all algorithms at 80<span class="thinspace"></span>% sea-ice concentration. None of the algorithms investigated performs best based on our investigation of data from summer 2009. We suggest that those algorithms which are more sensitive to melt ponds could be optimized more easily because the influence of unknown snow and sea-ice surface property variations is less pronounced.</p>
doi:10.5194/tc-10-2217-2016 fatcat:zgxfuzb7prbnjbfuxn55eouhc4