Day 4

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Paper title Examining extreme precipitation events from satellite altimetry
Authors
  1. Sebastian Bjerregaard Simonsen Technical University of Denmark Speaker
  2. Kirk M. Scanlan DTU Space
  3. Nicolaj Hansen DTU Space - Technical University of Denmark
  4. Heidi Ranndal DTU Space - Technical University of Denmark
  5. Ruth Mottram Danish Metrological Office
  6. Louise Sandberg Sørensen DTU Space
Form of presentation Poster
Topics
  • A9. Polar Science and Cryosphere
    • A9.04 Mass Balance of the Cryosphere
Abstract text A key component of the Greenland ice sheet surface mass balance is the occurrence of extreme precipitation (snowfall) events, during which warm air masses bring moist air onto the Greenland ice sheet and deposit massive amounts of snow in the affected area. These events are common in the southeastern parts of the ice sheet but are also observed in other places such as in the northwest. In October 2016 extra-tropical cyclones Matthew and Nicole hit Greenland over a two-week period near the town of Tasiilaq. Matthew gave record-high rainfall at Tasiilaq, whereas the precipitation from Nicole hit predominantly over the ice sheet as snow. The high-resolution numerical weather prediction (NWP) model HARMONIE-AROME results (displayed at Polarportal.dk) used to drive a surface mass budget (SMB) model show a peak in Greenland surface mass balance of 12 Gt/day during this event, mainly driven by the snowfall on the Greenland east coast. Another less observed event occurred in October 2019 near Thule in the northwestern part of the Greenland ice sheet. Here, the nearby meteorological station at Qaanaaq does not measure precipitation but did measure increased relative humidity that gives an indication of a large precipitation event on the ice sheet. The NWP model here estimates a deposition of about 4 Gt/day of snow in the area during the event.

The occurrence of extreme precipitation events is a difficult phenomenon to model at typical scales of existing regional climate models (RCM) and the limited in-situ observations of these events on ice sheets make it even harder to improve model estimates of accumulation in both space, time, and quantity. These problems are an order of magnitude bigger in Antarctica where extreme precipitation events also contribute disproportionately to Ice sheet mass budget. Luckily, we are now in a golden era of satellite radar altimetry with multiple satellites measuring elevation change at different radar frequencies. With the difference in frequency comes also differences in the ratio between volume and surface scattering observed by the individual missions. In addition to multiple satellite radar altimeters, we also have a massive lidar dataset available from ICESat-2. In Greenland, we are fortunate also to have the high quality PROMICE weather station data sets that allow us to calibrate and evaluate both satellite and model outputs in some specific areas.

Hence, it is time to unify this wealth of satellite data to provide a new source of observations to shed insight on the occurrence of extreme precipitation events and thereby improve the predictive capabilities of NWPs. As satellite altimeters are so diverse in their instrumental setup and sensing capabilities, we first divide our efforts along three parallel lines of work:

(1) Conventional radar altimeter (elevation retrieval) investigations. The raw elevation measurements of either Ku-/Ka-band radar altimetry are affected differently by changes in surface properties. Initial studies have shown how the range to the Greenland ice sheet changes differently in the two frequencies, which may be related to surface conditions varying throughout time. This difference is used to map the first order surface behavior during the extreme precipitation events.

(2) Enhanced radar altimeter (surface power modeling) investigations. The strength with which radar waves are reflected are affected by several physical factors, including the contrast in electromagnetic properties across the surface interface as well as the roughness of that interface. Both are expected to change during the occurrence of the extreme precipitation events and may serve as a secondary proxy for precipitated snow.

(3) Laser altimetry (ICESat-2). The multiple returns of the Lidar photons allow for further investigations into individual snow regimes before, during and after the occurrence of the extreme precipitation event. Examining the photons reflected off the subsurface snow, surface snow, and/or blowing snow; thereby provides further insight into the nature of the events.

Finally, combining all three pieces of the puzzle provided by satellite altimetry into a common view of the extreme precipitation events will provide the needed in situ observations to ensure improvements for the predictive capabilities of climate models in the future and truly make use of this current golden era of satellite altimetry. We apply this analysis in the first instance to evaluate high magnitude precipitation events over the Greenland ice sheet in the newly released Copernicus Arctic Regional Reanalysis. The reanalysis is run in unprecedented high resolution with 3d variational data assimilation with a state-of-the-art numerical weather prediction model.