... TRI BUF BTV CAR CPR CRW CAO CLE cos CMH CON DEN DSM DTW ERI FAR FLG FSM GLD GJT GTF GRB MDT B... more ... TRI BUF BTV CAR CPR CRW CAO CLE cos CMH CON DEN DSM DTW ERI FAR FLG FSM GLD GJT GTF GRB MDT BDL IND INL MCI LSE LND LNK SDF LBB LYH ... PDT PIA PHL PIT PIH PWM RAP RFD STC STL SLC SSM BFF sux FSD SBN SGF SYR TOP ICT IPT ISN WMC ...
The NASA announcement of record surface melting of the Greenland ice sheet in July 2012 led us to... more The NASA announcement of record surface melting of the Greenland ice sheet in July 2012 led us to examine the atmospheric and oceanic climatic anomalies that are likely to have contributed to these exceptional conditions and also to ask the question of how unusual these anomalies were compared to available records. Our analysis allows us to assess the relative contributions of these two key influences to both the extreme melt event and ongoing climate change. In 2012, as in recent warm summers since 2007, a blocking high pressure feature, associated with negative NAO conditions, was present in the mid-troposphere over Greenland for much of the summer. This circulation pattern advected relatively warm southerly winds over the western flank of the ice sheet, forming a ‘heat dome’ over Greenland that led to the widespread surface melting. Both sea-surface temperature and sea-ice cover anomalies seem to have played a minimal role in this record melt, relative to atmospheric circulation. Two representative coastal climatological station averages and several individual stations in south, west and north-west Greenland set new surface air temperature records for May, June, July and the whole (JJA) summer. The unusually warm summer 2012 conditions extended to the top of the ice sheet at Summit, where our reanalysed (1994–2012) DMI Summit weather station summer (JJA) temperature series set new record high mean and extreme temperatures in 2012; 3-hourly instantaneous 2-m temperatures reached an exceptional value of 2.2°C at Summit on 11 July 2012. These conditions translated into the record observed ice-sheet wide melt during summer 2012. However, 2012 seems not to be climatically representative of future ‘average’ summers projected this century.
ABSTRACT Atmospheric blocking commonly occurs over the high latitudes of the Northern Hemisphere,... more ABSTRACT Atmospheric blocking commonly occurs over the high latitudes of the Northern Hemisphere, resulting from the development of persistent areas of high pressure that lead to warmer-than-average surface temperatures west of the high centre. While the variability and trends in anticyclonic circulation patterns (including blocking) over Greenland have been previously documented, an analysis of the most extreme blocking events within the observational record is lacking. In this study, a historical climatology of extreme Greenland blocking episodes (GBEs) from 1958 to 2013 is examined within the context of anomalous anticyclonic circulation patterns over the North Atlantic region during recent years. Based on a combination of the ERA-40 (1958–1978) and ERA-Interim (1979–2013) reanalysis data sets, the Greenland Blocking Index (GBI) is used to quantify 500 hPa geopotential height anomalies for the identification of extreme GBEs. The annual rate of extreme blocking days has doubled since 1958, reaching an average of approximately 20 days per year by 2013. The frequency and, to some extent, duration of extreme GBEs were unprecedentedly high from 2007 to 2013 compared to the 56-year period of record, with a majority of the increase occurring during the spring (MAM) and summer (JJA). A multiple linear regression analysis reveals that interannual variability in extreme blocking and the Atlantic Multidecadal Oscillation (AMO) are the two predominant drivers of surface meltwater production across the entire Greenland ice sheet (GrIS), but Arctic sea ice extent and North Atlantic cyclone activity can also influence the extent of summer melting over portions of the GrIS. Thus, in addition to the larger-scale atmospheric and oceanic variability, smaller-scale features such as extratropical cyclones can play a significant role in modulating GrIS surface melting each summer.
Meteorological and glaciological models of snow accumulation and meltwater runoff are used with E... more Meteorological and glaciological models of snow accumulation and meltwater runoff are used with European Centre for Medium-Range Weather Forecasts climate analy- ses, to simulate the current/recent surface mass balance of the Greenland Ice Sheet and its interannual variations. The usefulness of the technique is demonstrated by compar- ison with shallow ice-core data (of net snow accumulation) and passive microwave satellite data (of net runoff). Our method will be used with the forthcoming ECMWF ERA-40 reanalysis to produce a novel dataset of temporal and spatial variations of SMB across the Ice Sheet for the past 40 years. It should help elucidate the cause(s) of recent changes in surface elevation of the Ice Sheet observed by aircraft/satellite laser/radar.
Multiple satellite-derived sources of snow cover extent (SCE), snow-cover fraction (SCF) and melt... more Multiple satellite-derived sources of snow cover extent (SCE), snow-cover fraction (SCF) and melting snow over Northern Hemisphere (NH) lands have been used to produce two integrated datasets. One dataset is long enough (over 30 years) to be considered a climate data record (CDR), a second has data of a quality suitable for a CDR but lacks the duration to be recognized as such. These datasets provide state-of-the-art NH snow information in multiple formats and on multiple time steps for the research community, decision-makers and stakeholders. To generate the CDR, a low-resolution, long-term SCE record available on a weekly timescale derived from a thorough reanalysis of NOAA satellite-derived maps of NH continental SCE dating back to the late 1966 was integrated with a newly generated 1979-present microwave SCE and melting snow product. A 100 km Equal-Area Scalable Earth version 2 grid (EASE2) developed at the National Snow and Ice Data Center is used for this CDR. The shorter-term...
... and Hu, H.(2007) Urban effects and summer thunderstorms in a tropical cyclone affected situat... more ... and Hu, H.(2007) Urban effects and summer thunderstorms in a tropical cyclone affected situation ... on the climate in Europe: Numerical experiments by the PSU-NCAR Mesoscale Model (MM5), Journal of ... 5. Van den Heever, SC and Cotton, WR (2007) Urban aerosol impacts on ...
ABSTRACT High-resolution dynamical downscaling is used to explore 2080-2090 peak-season hazardous... more ABSTRACT High-resolution dynamical downscaling is used to explore 2080-2090 peak-season hazardous convective weather as simulated from the Community Climate System Model version 3. Downscaling to 4 km grid spacing is performed using the Weather Research and Forecasting model. Tornadoes, damaging wind gusts, and large hail are simulated using a model proxy at hourly intervals for locations east of the U.S. Continental Divide. Future period results are placed into context using 1980-1990 output. While a limited sample size exists, a statistically significant increase in synthetic severe weather activity is noted in March, whereas event frequency is shown to slightly increase in April, and stay the same in May. These increases are primarily found in the Mississippi, Tennessee, and Ohio River valleys. Diurnally, most of the increase in hazardous convective weather activity is shown to be in the hours surrounding local sunset. Peak-season severe weather is also shown to be more variable in the future with a skewed potential toward larger counts. Finally, modeled proxy events are compared to environmental parameters known to generate hazardous convective weather activity. These environmental conditions explain over 80 % of the variance associated with modeled reports during March-May and show an increasing future tendency. Finally, challenges associated with dynamical downscaling for purposes of resolving severe local storms are discussed.
... TRI BUF BTV CAR CPR CRW CAO CLE cos CMH CON DEN DSM DTW ERI FAR FLG FSM GLD GJT GTF GRB MDT B... more ... TRI BUF BTV CAR CPR CRW CAO CLE cos CMH CON DEN DSM DTW ERI FAR FLG FSM GLD GJT GTF GRB MDT BDL IND INL MCI LSE LND LNK SDF LBB LYH ... PDT PIA PHL PIT PIH PWM RAP RFD STC STL SLC SSM BFF sux FSD SBN SGF SYR TOP ICT IPT ISN WMC ...
The NASA announcement of record surface melting of the Greenland ice sheet in July 2012 led us to... more The NASA announcement of record surface melting of the Greenland ice sheet in July 2012 led us to examine the atmospheric and oceanic climatic anomalies that are likely to have contributed to these exceptional conditions and also to ask the question of how unusual these anomalies were compared to available records. Our analysis allows us to assess the relative contributions of these two key influences to both the extreme melt event and ongoing climate change. In 2012, as in recent warm summers since 2007, a blocking high pressure feature, associated with negative NAO conditions, was present in the mid-troposphere over Greenland for much of the summer. This circulation pattern advected relatively warm southerly winds over the western flank of the ice sheet, forming a ‘heat dome’ over Greenland that led to the widespread surface melting. Both sea-surface temperature and sea-ice cover anomalies seem to have played a minimal role in this record melt, relative to atmospheric circulation. Two representative coastal climatological station averages and several individual stations in south, west and north-west Greenland set new surface air temperature records for May, June, July and the whole (JJA) summer. The unusually warm summer 2012 conditions extended to the top of the ice sheet at Summit, where our reanalysed (1994–2012) DMI Summit weather station summer (JJA) temperature series set new record high mean and extreme temperatures in 2012; 3-hourly instantaneous 2-m temperatures reached an exceptional value of 2.2°C at Summit on 11 July 2012. These conditions translated into the record observed ice-sheet wide melt during summer 2012. However, 2012 seems not to be climatically representative of future ‘average’ summers projected this century.
ABSTRACT Atmospheric blocking commonly occurs over the high latitudes of the Northern Hemisphere,... more ABSTRACT Atmospheric blocking commonly occurs over the high latitudes of the Northern Hemisphere, resulting from the development of persistent areas of high pressure that lead to warmer-than-average surface temperatures west of the high centre. While the variability and trends in anticyclonic circulation patterns (including blocking) over Greenland have been previously documented, an analysis of the most extreme blocking events within the observational record is lacking. In this study, a historical climatology of extreme Greenland blocking episodes (GBEs) from 1958 to 2013 is examined within the context of anomalous anticyclonic circulation patterns over the North Atlantic region during recent years. Based on a combination of the ERA-40 (1958–1978) and ERA-Interim (1979–2013) reanalysis data sets, the Greenland Blocking Index (GBI) is used to quantify 500 hPa geopotential height anomalies for the identification of extreme GBEs. The annual rate of extreme blocking days has doubled since 1958, reaching an average of approximately 20 days per year by 2013. The frequency and, to some extent, duration of extreme GBEs were unprecedentedly high from 2007 to 2013 compared to the 56-year period of record, with a majority of the increase occurring during the spring (MAM) and summer (JJA). A multiple linear regression analysis reveals that interannual variability in extreme blocking and the Atlantic Multidecadal Oscillation (AMO) are the two predominant drivers of surface meltwater production across the entire Greenland ice sheet (GrIS), but Arctic sea ice extent and North Atlantic cyclone activity can also influence the extent of summer melting over portions of the GrIS. Thus, in addition to the larger-scale atmospheric and oceanic variability, smaller-scale features such as extratropical cyclones can play a significant role in modulating GrIS surface melting each summer.
Meteorological and glaciological models of snow accumulation and meltwater runoff are used with E... more Meteorological and glaciological models of snow accumulation and meltwater runoff are used with European Centre for Medium-Range Weather Forecasts climate analy- ses, to simulate the current/recent surface mass balance of the Greenland Ice Sheet and its interannual variations. The usefulness of the technique is demonstrated by compar- ison with shallow ice-core data (of net snow accumulation) and passive microwave satellite data (of net runoff). Our method will be used with the forthcoming ECMWF ERA-40 reanalysis to produce a novel dataset of temporal and spatial variations of SMB across the Ice Sheet for the past 40 years. It should help elucidate the cause(s) of recent changes in surface elevation of the Ice Sheet observed by aircraft/satellite laser/radar.
Multiple satellite-derived sources of snow cover extent (SCE), snow-cover fraction (SCF) and melt... more Multiple satellite-derived sources of snow cover extent (SCE), snow-cover fraction (SCF) and melting snow over Northern Hemisphere (NH) lands have been used to produce two integrated datasets. One dataset is long enough (over 30 years) to be considered a climate data record (CDR), a second has data of a quality suitable for a CDR but lacks the duration to be recognized as such. These datasets provide state-of-the-art NH snow information in multiple formats and on multiple time steps for the research community, decision-makers and stakeholders. To generate the CDR, a low-resolution, long-term SCE record available on a weekly timescale derived from a thorough reanalysis of NOAA satellite-derived maps of NH continental SCE dating back to the late 1966 was integrated with a newly generated 1979-present microwave SCE and melting snow product. A 100 km Equal-Area Scalable Earth version 2 grid (EASE2) developed at the National Snow and Ice Data Center is used for this CDR. The shorter-term...
... and Hu, H.(2007) Urban effects and summer thunderstorms in a tropical cyclone affected situat... more ... and Hu, H.(2007) Urban effects and summer thunderstorms in a tropical cyclone affected situation ... on the climate in Europe: Numerical experiments by the PSU-NCAR Mesoscale Model (MM5), Journal of ... 5. Van den Heever, SC and Cotton, WR (2007) Urban aerosol impacts on ...
ABSTRACT High-resolution dynamical downscaling is used to explore 2080-2090 peak-season hazardous... more ABSTRACT High-resolution dynamical downscaling is used to explore 2080-2090 peak-season hazardous convective weather as simulated from the Community Climate System Model version 3. Downscaling to 4 km grid spacing is performed using the Weather Research and Forecasting model. Tornadoes, damaging wind gusts, and large hail are simulated using a model proxy at hourly intervals for locations east of the U.S. Continental Divide. Future period results are placed into context using 1980-1990 output. While a limited sample size exists, a statistically significant increase in synthetic severe weather activity is noted in March, whereas event frequency is shown to slightly increase in April, and stay the same in May. These increases are primarily found in the Mississippi, Tennessee, and Ohio River valleys. Diurnally, most of the increase in hazardous convective weather activity is shown to be in the hours surrounding local sunset. Peak-season severe weather is also shown to be more variable in the future with a skewed potential toward larger counts. Finally, modeled proxy events are compared to environmental parameters known to generate hazardous convective weather activity. These environmental conditions explain over 80 % of the variance associated with modeled reports during March-May and show an increasing future tendency. Finally, challenges associated with dynamical downscaling for purposes of resolving severe local storms are discussed.
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