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Richard I. Cullather and Michael G. Bosilovich

Abstract

The atmospheric moisture budget from the Modern Era Retrospective-Analysis for Research and Applications (MERRA) is evaluated in polar regions for the period 1979–2005 and compared with previous estimates, accumulation syntheses over polar ice sheets, and in situ Arctic precipitation observations. The system is based on a nonspectral background model and utilizes the incremental analysis update scheme. The annual moisture convergence from MERRA for the north polar cap is comparable to previous estimates using 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40) and earlier reanalyses but it is more than 50% larger than MERRA precipitation minus evaporation (PE) computed from physics output fields. This imbalance is comparable to earlier reanalyses for the Arctic. For the south polar cap, the imbalance is 20%. The MERRA physics output fields are also found to be overly sensitive to changes in the satellite observing system, particularly over data-sparse regions of the Southern Ocean. Comparisons between MERRA and prognostic fields from two contemporary reanalyses yield a spread of values from 6% of the mean over the Antarctic Ice Sheet to 61% over a domain of the Arctic Ocean. These issues highlight continued problems associated with the representation of cold-climate physical processes in global data assimilation models. The distribution of MERRA surface fluxes over the major polar ice sheets emphasizes larger values along the coastal escarpments, which agrees more closely with recent assessments of ice sheet accumulation using regional models. Differences between these results and earlier assessments illustrate a continued ambiguity in the surface moisture flux distribution over Greenland and Antarctica. The higher spatial and temporal resolution as well as the availability of all budget components, including analysis increments in MERRA, offer prospects for an improved representation of the high-latitude water cycle in reanalyses.

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Richard I. Cullather and Sophie M. J. Nowicki

Abstract

Melt area is one of the most reliably monitored variables associated with surface conditions over the full Greenland Ice Sheet (GrIS). Surface melt is also an important indicator of surface mass balance and has potential relevance to the ice sheet’s global sea level contribution. Melt events are known to be spatially heterogeneous and have varying time scales. To understand the forcing mechanisms, it is necessary to examine the relation between the existing conditions and melt area on the time scales that melt is observed. Here, the authors conduct a regression analysis of atmospheric reanalysis variables including sea level pressure, near-surface winds, and components of the surface energy budget with surface melt. The regression analysis finds spatial heterogeneity in the associated atmospheric circulation conditions. For basins in the southern GrIS, there is an association between melt area and high pressure located south of the Denmark Strait, which allows for southerly flow over the western half of the GrIS. Instantaneous surface melt over northern basins is also associated with low pressure over the central Arctic. Basins associated with persistent summer melt in the southern and western GrIS are associated with the presence of an enhanced cloud cover, a resulting decreased downwelling solar radiative flux, and an enhanced downwelling longwave radiative flux. This contrasts with basins to the north and east, where an increased downwelling solar radiative flux plays a more important role in the onset of a melt event. The analysis emphasizes the importance of daily variability in synoptic conditions and their preferred association with melt events.

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Richard I. Cullather and Michael G. Bosilovich

Abstract

Components of the atmospheric energy budget from the Modern-Era Retrospective Analysis for Research and Applications (MERRA) are evaluated in polar regions for the period 1979–2005 and compared with previous estimates, in situ observations, and contemporary reanalyses. Closure of the budget is reflected by the analysis increments term, which indicates an energy surplus of 11 W m−2 over the North Polar cap (70°–90°N) and 22 W m−2 over the South Polar cap (70°–90°S). Total atmospheric energy convergence from MERRA compares favorably with previous studies for northern high latitudes but exceeds the available previous estimate for the South Polar cap by 46%. Discrepancies with the Southern Hemisphere energy transport are largest in autumn and may be related to differences in topography with earlier reanalyses. For the Arctic, differences between MERRA and other sources in top of atmosphere (TOA) and surface radiative fluxes are largest in May. These differences are concurrent with the largest discrepancies between MERRA parameterized and observed surface albedo. For May, in situ observations of the upwelling shortwave flux in the Arctic are 80 W m−2 larger than MERRA, while the MERRA downwelling longwave flux is underestimated by 12 W m−2 throughout the year. Over grounded ice sheets, the annual mean net surface energy flux in MERRA is erroneously nonzero. Contemporary reanalyses from the Climate Forecast Center (CFSR) and the Interim Re-Analyses of the European Centre for Medium-Range Weather Forecasts (ERA-I) are found to have better surface parameterizations; however, these reanalyses also disagree with observed surface and TOA energy fluxes. Discrepancies among available reanalyses underscore the challenge of reproducing credible estimates of the atmospheric energy budget in polar regions.

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David H. Bromwich, Richard I. Cullather, and Robert W. Grumbine

Abstract

Analyses and medium-range numerical weather forecasts produced by the National Centers for Environmental Prediction are evaluated poleward of 50°S during the July 1994 special observing period of the Antarctic First Regional Observing Study of the Troposphere project. Over the Antarctic plateau, the poor representation of the continent’s terrain creates ambiguity in assessing the quality of surface variables. An examination of the vertical temperature profile, however, finds the near-surface temperature inversion strength to be substantially smaller than the observed climatology at the zero forecast hour. This arises from surface temperatures that are warmer than expected. Significant adjustment occurs in a variety of fields over the first few days of the medium-range forecast, which likely results from the initial hour’s suspect temperature profile. A spatially oscillating series of forecast anomalies in the zonally averaged temperature cross section stretches to middle latitudes by day 3. Near-surface and upper-troposphere values are found actually to improve at the South Pole with forecast time, although some fields continue to adjust through day 7. Although the examination presented here does not give a complete diagnosis, differences between observations and analyses suggest deficiencies with the model initial fields have a major role in producing the substantial model drift found. Atmospheric moisture over the continental interior does not change significantly with forecast hour, although the distinct contrast between nearshore and interior conditions lessens with forecast time. A spurious high-latitude wave pattern is found for a variety of variables. The pattern of this distortion remains constant with forecast hour. Over the ocean, large forecast pressure and height differences with analyses are associated with blocking conditions. However, it is unclear whether this results from deficiencies in the forecast model or the meager observational network over the Southern Ocean.

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Richard I. Cullather, David H. Bromwich, and Mark C. Serreze

Abstract

The atmospheric moisture budget is evaluated for the region 70°N to the North Pole using reanalysis datasets of the European Centre for Medium-Range Weather Forecasts (ECMWF; ERA: ECMWF Re-Analysis) and the collaborative effort of the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR). For the forecast fields of the reanalyses, the ERA annually averaged PE (precipitation minus evaporation/sublimation) field reproduces the major features of the basin perimeter as they are known, while the NCEP–NCAR reanalysis forecast fields contain a spurious wave pattern in both P and E. Comparisons between gauge data from Soviet drift camp stations and forecast P values of the reanalyses show reasonable agreement given the difficulties (i.e., gauge accuracy, translating location). When averaged for 70°–90°N, the ERA and NCEP–NCAR forecast PE are similar in the annual cycle. Average reanalysis forecast values of E for the north polar cap are found to be 40% or more too large based on comparisons using surface latent heat flux climatologies.

Differences between a synthesized average moisture flux across 70°N from rawinsonde data of the Historical Arctic Rawinsonde Archive (HARA) and the reanalysis data occur in the presence of rawinsonde network problems. It is concluded that critical deficiencies exist in the rawinsonde depiction of the summertime meridional moisture transport. However, it remains to be seen whether the rawinsonde estimate can be rectified with a different method. For 70°–90°N, annual moisture convergence (PE) values from the ERA and NCEP–NCAR are very similar; for both reanalyses, annual PE values obtained from forecast fields are much lower than those obtained from moisture flux convergence by about 60%, indicating severe nonclosure of the atmospheric moisture budget. The nonclosure primarily results from anomalously large forecast E values. In comparison with other studies, reanalyses moisture convergence values are much more reasonable. A synthesis of the reanalysis moisture convergence values and more recent studies yields a value of 18.9 ± 2.3 cm yr−1 for the north polar cap.

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Richard I. Cullather, David H. Bromwich, and Michael L. Van Woert

Abstract

The spatial and temporal variability of net precipitation (precipitation minus evaporation/sublimation) for Antarctica derived from the European Centre for Medium-Range Weather Forecasts operational analyses via the atmospheric moisture budget is assessed in comparison to a variety of glaciological and meteorological observations and datasets. For the 11-yr period 1985–95, the average continental value is 151 mm yr−1 water equivalent. Large regional differences with other datasets are identified, and the sources of error are considered. Interannual variability in the Southern Ocean storm tracks is found to be an important mechanism for enhanced precipitation minus evaporation (PE) in both east and west Antarctica. In relation to the present findings, an evaluation of the rawinsonde method for estimating net precipitation in east Antarctica is conducted. Estimates of PE using synthetic rawinsondes derived from the analyses are found to compare favorably to glaciological estimates. A significant upward trend of 2.4 mm yr−1 is found for the Antarctic continent that is consistent with findings from the National Centers for Environmental Prediction, formerly the National Meteorological Center, and the National Center for Atmospheric Research Reanalysis precipitation dataset. Despite large regional discrepancies, the general agreement on the main features of Antarctic precipitation between studies suggests that a threshold has been reached, where the assessment of the smaller terms including evaporation/sublimation and drift snow loss is required to explain the differences.

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Allison B. Marquardt Collow, Richard I. Cullather, and Michael G. Bosilovich

Abstract

Surface air temperatures have recently increased more rapidly in the Arctic than elsewhere in the world, but large uncertainty remains in the time series and trend. Over the data-sparse sea ice zone, the retrospective assimilation of observations in numerical reanalyses has been thought to offer a possible, but challenging, avenue for adequately reproducing the historical time series. Focusing on the central Arctic Ocean, output is analyzed from 12 reanalyses with a specific consideration of two widely used products: the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), and the European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim, hereafter ERA-I). Among the reanalyses considered, a trend of 0.9 K decade−1 is indicated but with an uncertainty of 6%, and a large spread in mean values. There is a partitioning among those reanalyses that use fractional sea ice cover and those that employ a threshold, which are colder in winter by an average of 2 K but agree more closely with in situ observations. For reanalyses using fractional sea ice cover, discrepancies in the ice fraction in autumn and winter explain most of the differences in air temperature values. A set of experiments using the MERRA-2 background model using MERRA-2 and ERA-I sea ice and sea surface temperature indicates significant effects of boundary condition differences on air temperatures, and a preferential warm bias inherent in the MERRA-2 model sea ice representation. Differences between experiments and reanalyses suggest the available observations apply a significant constraint on reanalysis mean temperatures.

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David H. Bromwich, Frank M. Robasky, Richard I. Cullather, and Michael L. Van Woert

Abstract

Moisture budget calculations for Antarctica and the Southern Ocean (40°–72deg;S) are performed using operational numerical analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF), the National Meteorological Center (NMC), and the Australian Bureau of Meteorology (ABM). The analyses are intetcompared for an 8-yr period from 1985 to 1992 and are evaluated against representative rawinsonde sites, which are considered accurate depictions of moisture transport at these sites.

The comparisons to East Antarctic rawinsondes and those from Macquarie Island show the ECMWF analyses to be superior in reproducing sounding values at each level. While results are highly variable depending on the station location, agreement of the ECMWF analyses to zonally averaged sounding moisture flux values along the East Antarctic coast is very close. The zonally averaged annual meridional moisture flux, for example, is within as little as 0.03 g kg−1 m s−1, or 2% at the surface. This is particularly good considering the highly variable inflow and outflow patterns along the Antarctic perimeter. The NMC and ABM analyses generally underestimate transport at each level; error cancellation occurs during vertical integration however. A comparison of moisture convergence for East Antarctica with values calculated from rawinsonde data indicates the ECMWF analysis is within 5 mm yr−1 of the observed value, while the NMC result is severely deficient. Overall these results are not surprising given the coarse resolution and spectral nature of the analyses. The ability of the ECMWF analyses to reproduce the observed moisture transport at each level is reassuring.

Comparison of the moisture transport convergence derived from the numerical analyses with previous moisture flux studies over the Southern Ocean reveals general agreement in the location of the boundary between the moisture source and sink. The ECMWF and NMC analyses place the convergence maximum slightly farther south than has been previously found. It is inferred that this results from the blocking effect of the Antarctic coastal topography. At full resolution this point is at approximately 64°S.

Long-term net precipitation (precipitation minus sublimation/evaporation) derived from the numerical analyses is somewhat smaller than values determined by glaciological methods. Net precipitation varies interannually by 25%, with most of the variation concentrated in the South Pacific sector, the region of greatest poleward moisture transport.

The results presented here offer a substantially more positive outlook on the prospects of determining continental precipitation trends in Antarctica through atmospheric methods than has been previously found and demonstrate that the ECMWF analyses provide generally good estimates.

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Keith M. Hines, Robert W. Grumbine, David H. Bromwich, and Richard I. Cullather

Abstract

The surface energy budget in Antarctic latitudes is evaluated for the medium-range numerical weather forecasts produced by the National Centers for Environmental Prediction (NCEP) and for the NCEP–National Center for Atmospheric Research reanalysis project during the winter, spring, and summer special observing periods (SOPs) of the Antarctic First Regional Observing Study of Troposphere project. A significant change in the energy balance resulted from an extensive model update beginning with the forecasts initialized on 11 January 1995 during the summer SOP. Both the forecasts and the reanalysis include significant errors in the surface energy balance over Antarctica. The errors often tend to cancel and thus produce reasonable surface temperature fields. General errors include downward longwave radiation about 30–50 W m−2 too small. Lower than observed cloudiness contributes to this error and to excessive downward shortwave radiation at the surface. The model albedo over Antarctica, about 75%, is lower than that derived from observations, about 81%. During the polar day, errors in net longwave and net shortwave radiation tend to cancel. The energy balance over Antarctica in the reanalysis is, in general, degraded from that of the forecasts.

Seasonal characteristics of the surface energy balance include cooling over East Antarctica and slight warming over West Antarctica during NCEP forecasts for the winter SOP. Wintertime surface warming by downward sensible heat flux is larger than observations by 21–36 W m−2 and tends to balance the excessive longwave cooling at the surface. During the spring SOP, forecast sensible heat flux produces an excessive heating contribution by about 20 W m−2. Latent heat flux during the Antarctic winter for the reanalysis is at least an order of magnitude larger than the very small observed values.

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Irina V. Gorodetskaya, L-Bruno Tremblay, Beate Liepert, Mark A. Cane, and Richard I. Cullather

Abstract

The impact of Arctic sea ice concentrations, surface albedo, cloud fraction, and cloud ice and liquid water paths on the surface shortwave (SW) radiation budget is analyzed in the twentieth-century simulations of three coupled models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report. The models are the Goddard Institute for Space Studies Model E-R (GISS-ER), the Met Office Third Hadley Centre Coupled Ocean–Atmosphere GCM (UKMO HadCM3), and the National Center for Atmosphere Research Community Climate System Model, version 3 (NCAR CCSM3). In agreement with observations, the models all have high Arctic mean cloud fractions in summer; however, large differences are found in the cloud ice and liquid water contents. The simulated Arctic clouds of CCSM3 have the highest liquid water content, greatly exceeding the values observed during the Surface Heat Budget of the Arctic Ocean (SHEBA) campaign. Both GISS-ER and HadCM3 lack liquid water and have excessive ice amounts in Arctic clouds compared to SHEBA observations. In CCSM3, the high surface albedo and strong cloud SW radiative forcing both significantly decrease the amount of SW radiation absorbed by the Arctic Ocean surface during the summer. In the GISS-ER and HadCM3 models, the surface and cloud effects compensate one another: GISS-ER has both a higher summer surface albedo and a larger surface incoming SW flux when compared to HadCM3. Because of the differences in the models’ cloud and surface properties, the Arctic Ocean surface gains about 20% and 40% more solar energy during the melt period in the GISS-ER and HadCM3 models, respectively, compared to CCSM3.

In twenty-first-century climate runs, discrepancies in the surface net SW flux partly explain the range in the models’ sea ice area changes. Substantial decrease in sea ice area simulated during the twenty-first century in CCSM3 is associated with a large drop in surface albedo that is only partly compensated by increased cloud SW forcing. In this model, an initially high cloud liquid water content reduces the effect of the increase in cloud fraction and cloud liquid water on the cloud optical thickness, limiting the ability of clouds to compensate for the large surface albedo decrease. In HadCM3 and GISS-ER, the compensation of the surface albedo and cloud SW forcing results in negligible changes in the net SW flux and is one of the factors explaining moderate future sea ice area trends. Thus, model representations of cloud properties for today’s climate determine the ability of clouds to compensate for the effect of surface albedo decrease on the future shortwave radiative budget of the Arctic Ocean and, as a consequence, the sea ice mass balance.

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