Comments On The Observatonal Paper “Recent Changes In Tropospheric Water Vapor Over The Arctic As Assessed From Radiosondes And Atmospheric Reanalyses” By Serreze Et Al 2012

Figure 5 from Serreze et al 2012 [Time series (1979–2010) of monthly standardized anomalies (z-scores) in surface to 500 hPa precipitable water at the nine radiosonde sites, along with the linear trend line (shown in black), slope (z score per decade) and (in parentheses) the statistical significance.].

Serreze, M. C., A. P. Barrett, and J. Stroeve (2012), Recent changes in tropospheric water vapor over the Arctic as assessed from radiosondes and atmospheric reanalyses, J. Geophys. Res., 117, D10104, doi:10.1029/2011JD017421.

The abstract reads [highlight added]

Changes in tropospheric water vapor over the Arctic are examined for the period 1979 to 2010 using humidity and temperature data from nine high latitude radiosonde stations north of 70°N with nearly complete records, and from six atmospheric reanalyses, emphasizing the three most modern efforts, MERRA, CFSR and ERA-Interim. Based on comparisons with the radiosonde profiles, the reanalyses as a group have positive cold-season humidity and temperature biases below the 850 hPa level and consequently do not capture observed low-level humidity and temperature inversions. MERRA has the smallest biases. Trends in column-integrated (surface to 500 hPa) water vapor (precipitable water) computed using data from the radiosondes and from the three modern reanalyses at the radiosonde locations are mostly positive, but magnitudes and statistical significance vary widely between sites and seasons. Positive trends in precipitable water from MERRA, CFSR and ERA-Interim, largest in summer and early autumn, dominate the northern North Atlantic, including the Greenland, Norwegian and Barents seas, the Canadian Arctic Archipelago and (on the Pacific side) the Beaufort and Chukchi seas. This pattern is linked to positive anomalies in air and sea surface temperature and negative anomalies in end-of-summer sea ice extent. Trends from ERA-Interim are weaker than those from either MERRA or CFSR. As assessed for polar cap averages (the region north of 70°N), MERRA, CFSR and ERA-Interim all show increasing surface-500 hPa precipitable over the analysis period encompassing most months, consistent with increases in 850 hPa air temperature and 850 hPa specific humidity. Data from all of the reanalyses point to strong interannual and decadal variability. The MERRA record in particular shows evidence of artifacts likely introduced by changes in assimilation data streams. A focus on the most recent decade (2001–2010) reveals large differences between the three reanalyses in the vertical structure of specific humidity and temperature anomalies.

The conclusions include the text

On the basis of radiosonde profiles and output from the three latest generation atmospheric reanalyses (MERRA, CFSR and ERA-I), statistically significant trends in precipitable water over the Arctic as assessed over the period 1979–2010 are mostly positive. Trends from the three reanalyses are variously larger or smaller than radiosonde-based estimates. Trends are highly heterogeneous in space and time. The most consistent pattern between months and between the reanalyses is increasing precipitable water over the open waters of the northern North Atlantic, consistent with observed increases in sea surface temperature. Increases are also prominent over the Canadian Arctic Archipelago, especially in the summer months; the strong summer trends in this region are also seen in the radiosonde data. A feature common to all of the reanalyses is a region of positive trends in precipitable water centered over the Beaufort and Chuckchi seas in August and September, corresponding to where negative trends in end-of-summer summer sea ice extent have been most pronounced. These trend patterns mask considerable variability from year to year and from decade to decade.

The results presented here must be viewed with the caveat of uncertainties in both the radiosonde and the reanalysis data Obtaining accurate humidity data in polar regions from radiosondes has and will remain to be a daunting problem. Pointing to challenges of data assimilation in high latitudes, we have also shown that the reanalyses have moist and warm biases at and near the surface from autumn through spring, with smaller biases in summer.None of the reanalyses correctly capture the cold season humidity and temperature inversions seen in the radiosonde data. There are some substantial differences between MERRA, CFSR and ERA-I with respect to the vertical structure of recent (2001–2010 decade) anomalies in specific humidity and air temperature. We see evidence of unphysical features in the MERRA record, and numerous past studies have identified a slate of potential inconsistencies related to changes in data streams.

My Comments:

We need more such observational based studies. There are further implications from what Serreze et al 2012 have found:

1. An increase in water vapor in the Arctic and sub-Arctic (and its influence on the long-wave radiative fluxes) is one of the effects that is shown in

McNider, R.T., G.J. Steeneveld, B. Holtslag, R. Pielke Sr, S.   Mackaro, A. Pour Biazar, J.T. Walters, U.S. Nair, and J.R. Christy, 2012: Response and sensitivity of the nocturnal boundary layer over land to added longwave radiative forcing. J. Geophys. Res., doi:10.1029/2012JD017578, in press.

to result in a greater temperature increase in the air near the surface, than higher in the troposphere. Such a temperature increase near the surface overstates the magnitude of the top of the atmosphere radiative imbalance (i.e. global warming) when the surface temperature data is used for this purpose.

The importance of the stable boundary layer in the Arctic, and that even slight changes in vertical mixing can result in significant changes in near surface temperature without much temperature change elsewhere in the troposphere, appears to be a main reason for the divergence of the surface and lower tropospheric temperature trends that we documented in

Klotzbach, P.J., R.A. Pielke Sr., R.A. Pielke Jr.,  J.R. Christy, and R.T. McNider, 2009: An alternative explanation for differential temperature trends at the  surface and in the lower troposphere. J. Geophys. Res., 114, D21102, doi:10.1029/2009JD011841.

Klotzbach, P.J., R.A. Pielke Sr., R.A. Pielke Jr.,  J.R. Christy, and R.T. McNider, 2010: Correction to: “An alternative explanation for differential temperature trends at the  surface and in the lower troposphere. J. Geophys. Res., 114, D21102, doi:10.1029/2009JD011841″, J. Geophys. Res.,  115, D1, doi:10.1029/2009JD013655.

2. That Serreze et al 2012 found that “the reanalyses have moist and warm biases at and near the surface from autumn through spring, with smaller biases in summer“, indicates that their use to quantify the magnitude of global warming using their surface temperature data analyses, at least in this part of the world, is inappropriate.

3. We have examined the effect of an incremental increase in water vapor in the subarctic using a 1-D radiative transfer model as reported in the post

Relative Roles of CO2 and Water Vapor in Radiative Forcing

This analysis, completed by Norm Woods, concluded that with respect to an imposed 5% increase in water vapor for subarctic (summer and winter) and tropical climatological clear sky profiles that

The downwelling fluxes at the surface for the subarctic profile appear less sensitive to changes in carbon dioxide and water vapor concentrations than do the fluxes for the tropical and subarctic summer profiles.  The subarctic winter profile has a relatively weak lapse rate in the lowest part of the troposphere, so changes in the  position of the weighting function may have had little effect on the  downwelling fluxes. In addition, the water vapor amounts in the subarctic winter profile are considerably smaller than those in the two other profiles…due to the much lower atmospheric concentrations of water vapor in the subarctic winter sounding, the change from a zero concentration to its current value results in an increase of 116.46 Watts per meter squared, while adding 5% to the current value results in a 0.70 Watts per meter squared increase.

4.  The spatial analyses of global precipitable water reported in

Vonder Haar, T. H., J. Bytheway, and J. M. Forsythe (2012), Weather and climate analyses using improved global water vapor observations,
Geophys. Res. Lett.,doi:10.1029/2012GL052094, in press.

in which no global trend is seen in recent year; see

needs to assessed to ascertain if the Serreze et al radiosonde results are consistent with what is found from the satellite data with its greater spatial coverage.

Comments Off

Filed under Climate Change Metrics, Research Papers

Comments are closed.