Paul C. Knappenberger

A climatology of snowfall-temperature relationships in Canada

A better understanding of potential climate change impacts on the global hydrological cycle requires knowledge of the interaction between air temperature and water in its various forms. One important example is the effect of air temperature on snowfall. Proper parameterization of the snowfall-temperature relationship in climate models is essential for accurate prediction of future snowfall changes that might arise from high-latitude warming. On a climatological basis, at any location, air temperature and snowfall can be correlated positively (higher temperatures increase atmospheric moisture and snowfall through the Clausius-Clapeyron relationship) or negatively (precipitation falls as rain instead of snow). Examination of 50 years of monthly snowfall water equivalent and mean temperature data indicates that the snowfall-temperature relationship is positive in the high latitudes and negative in southern Canada, along both coasts, and east of the Rockies. The “zero line” (the transition zone north of which warmer months receive more snowfall than colder months) migrates southward from autumn to winter so that by January most of eastern and northwestern Canada has a positive snowfall-temperature slope. The primary exception to a straightforward relationship between slope and latitude occurs east of the Rockies, where anomalous negative slopes extend far to the north. In this region, dry, adiabatically warmed air from downslope or circulation effects may reduce the number of snow events and modify the slope of the snowfall-temperature curve. Based on first principles and Canadian observations, we develop a function relating temperature to snowfall that attempts to account for the complex spatial and seasonal nature of the snowfall-temperature relationship. Given the importance of snowfall in the global radiation balance, this analysis could be used to improve climate change predictions.

Comments on “Impacts of CO2-Induced Warming on Simulated Hurricane Intensity and Precipitation: Sensitivity to the Choice of Climate Model and Convective Scheme”

Abstract In a simulation of enhanced tropical cyclones in a warmer world, Knutson and Tuleya make several assumptions that are not borne out in the real world. They include an unrealistically large carbon dioxide growth rate, an overly strong relationship between sea surface temperature and hurricane intensity, and the use of a mesoscale model that has shown little to no useful skill in predicting current-day hurricane intensity. After accounting for these inaccuracies, a detectable increase in Atlantic hurricane intensity in response to growing atmospheric greenhouse gas levels during this century becomes unlikely.

Natural signals in the MSU lower tropospheric temperature record

Volcanic and ENSO forcings explain nearly two-thirds (63.5%) of the variance in monthly global temperature anomalies in the MSU lower tropospheric temperature observations from 1979 through 1999. While the raw trend in the drift-adjusted 1979–99 global data is a statistically significant warming of 0.055°C/decade, the combination of volcanism and ENSO account for 25.4% of this trend, or 0.014°C/decade. The remaining trend, 0.041°C/decade, is left unexplained, likely comprising a combination of factors including the anthropogenic alterations to the collection of the earths greenhouse gases, solar variability, and other forcings internal to the earth/atmosphere system. A recent widely cited attempt to explain the discrepancy between the rate of the observed increase in lower tropospheric MSU temperatures and the much larger value predicted by general circulation models weakens considerably when the known natural forcing mechanisms are more completely specified.

Observed changes in the diurnal temperature and dewpoint cycles across the United States

We analyzed hourly coterminous U.S. airport data, beginning in 1948, for changes in daily temperature and dewpoint regimes. We found an ubiquitous phase shift in the daytime cycle of warming, towards later in the day. Overall day and night temperature changes were very consistent with the results of {ital Karl} {ital et} {ital al}. [1993] even though this is an entirely different source of data, and one that is not specifically deurbanized, although we attempted to use stations that were not from downtown airports. We divided our results into eastern and western subsets as a first approximation for high and low sulfate aerosol conditions, and found evidence that was consistent with a sulfate effect on overall temperature, but inconsistent with modeled estimates of the effect of sulfates on the intradiurnal regime. It is difficult to establish the causes of the observed intradiurnal phase shift of warming time although urbanization and anthropogenic emissions are likely to be involved. The nature of our finding is subtle, and is consistent with other results that show very modest (and nonobvious) responses to greenhouse changes. Further research will ultimately clarify its causes and effects. {copyright} American Geophysical Union 1996

Disparity of tropospheric and surface temperature trends: New evidence

Received 9 April 2004; revised 27 May 2004; accepted 7 June 2004; published 9 July 2004. [1] Observations suggest that the earth’s surface has been warming relative to the troposphere for the last 25 years; this is not only difficult to explain but also contrary to the results of climate models. We provide new evidence that the disparity is real. Introducing an additional data set, R2 2-meter temperatures, a diagnostic variable related to tropospheric temperature profiles, we find trends derived from it to be in close agreement with satellite measurements of tropospheric temperature. This suggests that the disparity likely is a result of near-surface processes. We find that the disparity does not occur uniformly across the globe, but is primarily confined to tropical regions which are primarily oceanic. Since the ocean measurements are sea surface temperatures, we suggest that the disparity is probably associated with processes at the ocean-atmosphere interface. Our study thus makes unlikely some of the explanations advanced to account for the disparity; it also demonstrates the importance of distinguishing between land, sea and air measurements. INDEX TERMS: 1610 Global Change: Atmosphere (0315, 0325); 1699 Global Change: General or miscellaneous; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 4504 Oceanography: Physical: Air/sea interactions (0312). Citation: Douglass, D. H., B. D. Pearson, S. F. Singer, P. C. Knappenberger, and P. J. Michaels (2004), Disparity of tropospheric and surface temperature trends: New evidence, Geophys. Res. Lett., 31, L13207, doi:10.1029/ 2004GL020212.

Observed changes in the diurnal dewpoint cycles across North America

We analyzed the record of hourly dewpoint data from 92 climatological stations in North America. The data were centered with respect to sunrise and sunset and divided into four geographic regions of approximately equal area in an attempt to uncover regional trends. Few statistically significant regional trends in mean dewpoint temperatures were found. However, the within-day variations were very well behaved and consistent across regions. They showed a relative decline in dewpoints in the late afternoon at most stations, and small declines just after sunrise. The results are consistent with urbanization signals described by Oke [1987]. An observed rise in midday dewpoint, followed by a fall in late afternoon, indicates some regional enhancement of convection, which may in part be from the urban effect, but other causes are possible. Our dewpoint changes are somewhat consistent with precipitation changes that have described by other researchers.

Development of a Discriminant Analysis Mixed Precipitation (DAMP) Forecast Model for Mid-Atlantic Winter Storms

Abstract The frequency of “wintry mix” precipitation—freezing rain and ice pellets—is considerable in the mid-Atlantic region of the United States. Despite the fact that the general conditions necessary to support the various winter precipitation types have been known for years, in that region, the proper forecast of type and duration of winter precipitation is one of the most difficult challenges in operational meteorology, with extensive public safety and economic ramifications. The purpose of this project is to report on an improved methodology for winter precipitation forecasts. This study analyzed precipitation type and surface temperature data from NOAA’s hourly surface airway observations and temperatures and heights for all mandatory and significant levels from NOAA’s Radiosonde Data of North America from Washington Dulles International Airport, Virginia (1962–95), and Greensboro, North Carolina (1948–95). Precipitation that occurred within 2 h of a sounding for the months November through March w...

The United Nations Intergovernmental Panel on Climate Change and the Scientific “Consensus†on Global Warming

Climate data support the “moderate” prediction of climate change (l-1.5°C) rather than the more extreme scenario (4°C or more). The moderate point of view was originally marginalized in the IPCC “consensus” process in both the 1990 First Assessment on Climate Change and in the 1992 Update prepared specifically for the Earth Summit and to provide backing for the Rio Framework Convention on Climate Change. It is now accepted, based on ground-based data, that the errors in those models are currently between 160% and 360%. If one compares them to the satellite data combined with the land record, the error rises to a maximum of 720%. In some recognition of this massive error, the 1995 IPCC “consensus” is that warming has been mitigated by sulfate aerosols. However, when that hypothesis is specifically tested, it fails. Further, data required to test the validity of the sulfate enhanced greenhouse models was withheld by the IPCC. despite repeated requests.

Reply to comment by K. Emanuel on “Sea‐surface temperatures and tropical cyclones in the Atlantic basin”

[1] Most recent studies of tropical cyclones and their relationship to changing environmental conditions, including sea surface temperatures (SST), have examined regionalscale seasonal averages in comparison with basin-wide seasonal tropical cyclone characteristics [e.g., Emanuel, 2005; Webster et al., 2005; Hoyos et al., 2006]. In our recent paper [Michaels et al., 2006], we took a different approach. We examined the local SST (weekly averages contained in 1 1 degree gridcells) associated with each named tropical cyclone as it traversed the North Atlantic Ocean during the period 1982–2005. Aggregating the SST associated with the maximum storm intensity (as measured by wind speed) attained in each storm, we found that while overall (across the full range of SST) there exists a positive relationship between maximum storm intensity and SST, this relationship breaks down above 28.25C. We identified this temperature (28.25C) as both a threshold necessary to be reached for a tropical cyclone to reach major hurricane status (category 3 or higher on the Saffir-Simpson hurricane scale, sustained surface winds of at least 50 m/s), as well as the SST beyond which there is no longer a statistically significant relationship between maximum storm intensity and local SST in the collection of Atlantic tropical cyclones that make up our data set. We proposed that in a warming climate, higher SST would lead to a modest increase in mean maximum tropical cyclone wind speeds as an increasing number of storms encountered SST necessary to develop into major hurricanes. We noted that while we expected an increase in the number of major hurricanes, we did not expect to see an increase in the intensity of these storms. [2] In his comment to our paper, Emanuel [2007] has concerns as to whether spatial patterns of storm tracks may have led us to overestimate the SST/intensity relationship at low SST, and that the available sample size may be insufficient to allow us to detect a significant relationship between SST and storm intensity at higher SST. New analysis presented here, along with the discussion and analysis provided by Emanuel, have led to a better understanding of these issues.