Sean M. Davis

Contributions of Stratospheric Water Vapor to Decadal Changes in the Rate of Global Warming

Dropping a Notch Between 2000 and 2001, the concentration of water vapor in the stratosphere dropped by about 10%. Water vapor is an important greenhouse gas, so did the decrease affect climate and slow global warming? Solomon et al. (p. 1219, published online 28 January) used a combination of data and models to show that lower stratospheric water vapor probably has contributed to the flattening of global average temperatures since 2000, by acting to slow the rate of warming by about 25%. Furthermore, the amount of water vapor in the stratosphere probably increased between 1980 and 2000, a period of more rapid warming, suggesting how important the concentration of stratospheric water vapor might be to climate. Decreases in stratospheric water vapor after the year 2000 slowed the rate of increase in global surface temperature. Stratospheric water vapor concentrations decreased by about 10% after the year 2000. Here we show that this acted to slow the rate of increase in global surface temperature over 2000–2009 by about 25% compared to that which would have occurred due only to carbon dioxide and other greenhouse gases. More limited data suggest that stratospheric water vapor probably increased between 1980 and 2000, which would have enhanced the decadal rate of surface warming during the 1990s by about 30% as compared to estimates neglecting this change. These findings show that stratospheric water vapor is an important driver of decadal global surface climate change.

Global-scale black carbon profiles observed in the remote atmosphere and compared to models

[1] Refractory black carbon (rBC) aerosol loadings and mass size distributions have been quantified during the HIPPO campaign above the remote Pacific from 80N to 67S. Over 100 vertical profiles of rBC loadings, extending from ∼0.3 to ∼14 km were obtained with a Single-Particle Soot Photometer (SP2) during a two-week period in January 2009. The dataset provides a striking, and previously unobtainable, pole-to-pole snapshot of rBC mass loadings. rBC vertical concentration profiles reveal significant dependences on latitude, while associated rBC mass size distributions were highly uniform. The vertical profiles averaged in five latitude zones were compared to an ensemble of AEROCOM model fields. The model ensemble spread in each zone was over an order of magnitude, while the model average over-predicted rBC concentrations overall by a factor five. The comparisons suggest that rBC removal in global models may need to be evaluated separately in different latitude regions and perhaps enhanced.

A Multidiagnostic Intercomparison of Tropical-Width Time Series Using Reanalyses and Satellite Observations

Polewardmigrationofthe latitudinaledgeofthe tropicsof0.258‐3.08decade 21 hasbeenreportedin several recent studies basedon satellite andradiosondedata and reanalysis output coveringthe past ;30 yr. The goal ofthis paperistoidentifythe extenttowhichthis largerangeoftrendscanbeexplainedbytheuse ofdifferent datasources,timeperiods,andedgedefinitions,aswellashowthewideningvariesasafunctionofhemisphere and season. Toward this end, a suite of tropical edge latitude diagnostics based on tropopause height, winds, precipitation‐evaporation, and outgoing longwave radiation (OLR) are analyzed using several reanalyses and satellite datasets. These diagnostics include both previously used definitionsand new definitions designed for more robust detection. The wide range of widening trends is shown to be primarily due to the use of different datasets and edge definitions and only secondarily due to varying start‐end dates. This study also shows that the large trends (.;18 decade 21 ) previously reported in tropopause and OLR diagnostics are due to theuse of subjectivedefinitions basedon absolutethresholds.Statistically significant Hadleycell expansion based on the mean meridional streamfunction of 1.08‐1.58 decade 21 is found in three of four reanalyses that cover the full time period (1979‐2009), whereas other diagnostics yield trends of 20.58‐0.88 decade 21 that are mostly insignificant. There are indications of hemispheric and seasonal differences in the trends, but the differences are not statistically significant.

Stratospheric water vapor feedback

Significance We show observational evidence for a stratospheric water vapor feedback—a warmer climate increases stratospheric water vapor, and because stratospheric water vapor is itself a greenhouse gas, this leads to further warming. An estimate of its magnitude from a climate model yields a value of +0.3 W/(m2⋅K), suggesting that this feedback plays an important role in our climate system. We show here that stratospheric water vapor variations play an important role in the evolution of our climate. This comes from analysis of observations showing that stratospheric water vapor increases with tropospheric temperature, implying the existence of a stratospheric water vapor feedback. We estimate the strength of this feedback in a chemistry–climate model to be +0.3 W/(m2⋅K), which would be a significant contributor to the overall climate sensitivity. One-third of this feedback comes from increases in water vapor entering the stratosphere through the tropical tropopause layer, with the rest coming from increases in water vapor entering through the extratropical tropopause.

Influence of Tropical Tropopause Layer Cooling on Atlantic Hurricane Activity

AbstractVirtually all metrics of Atlantic tropical cyclone activity show substantial increases over the past two decades. It is argued here that cooling near the tropical tropopause and the associated decrease in tropical cyclone outflow temperature contributed to the observed increase in tropical cyclone potential intensity over this period. Quantitative uncertainties in the magnitude of the cooling are important, but a broad range of observations supports some cooling. Downscalings of the output of atmospheric general circulation models (AGCMs) that are driven by observed sea surface temperatures and sea ice cover produce little if any increase in Atlantic tropical cyclone metrics over the past two decades, even though observed variability before roughly 1970 is well simulated by some of the models. Part of this shortcoming is traced to the failure of the AGCMs examined to reproduce the observed cooling of the lower stratosphere and tropical tropopause layer (TTL) over the past few decades. The authors ...

Variations of stratospheric water vapor over the past three decades

We examine variations in water vapor in air entering the stratosphere through the tropical tropopause layer (TTL) over the past three decades in satellite data and in a trajectory model. Most of the variance can be explained by three processes that affect the TTL: the quasi-biennial oscillation, the strength of the Brewer-Dobson circulation, and the temperature of the tropical troposphere. When these factors act in phase, significant variations in water entering the stratosphere are possible. We also find that volcanic eruptions, which inject aerosol into the TTL, affect the amount of water entering the stratosphere. While there is clear decadal variability in the data and models, we find little evidence for a long-term trend in water entering the stratosphere through the TTL over the past 3 decades.

Improving stratospheric transport trend analysis based on SF6 and CO2 measurements

In this study we reexamine nearly four decades of in situ balloon-based stratospheric observations of SF6 and CO2 with an idealized model and reanalysis products. We use new techniques to account for the spatial and temporal inhomogeneity of the sparse balloon profiles and to calculate stratospheric mean ages of air more consistently from the observations with the idealized model. By doing so we are able to more clearly show and account for the variability of mean age of air throughout the bulk of the depth of the stratosphere. From an idealized model guided by the observations, we identify variability in the mean age due to the seasonal cycle of stratospheric transport, the quasi-biennial oscillation in tropical zonal winds, major volcanic eruptions, and linear trends that vary significantly with altitude. We calculate a negative mean age trend in the lowest 5 km of the stratosphere that agrees within uncertainties with a trend calculated from a set of chemistry climate model mean ages in this layer. The mean age trends reverse sign in the middle and upper stratosphere and are in agreement with a previous positive trend estimate using the same observational data set, although we have substantially reduced the uncertainty on the trend. Our analysis shows that a long time series of in situ profile measurements of trace gases such as SF6 and CO2 can be a unique and useful indicator of stratospheric circulation variability on a range of time scales and an important contributor to help validate the stratospheric portion of global chemistry climate models. However, with only SF6 and CO2 measurements, the competing effects on mean age between mean circulation and mixing (tropical entrainment) are not uniquely separable.

Measurement of Total Water with a Tunable Diode Laser Hygrometer: Inlet Analysis, Calibration Procedure, and Ice Water Content Determination

Abstract The University of Colorado closed-path tunable diode laser hygrometer (CLH), a new instrument for the in situ measurement of enhanced total water (eTW, the sum of water vapor and condensed water enhanced by a subisokinetic inlet), has recently been flown aboard the NASA DC-8 and WB-57F aircrafts. The CLH has the sensitivity necessary to quantify the ice water content (IWC) of extremely thin subvisual cirrus clouds (∼0.1 mg m−3), while still providing measurements over a large range of conditions typical of upper-tropospheric cirrus (up to 1 g m−3). A key feature of the CLH is its subisokinetic inlet system, which is described in detail in this paper. The enhancement and evaporation of ice particles that results from the heated subisokinetic inlet is described both analytically and based on computational fluid dynamical simulations of the flow around the aircraft. Laboratory mixtures of water vapor with an accuracy of 2%–10% (2σ) were used to calibrate the CLH over a wide range of water vapor mixi...

Comparisons of in situ measurements of cirrus cloud ice water content

Experiment (MidCiX) in 2004. At larger values of IWC (IWC > 10 mg m � 3 ), the three instruments agree, on average, to within 20%, which is of the order of their estimated instrumental uncertainties. At smaller IWC values ( 1), implying that IWC measurements in thicker clouds are of sufficient accuracy for validation studies.

Climatology of long‐range transported Asian dust along the West Coast of the United States

The contribution of trans-Pacific dust estimated from satellite observations has been shown to be 3 times greater than domestic dust in North America throughout the year. Thus, a quantitative understanding of the frequency and locations where Asian dust is transported is necessary to improve global dust modeling for weather and climate predictions. This work presents a 10 year record (2002–2011) of dust along the U.S. West Coast estimated from the Interagency Monitoring of Protected Visual Environments network in an effort to characterize the seasonal cycle and interannual variability of Asian dust transport. In addition, observations of dust exported from East Asia were analyzed along with air mass trajectories and satellite and ground-based precipitation data to investigate seasonal variability of Asian dust transport. On average, Asian dust concentrations (0.08–0.60 µg m−3) from ground-based observations were 1.7 times those of local dust (0.00–0.53 µg m−3) and 23% (up to 44%) of fine particulate matter (particles with diameters ≤ 2.5 micrometers, or PM2.5) mass concentrations at high elevations in the spring. The maximum in springtime Asian dust on the U.S. West Coast was attributed to higher source concentrations (10.98–36.27 µg m−3) and reduced potential for wet removal over the Pacific Ocean and U.S. West Coast. Although trans-Pacific transport was more favorable during the winter, minimum concentrations of Asian dust were observed on the U.S. West Coast (0.11 µg m−3) due to a lower source influence and higher potential for wet removal during transport. Multiobservational approaches such as these should be taken into account when modeling transport of Asian dust to the western U.S.