Eric S. Posmentier

Influence of sea ice on Arctic precipitation

Significance There has been a growing consensus that a decrease in sea ice would cause an increase in Arctic precipitation because of the potential for increased local evaporation. We quantify the effect of sea ice on the percentage of moisture sourced from the Arctic, using measurements of the isotopic composition of precipitation at six sites across the Arctic. These moisture proportion changes are important in that they indicate systematic adjustment and/or reorganization of the global hydrological cycle with climate change and provide validation for climate models. We explore how much these changes may increase Arctic precipitation and its impact on the energy balance. Global climate is influenced by the Arctic hydrologic cycle, which is, in part, regulated by sea ice through its control on evaporation and precipitation. However, the quantitative link between precipitation and sea ice extent is poorly constrained. Here we present observational evidence for the response of precipitation to sea ice reduction and assess the sensitivity of the response. Changes in the proportion of moisture sourced from the Arctic with sea ice change in the Canadian Arctic and Greenland Sea regions over the past two decades are inferred from annually averaged deuterium excess (d-excess) measurements from six sites. Other influences on the Arctic hydrologic cycle, such as the strength of meridional transport, are assessed using the North Atlantic Oscillation index. We find that the independent, direct effect of sea ice on the increase of the percentage of Arctic sourced moisture (or Arctic moisture proportion, AMP) is 18.2 ± 4.6% and 10.8 ± 3.6%/100,000 km2 sea ice lost for each region, respectively, corresponding to increases of 10.9 ± 2.8% and 2.7 ± 1.1%/1 °C of warming in the vapor source regions. The moisture source changes likely result in increases of precipitation and changes in energy balance, creating significant uncertainty for climate predictions.

The effect of acidification on the determination of elemental carbon, char-, and soot-elemental carbon in soils and sediments

We studied the influence of acid pretreatment on the effective distinction between elemental carbon (EC) and organic carbon (OC), and between char-EC and soot-EC. Though widely employed in the pretreatment of soils and sediments for EC quantification, the use of HCl, HF, and HNO(3) could decrease soot thermal stability as acid remains, leading to an underestimation of soot-EC by thermal methods. We compared thermal optical reflectance (TOR) measurements of EC concentrations in char reference materials and in lacustrine and marine sediments following pretreatment with various acids. The results showed that pretreatment with 2M HCl, concentrated HNO(3), 7 M HNO(3), and 1 M HNO(3) did not result in EC oxidation. However, hot concentrated HNO(3) oxidized EC significantly, leading to lower concentrations of EC, char-EC and soot-EC. By comparing the removal of potentially interfering materials, which contain little fire-derived carbon, with different acid pretreatments, we recommend the HCl-HF-HCl and concentrated (not hot) HNO(3)-HF-HCl pretreatments for the determination of EC, char-EC, and soot-EC in soils and sediments using the TOR method.

The Changes in North American atmospheric circulation patterns indicated by wood cellulose

General circulation model simulations suggest that during the Last Glacial Maximum, the northern circumpolar vortex intensifi ed and enlarged, a glacial anticyclone developed over the Laurentide Ice Sheet, and the position of the jet stream was shifted southward. However, observations directly related to shifts in wind patterns across the North American continent have not yet been reported. We examined tree-ring cellulose from the Holocene and the last glacial period for: (1) covariation between precipitation δ 18 O (and δD) and relative humidity, and (2) variation of cellulose δ 18 O and δD with longitude. Holocene isotopic features are consistent with modern moisture trajectories. The isotopic features during the last glaciation are dissimilar to those in the Holocene, and constitute direct evidence for an expansion of the polar easterlies to latitudes as low as 40°N. This is the fi rst time that moisture transport patterns have been inferred from covariation between isotopic composition in precipitation and relative humidity, a technique that holds much promise for future studies of atmospheric circulation.

Infrasound at Long Range from Saturn V, 1967

Two distinct groups of infrasonic waves from Saturn V, 1967, were recorded at Palisades, New York, 1485 kilometers from the launch site. The first group, of 10-minute duration, began about 70 minutes after launch time; the second, having more than twice the amplitude and a duration of 9 minutes, commenced 81 minutes after launch time. From information on the Saturn V trajectory and analysis of recorded data, it is established that the first group represents sound emitted either by the first stage reentry or by the second stage when its elevation was above 120 kilometers. The second, more intense wave group represents the sound from the powered first stage. A reversal of signal occurs because the rocket outran its own sound. Fourier analyses indicate that the energy extends to relatively long periods—10 seconds for the first stage and 7 seconds for the second. Trapping of sound in the upper atmospheric sound channel can be the cause of the separation of the signal into two distinct groups.

The diel cycle of water vapor in west Greenland

We present a study of the dynamics of small-scale (~100 km) atmospheric circulation in west Greenland which is dominated by interactions of marine and continental air masses. Water vapor concentration and isotopic ratios measured continuously over a 25 day period in Kangerlussuaq, Greenland were used to monitor the convergence of easterly katabatic winds and westerly sea breezes that form a front between the dry, isotopically depleted, glacial air mass and the moist, isotopically enriched, marine air mass. During the latter 16 days of the measurement period, an interval with no large-scale synoptic interference, the inland penetration of the sea breeze controlled the largest day-to-day humidity and vapor isotopic variations. Kangerlussuaq experienced sea breezes in the afternoon on 9 days, consistent with the long-term average of such occurrences on 56% of days in July and August. The inland position of the sea breeze front is controlled by the katabatic wind strength, which is stronger during times of reduced cloud coverage and/or higher-pressure gradient between the coast and the Greenland ice sheet. The position and movement of the front will likely respond to changes in the general atmospheric circulation and regional radiation balance resulting from global warming, which will, in turn, impact the local hydrological cycle and ecosystem processes.

Patterns of Evaporation and Precipitation Drive Global Isotopic Changes in Atmospheric Moisture

Because water isotope ratios respond to phase changes during evaporation (E) and precipitation (P), they are candidate fingerprints of changing atmospheric hydrology. Moreover, through preservation in ice cores and other paleoproxies, they provide important insight into the past. Still, there is disagreement over what specific attributes of hydroclimate variability isotopes reveal. Here we argue that variations in zonal mean isotope ratios of water vapor and precipitation are largely a response to geographically shifting patterns of E and P. Differences in the relative importance of local versus remote changes in these moisture variables explain the apparent distinct isotopic sensitivities to temperature and precipitation amount in high and low latitudes, respectively. Not only does our work provide a unified framework for interpreting water isotopic measurements globally, but it also presents a novel approach for diagnosing water cycle changes in a warmer world. Plain Language Summary Observations that track changes in the water cycle are critical for improving our understanding of the climate system. Particularly important are measurements that can verify whether imbalances in evaporation and precipitation increase in response to global warming. Because the isotope ratios of hydrogen and oxygen in water vapor and precipitation vary with rates of evaporation and precipitation, they are candidate fingerprints of water cycle changes. Here we use a simple mass balance model to evaluate the isotopic response of water vapor in the atmosphere to spatial variations in evaporation and precipitation.We find that these spatial patterns shape the isotope ratios by changing two critical factors: the efficiency with which precipitation dries the atmosphere and the probability that moisture is transported downwind (rather than rained out). These two factors suggest that isotope ratios are influenced both by local and by remote changes in evaporation and precipitation. Low-latitude isotope ratios respond largely to local imbalances in evaporation and precipitation, while high-latitude isotope ratios depend more on what happens “upstream.” These findings provide key guidance for interpreting climate changes of the past and offer a novel approach for diagnosing water cycle changes in a warmer future.

Testing a Novel Method for Initializing Air Parcel Back Trajectories in Precipitating Clouds Using Reanalysis Data

AbstractLagrangian air parcel tracking is a powerful tool for estimating vapor source locations, particularly for isotope hydrology applications. Identified vapor source regions may be sensitive to the distribution of altitudes at which back trajectories are initiated. Ideally, those initial altitudes should reflect the altitudes where precipitation forms. This paper introduces a novel method for estimating these heights from reanalysis data and an air parcel lofting routine, which is referred to as the “Reanalysis” method. Using Barrow, Alaska (now known as Utqiaġvik), as a test site, the study compares the distribution of air parcel initiation heights and vapor source conditions from back trajectories initiated at 1) heights determined by the Reanalysis method and 2) heights acquired from 35-GHz vertically resolved cloud radar, termed the “Cloud Radar” method. Only 2 of the 70 events failed to produce condensation at any elevation. The distribution of air parcels generated by each method was compared on...