bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface albedo feedbacks from climate variability and change
Authors: Colman R. A.
Published: April 2013

Snow and sea ice surface albedo feedback is evaluated in models from the World Climate Research Program Coupled Model Intercomparison Project phase 3 using the radiative “kernel” technique. A comparison is carried out between feedbacks at climate change timescales and those operating under seasonal, interannual, and decadal variability. A primary motivation is to examine possible relationships across these timescales. Similarities are found in the mean model meridional distribution of feedback, although uncertainties are large due to model spread and uncertainties from regressions. Climate change feedback appears stronger over the Arctic north of 75°N. Consistent with findings elsewhere, climate change feedback strengths over Northern Hemisphere (NH) snow and sea ice are uncorrelated, although a weak correlation is found between NH and Southern Hemisphere (SH) sea ice. By contrast, at other timescales, moderate positive correlations are found between NH snow and sea ice albedo feedbacks, as well as between NH and SH sea ice albedo feedbacks. Statistically significant correlations are not found between surface albedo feedbacks at climate change and other timescales, either for global feedback or for NH and SH sea ice. Notably, however, correlations are found for NH snow albedo feedback against both seasonal and interannual feedbacks. This suggests that NH snow albedo feedback is a “special case,” potentially revealing short‐term analogues appropriate for close comparison with climate change, with such relationships not holding for other components of the surface albedo feedback.
Multi‐model analysis of Northern Hemisphere winter blocking: Model biases and the role of resolution
Authors: Anstey James A. , Davini Paolo , Gray Lesley J. , Woollings Tim J. , Butchart Neal , Cagnazzo Chiara , Christiansen Bo , Hardiman Steven C. , Osprey Scott M. , Yang Shuting
Published: April 2013

Blocking of the tropospheric jet stream during Northern Hemisphere winter (December‐January‐February) is examined in a multi‐model ensemble of coupled atmosphere‐ocean general circulation models (GCMs) obtained from the Coupled Model Intercomparison Project Phase 5 (CMIP5). The CMIP5 models exhibit large biases in blocking frequency and related biases in tropospheric jet latitude, similar to earlier generations of GCMs. Underestimated blocking at high latitudes, especially over Europe, is common. In general, model biases decrease as model resolution increases. Increased blocking frequency at high latitudes in both the Atlantic and Pacific basins, as well as more realistic variability of Atlantic jet latitude, are associated with increased vertical resolution in the mid‐troposphere to lowermost stratosphere. Finer horizontal resolution is associated with higher blocking frequency at all latitudes in the Atlantic basin but appears to have no systematic impact on blocking near Greenland or in the Pacific basin. Results from the CMIP5 analysis are corroborated by additional controlled experiments using selected GCMs.
BLUE‐based NO 2 data assimilation at urban scale
Authors: Tilloy Anne , Mallet Vivien , Poulet David , Pesin Céline , Brocheton Fabien
Published: April 2013

We aim at optimally combining air quality computations, from the Gaussian model ADMS Urban, and ground observations at urban scale. An ADMS simulation generated NO2 concentration fields across Clermont‐Ferrand (France) down to street level, every 3 h for the full year 2008. A monitoring network composed of nine fixed stations provided hourly observations to be assimilated. Every 3 h, we compute the so‐called BLUE (best linear unbiased estimator), which is a concentration field merging ADMS outputs and ground observations. Its error variance is supposed to be minimal under given assumptions regarding the errors on observations and model simulations. A key step lies in the modeling of error covariances between the computed NO2 concentrations across the city. We introduce a parameterized covariance which heavily relies on the road network. The covariance between two locations depends on the distance of each location to the road network and on the distance between the locations along the road network. Efficient parameters for the covariances are primarily chosen according to prior assumptions, χ2 diagnosis and leave‐one‐out cross‐validations. According to the cross‐validations, the improvements due to the assimilation seem moderately far from the observation network, but the root mean square error roughly decreases by 30–50% in the main city where the station density is high. The method is computationally tractable for the generation of improved concentration fields over a long period, or for day‐to‐day forecasts.
Observed vertical redistribution of black carbon and other insoluble light‐absorbing particles in melting snow
Authors: Doherty Sarah J. , Grenfell Thomas C. , Forsström Sanja , Hegg Dean L. , Brandt Richard E. , Warren Stephen G.
Published: April 2013

Light‐absorbing impurities in snow reduce snow albedo, producing a positive radiative forcing, warming the surface air and snowpack, and accelerating snow melt. As the snow melts, black carbon (BC) and other insoluble light‐absorbing particulate impurities (ILAP) are retained at the snow surface because their scavenging efficiency with meltwater is <100%, so concentrations of ILAP in surface snow increase with snow melt, further reducing snow albedo. The magnitude of this positive feedback depends on the scavenging efficiency of BC and other ILAP with snow meltwater. We present results from field measurements of the vertical distribution of BC and other ILAP in snow near Barrow (Alaska), the Dye‐2 station in Greenland and Tromsø (Norway) during the melt season. Amplification factors due to melt are calculated for the concentrations in surface snow of BC and all ILAP. At Barrow and Dye‐2, melt scavenging rates are estimated. Melt amplification appears generally to be confined to the top few centimeters of the snowpack, where it increases concentrations of BC and other ILAP by up to a factor of about five. Scavenging fractions of ILAP due to percolation of meltwater are estimated at 10–30%, with the rates for BC being comparable or a few percent lower. The lack of distinction may result from the particles in snow being internal mixtures of both BC and other ILAP, so that scavenging efficiencies for these internally mixed particles are determined by the total particle size and hydrophobicity rather than being different for individual particle components.
Geomagnetic activity signatures in wintertime stratosphere wind, temperature, and wave response
Authors: Seppälä A. , Lu H. , Clilverd M. A. , Rodger C. J.
Published: April 2013

We analyzed ERA‐40 and ERA Interim meteorological re‐analysis data for signatures of geomagnetic activity in zonal mean zonal wind, temperature, and Eliassen‐Palm flux in the Northern Hemisphere extended winter (November–March). We found that for high geomagnetic activity levels, the stratospheric polar vortex becomes stronger in late winter, with more planetary waves being refracted equatorward. The statistically significant signals first appear in December and continue until March, with poleward propagation of the signals with time, even though some uncertainty remains due to the limited amount of data available ( ∼ 50 years). Our results also indicated that the geomagnetic effect on planetary wave propagation has a tendency to take place when the stratosphere background flow is relatively stable or when the polar vortex is stronger and less disturbed in early winter. These conditions typically occur during high solar irradiance cycle conditions or westerly quasi‐biennial oscillation conditions.
Statistical decision analysis for flight decision support: The SPartICus campaign
Authors: Hanlon Christopher J. , Stefik Jason B. , Small Arthur A. , Verlinde Johannes , Young George S.
Published: April 2013

Field campaigns in atmospheric science typically require making challenging decisions about how best to deploy limited resources, especially aircraft flight hours. Algorithmic decision tools have shown the potential to outperform traditional heuristic approaches to allocating limited flight hours in field campaigns. The present study examines the utility of algorithmic decision tools in an application to the Atmospheric Radiation Measurement (ARM) Small Particles in Cirrus (SPartICus) campaign, which sampled cirrus clouds over the ARM Southern Great Plains (SGP) site between January and June 2010. Probabilistic forecasts of suitable data collection conditions were generated using relative humidity forecasts from the Global Forecast System (GFS) and self‐organizing maps. An optimization procedure based on dynamic programming was then used to generate day‐ahead fly/no‐fly decisions for research flights over the SGP site. The quality of flight decisions thus generated were compared with those made by the SPartICus science team. Results showed that the algorithmic decision tool would have delivered 11% more optimal data while shortening the length of the campaign season by 29 days and reducing the per‐day expenditure of investigator time on activities of forecasting and decision‐making.
Polar lows over the Nordic and Labrador Seas: Synoptic circulation patterns and associations with North Atlantic‐Europe wintertime weather regimes
Authors: Mallet Paul‐Etienne , Claud Chantal , Cassou Christophe , Noer Gunnar , Kodera Kunihiko
Published: April 2013

Polar Lows (PLs) are intense meso‐cyclones forming in winter at high latitudes over open water. Using several datasets of PLs over the North Atlantic, the synoptic environment conducive to their development is determined. The 500 hPa geopotential height, the difference between the sea surface temperature and the 500 hPa air temperature, the near‐surface wind and air temperature, and the 300 hPa potential vorticity present significant anomaly patterns over large areas centered over PL genesis zones, suggesting cold air outbreaks and stratospheric intrusions. PLs develop within a northerly flow in the Norwegian Sea, a northeasterly flow in the Barents Sea and a westerly flow in the Labrador Sea. PLs form after a certain build‐up, the outbreak day being marked by strong winds and PV intensification. The relationship between PLs and daily weather regimes over North Atlantic‐Europe is then investigated. Regimes have a typical lifetime of 8–10 days, similar to the large‐scale anomalies associated with PLs. Over the Norwegian and Barents seas from 1999 to 2011, 37% of PLs are observed during the Atlantic Ridge regime (AR) and 28% in the negative phase regime of the North Atlantic Oscillation (NAO), whereas PL probability of occurrence is reduced by half for the positive phase of NAO and the Scandinavian blocking (SB) regimes. Over the Labrador Sea, most PLs occur during NAO+ while they are almost absent during NAO−. Demonstrating the temporal variation of key factors based on an updated dataset and relating PLs to weather regimes will introduce novel and important elements in PL forecasting methodology.
The radar tropopause above Svalbard 2008–2012: Characteristics at various timescales
Authors: Hall C. M.
Published: April 2013

Determinations of the tropopause altitude over Adventdalen, Svalbard (78°N, 16°E) have been assembled for the period 2008–2012. These reveal characteristics at various timescales: interannual, seasonal, and stochastic. First, it is established that the inclusion of considerably more data does not alter earlier findings of good temporal agreement with radiosonde measurements, that the radar tropopause is located at a fairly constant altitude above the meteorological tropopause, and that the seasonal variation lags the surface temperature variation by approximately 1 month, indicating a degree of top‐down control from the stratosphere. Over the longest timescale available, i.e., 5 years, we find an increase in tropopause altitude and identify this with increasing solar flux during the growth phase of solar cycle 24, and contrary to the seasonal variation, there is evidence that this is controlled by troposphere warming. At stochastic timescales, the scaling characteristics of the signal are examined, and in an exploratory investigation, the Hurst exponent is found to be ~0.82, which is in excellent agreement with independent findings for high latitude surface temperature data and therefore supportive of a complexity matching approach to identification of causal relationships between atmospheric metrics.
Distinct impact of tropical SSTs on summer North Pacific high and western North Pacific subtropical high
Authors: Yun Kyung‐Sook , Yeh Sang‐Wook , Ha Kyung‐Ja
Published: April 2013

The distinct impact of tropical Indian Ocean (IO) and western Pacific (WP) sea surface temperatures (SSTs) after the El Niño winter has been investigated in relation to the summer North Pacific high (NPH) and western North Pacific subtropical high (WNPSH). After the El Niño winter, warming of the IO leads to a summer eastern Pacific (EP) SST anomaly distinct from the cooling of WP; EP cooling occurs in the extreme IO warming case and EP warming in the WP cooling case. Both the warming of the IO and cooling of the WP are responsible for the development of the WNPSH, whereas the summer EP cooling induces an enhanced NPH, especially if it coexists with IO warming. The IO warming triggers an abrupt termination of the El Niño event by causing the easterly anomaly in the WP, which leads to the coexistence of IO warming and EP cooling during the boreal summer. The tropical EP cooling may change the North Pacific SST anomalies via the atmospheric bridge and consequently strengthen the extratropical NPH. The experimental results, which have been obtained from the use of atmospheric general circulation model, support the distinct roles of EP cooling on the NPH and of IO warming and WP cooling on the WNPSH. This finding suggests that the combined effect of IO warming and EP cooling generates a coupled pattern of NPH and WNPSH.
Influence of urban morphometric modification on regional boundary‐layer dynamics
Authors: Chan Allen , Fung Jimmy C. H. , Lau Alexis K. H.
Published: April 2013

Fidelity in simulating urban boundary‐layer (UBL) physics is recognized to prescribe the prognostic skill of subsequent regional air pollutant transport modeling. Conventional mesoscale meteorological models (MMM) deployed over the South China coast among urban locales have often yielded positive bias in surface wind speed. This bias has been hypothetically attributed to model parameterizations that yield inaccurate meteorological predictions due to underrepresentation of urban aerodynamic roughness. Chemical transport model (CTM) simulations that are forced by the overestimated UBL wind field may undergo excessive advection which results in negative bias in predicted pollutant concentration. This study aimed to corroborate the proposed causality between parameterized urban morphometry and UBL meteorology. Focus was placed on the urban meteorological adjustments induced by urban morphometry modifications rather than prediction improvements attributable to urban canopy parameterization (UCP). Case studies were devised to assess the sensitivity of an urban‐meteorology model to a pervasive, region‐wide urban morphometry modification. Performance of a UCP scheme was evaluated for the Pearl River Delta (PRD) region, a meso‐ β‐scale subtropical coastal megalopolis. To benchmark the limits of UBL adjustments that were predominantly attributable to urban morphometric transformation, numerical experiments were conducted against two urban fabrics of vastly dissimilar morphometric compositions, each occupying identical topographic tracts. Differences in the diurnal evolution of UBL structure and in the mean and turbulent flow characteristics were analyzed. This UCP sensitivity study suggests that improved urban morphological realism is able to reduce positive wind speed bias observed in conventional mesoscale meteorological models when applied to the PRD region.
Tree ring–based seven‐century drought records for the Western Himalaya, India
Authors: Yadav Ram R.
Published: April 2013

The paucity of available instrumental climate records in cold and arid regions of the western Himalaya, India, hampers our understanding of the long‐term variability of regional droughts, which seriously affect the agrarian economy of the region. Using ring width chronologies of Cedrus deodara and Pinus gerardiana together from a network of moisture‐stressed sites, Palmer Drought Severity Index values for October–May back to 1310 A.D. were developed. The twentieth century features dominant decadal‐scale pluvial phases (1981–1995, 1952–1968, and 1918–1934) as compared to the severe droughts in the early seventeenth century (1617–1640) as well as late fifteenth to early sixteenth (1491–1526) centuries. The drought anomalies are positively (negatively) associated with central Pacific (Indo‐Pacific Warm Pool) sea surface temperature anomalies. However, non‐stationarity in such relationships appears to be the major riddle in the predictability of long‐term droughts much needed for the sustainable development of the ecologically sensitive region of the Himalayas.
A score‐based method for assessing the performance of GCMs: A case study of southeastern Australia
Authors: Fu Guobin , Liu Zhaofei , Charles Stephen P , Xu Zongxue , Yao Zhijun
Published: April 2013

A multi‐criteria score‐based method is developed to assess General Circulation Model (GCM) performance at the regional scale. Application of the method assessing 25 GCM simulations of monthly mean sea level pressure (MSLP) and air temperature, and monthly and annual rainfall over the southeastern Australia region for 1960/1961–1999/2000 indicate that GCMs usually simulate monthly temperature better than monthly rainfall and mean sea level pressure. For example, the mean observed annual temperature for the study region is 16.7°C while the median and mean values of 25 GCMs are 16.8 and 16.9°C, respectively, and 24 GCMs (except BCC:CM1) can reproduce the annual cycle of temperature accurately, with a minimum correlation coefficient of 0.99. In contrast, the mean observed annual rainfall for the study region is 502 mm, whereas the GCM values vary from 195 to 807 mm, and 12 out of 25 GCMs produce a negative correlation coefficient of monthly rainfall annual cycle. However, GCMs overestimate trend magnitude for temperature, but underestimate for rainfall. The observed annual temperature trend is +0.007°C/yr, while both the median and mean GCM values are +0.013°C/yr, which is almost double the observed magnitude. The observed annual rainfall trend is +0.62mm/yr, while the median and mean values of 25 GCMs are 0.21and 0.36mm/yr, respectively. This demonstrates the advantages of using multi‐criteria to assess GCMs performance. The method developed in this study can easily be extended to different study regions and results can be used for better informed regional climate change impact analysis.
Luminosity of initial breakdown in lightning
Authors: Stolzenburg M. , Marshall T. C. , Karunarathne S. , Karunarathna N. , Vickers L. E. , Warner T. A. , Orville R. E. , Betz H.‐D.
Published: April 2013

Time correlated high‐speed video and electromagnetic data for 15 cloud‐to‐ground and intracloud lightning flashes reveal bursts of light, bright enough to be seen through intervening cloud, during the initial breakdown (IB) stage and within the first 3 ms after flash initiation. Each sudden increase in luminosity is coincident with a CG type (12 cases) or an IC type (3 cases) IB pulse in fast electric field change records. The E‐change data for 217 flashes indicate that all CG and IC flashes have IB pulses. The luminosity bursts of 14 negative CG flashes occur 11–340 ms before the first return stroke, at altitudes of 4–8 km, and at 4–41 km range from the camera. In seven cases, linear segments visibly advance away from the first light burst for 55–200 µs, then the entire length dims, then the luminosity sequence repeats along the same path. These visible initial leaders or streamers lengthen intermittently to about 300–1500 m. Their estimated 2‐D speeds are 4–18 × 105 m s−1 over the first few hundred microseconds and decrease by about 50% over the first 2 ms. In other cases, only a bright spot or a broad area of diffuse light, presumably scattered by intervening cloud, is visible. The bright area grows larger over 20–60 µs before the luminosity fades in about 100 µs, then this sequence may repeat several times. In several flashes, a 1–2 ms period of little or no luminosity and small E‐change is observed following the IB stage prior to stepped leader development.
High‐resolution multisite daily rainfall projections in India with statistical downscaling for climate change impacts assessment
Authors: Salvi Kaustubh , S. Kannan , Ghosh Subimal
Published: April 2013

Climate change impacts assessment involves downscaling of coarse‐resolution climate variables simulated by general circulation models (GCMs) using dynamic (physics‐based) or statistical (data‐driven) approaches. Here we use a statistical downscaling technique for projections of all‐India monsoon rainfall at a resolution of 0.5° in latitude/longitude. The present statistical downscaling model utilizes classification and regression tree, and kernel regression and develops a statistical relationship between large‐scale climate variables from reanalysis data and fine‐resolution observed rainfall, and then applies the relationship to coarse‐resolution GCM outputs. A GCM developed by the Canadian Centre for Climate Modeling and Analysis is employed for this study with its five ensemble runs for capturing intramodel uncertainty. The model appears to effectively capture individual station means, the spatial patterns of the standard deviations, and the cross correlation between station rainfalls. Computationally expensive dynamic downscaling models have been applied for India. However, our study is the first to attempt statistical downscaling for the entire country at a resolution of 0.5°. The downscaling model seems to capture the orographic effect on rainfall in mountainous areas of the Western Ghats and northeast India. The model also reveals spatially nonuniform changes in rainfall, with a possible increase for the western coastline and northeastern India (rainfall surplus areas) and a decrease in northern India, western India (rainfall deficit areas), and on the southeastern coastline, highlighting the need for a detailed hydrologic study that includes future projections regarding water availability which may be useful for water resource policy decisions.
A study of the long‐term variability of thunderstorm days in southeast Brazil
Authors: Pinto O. , Pinto I. R. C. A. , Ferro M. A. S.
Published: April 2013

In this report thunderstorm day monthly records obtained in three cities of southeast Brazil (São Paulo, Campinas, and Rio de Janeiro) since the 19th century are analyzed. The analysis is complemented by the spatial distribution of lightning in the last decade. For São Paulo and Campinas, data indicate a significant increase in thunderstorm activity during the period from the end of the 19th century to the present, simultaneously to an increase in the surface temperature well correlated to the population growth of the cities. This result did not match anything expected from natural climate cycles and gives strong observational evidence for the anthropogenic influence on the thunderstorm activity. For Rio de Janeiro, data did not show a significant positive trend from the middle of the 19th century to the present in spite of the increase in the surface temperature, suggesting that variations are most probably a result of a complex combination of local and large‐scale features. In addition, a statistical analysis of the data after 1951 shows that a significant increase (by a factor of 3.7) in the thunderstorm activity in Rio de Janeiro occurs for the simultaneous occurrence of a positive anomaly of the South Atlantic sea surface temperature and La Niña, compared to the simultaneous occurrence of a negative anomaly and El Niño, even though no significant variation was found when each large‐scale phenomena occurs isolated. The same occurs for São Paulo and Campinas, although with a lower amplitude.
Impacts of global warming on Northern Hemisphere winter storm tracks in the CMIP5 model suite
Authors: Eichler Timothy Paul , Gaggini Natalie , Pan Zaitao
Published: April 2013

A key question in assessing how global warming may affect climate is how it may impact day‐to‐day weather. To help answer this question, we evaluate the frequency and intensity of northern hemisphere storm tracks in the National Center for Climate Prediction reanalysis I dataset, and the historical, RCP4.5, and RCP8.5 climate scenarios featured in the CMIP5 simulations. We found that a warmer climate resulted in a general decrease in storm frequency in midlatitudes, especially in RCP8.5. In contrast, frequency trends in the reanalysis data reflected an increase in the North Pacific consistent with a shift towards a positive Pacific Decadal Oscillation and more frequent El Niño events post mid‐1970s. An examination of frequency and intensity trends in the active storm track regions of the North Pacific and North Atlantic showed that a significant decrease in storm track frequency was evident for RCP8.5. In contrast, intensity trends were dichotomous, with RCP8.5 exhibiting an increase in intensity in the North Atlantic active storm track region and a decrease in intensity in the North Pacific active storm track region. Poleward of these regions, a significant decrease in storm intensity in the North Atlantic and a significant increase in intensity in the North Pacific in RCP8.5 occurred. We also examined the intensity distribution of storms in the active storm track regions of the North Atlantic and North Pacific and determined that the models produced weaker storms with reduced variability relative to reanalysis data regardless of external climate forcing.
A climatology of polar winter stratopause warmings and associated planetary wave breaking
Authors: Greer K. , Thayer J. P. , Harvey V. L.
Published: April 2013

This work presents a climatology of synoptic‐scale disturbances in the upper stratosphere lower mesosphere (USLM) based on 20.5 years of assimilated data analyses from the U. K. Meteorological Office (1991–2012). USLM disturbance criteria are established, based on stratopause warmings at the 2 hPa level, to create climatologies in both hemispheres that delineate their timing, frequency, and geographic location. USLM disturbances occur on average 2.3 times per winter in the Northern Hemisphere (NH) (November through March) and 1.6 times per winter in the Southern Hemisphere (SH) (May through September), persist on average for 8 days in the NH and only 4 days in the SH, occur most frequently in December (July) in the Northern (Southern) Hemisphere, and are predominantly located in the longitude sector between 0oE and 90oE in both hemispheres. This is the first work to show that all major Sudden Stratospheric Warmings (SSWs) over the 20.5 year data record are preceded by USLM disturbances. One third of USLM disturbances evolve into a major SSW; only 22% of minor SSWs evolve into a major SSW. USLM disturbances and minor SSWs illustrate, at times, similar occurrence statistics, but the minor warming criteria seem to include a more diverse range of dynamical conditions. USLM disturbances are more specific in their dynamical construct with strong baroclinicity being a necessary condition. Potential vorticity analysis indicates that all USLM events occur with planetary wave breaking and that subsequent baroclinic instability may lead to the development of USLM disturbances.
Observations of compact intracloud lightning discharges in the northernmost region (51°N) of China
Authors: Lü Fanchao , Zhu Baoyou , Zhou Helin , Rakov Vladimir A. , Xu Weiwei , Qin Zilong
Published: April 2013

Using a three‐station wideband electric and magnetic field measuring network in conjunction with VHF receivers, we recorded the so‐called compact intracloud discharges (CIDs) in the Greater Khingan Range (51°N) in Northeast China. This type of lightning discharge had never been observed in such high‐latitude regions before. During the summer seasons of 2009 and 2010, a total of 493 CIDs with positive electric field pulses (negative charge moving up) occurring in 31 thunderstorms was recorded, with 87% (27/31) of CID‐producing storms generating less than 20 CIDs. The CID occurrence observed at 51°N appears to differ significantly from that in most lower latitude regions. Specifically, CIDs accounted only for 0.034% of the total of 1.4 million recorded lightning events, and no CIDs with negative electric field pulses (positive charge moving up) were observed. A total of 202 CIDs was located and they clustered at altitudes ranging from 5 to 12 km above ground level (agl), with a median height of 7.9 km agl. The effective ionosphere heights ranged from 75 to 95 km during the time period of 15:00–06:00 (local time). The median maximum heights of the 30 dBZ radar echo for convective cells with CIDs and without CIDs were 10.8 and 9.4 km, respectively. We infer that a relatively small vertical extent of thunderclouds, limited by the height of the tropopause, and a lack of vigorous convective surges in higher‐latitude regions might be responsible for the paucity or absence of CIDs in general and apparent suppression of those with negative electric field pulses in particular.
Projections of extreme precipitation events in regional climate simulations for Europe and the Alpine Region
Authors: Rajczak J. , Pall P. , Schär C.
Published: April 2013

Regional climate models (RCMs) from the ENSEMBLES project are analyzed to assess projected changes in 21st century heavy and extreme precipitation events over Europe. A set of 10 RCMs with horizontal grid spacing of 25 km is considered, which are driven by six GCMs under an A1B greenhouse gas scenario. The diagnostics include basic precipitation indices (including mean, wet‐day frequency, intensity, and percentile exceedance) and application of generalized extreme value theory for return periods up to 100 years. Changes in precipitation climate between present (1970–1999) and future (2070–2099) conditions are presented on a European scale and in more detail for 11 European regions (mostly in supplemental figures). On the European scale, projections show increases (decreases) in mean amounts and wet‐day frequency in northern (southern) Europe. This pattern is oscillating with the seasonal cycle. Changes in extremes exhibit a similar pattern, but increases in heavy events reach much further south. For instance, during spring and fall, much of the Mediterranean is projected to experience decreases in mean precipitation but increases in heavy events. Thus, projected changes in mean and extremes may show different signals. The inter‐model spread is partly attributable to a GCM‐dependent clustering of the climate change signal, but also affected by RCM uncertainties, in particular in summer. Despite these uncertainties, many of the projected changes are statistically significant and consistent across models. For instance, for the Alps, all models project an intensification of heavy events during fall, and these changes are statistically significant for a majority of the models considered.
Soil respiration in an old‐growth subtropical forest: Patterns, components, and controls
Authors: Tan Zheng‐Hong , Zhang Yi‐Ping , Liang Naishen , Song Qing‐Hai , Liu Yu‐Hong , You Guang‐Yong , Li Lin‐Hui , Yu Lei , Wu Chuan‐Shen , Lu Zhi‐Yun , Wen Han‐Dong , Zhao Jun‐Fu , Gao Fu , Yang Lian‐Yan , Song Liang , Zhang Yong‐Jiang , Munemasa Teramoto , Sha Li‐Qing
Published: April 2013

The patterns, components, and controls of soil respiration in an old‐growth subtropical forest were investigated using an automatic chamber system. We measured soil respiration in three treatments (control, trenching, litter removal) over 15 months. The annual total soil respiration (1248 gC m–2 yr–1) showed considerable spatial variation (coefficient of variation = 27.8%) within the forest. Thirty samples were required to obtain results within 10% of the mean value at a 95% confidential level. A distinctive cosine‐like diel pattern of soil respiration was observed; the time lag between gross primary production and soil respiration at this scale was calculated to be 4–5 h. Seasonality of soil respiration was strong (~1 µmol m–2 s–1 near the end of winter; ~6 µmol m–2 s–1 in midsummer). No time lag was discerned between gross primary production and soil respiration at the seasonal scale. Soil temperature at 5 cm below surface can explain most (>91%) of the observed annual variation in soil respiration. The apparent respiration temperature sensitivity index (Q10) was 3.05. The lowest Q10 value was observed in winter, when soil moisture was low. Soil respiration was overestimated by a Q10 function during both dry and wet periods. The relative contributions of soil organic matter (RSOM), litterfall decomposition (RL), and root respiration (RR) to total soil respiration are 65.25%, 18.73%, and 16.01%, respectively; the temperature sensitivity of these components differ: RL (Q10 = 7.22) > RSOM (2.73) > RR (1.65). This relationship between Q10 values for litter respiration, soil organic matter decomposition, and root respiration still holds after minimizing the confounding effect of moisture. A relatively constant substrate supply and/or thermal acclimation could account for the observed low‐temperature sensitivity in root respiration. Given the high carbon stocks and fluxes, the old‐growth subtropical forests of China seem important in the global carbon budget and climate change.
Obtaining diverse behaviors in a climate model without the use of flux adjustments
Authors: Yamazaki Kuniko , Rowlands Daniel J. , Aina Tolu , Blaker Adam T. , Bowery Andy , Massey Neil , Miller Jonathan , Rye Cameron , Tett Simon F. B. , Williamson Daniel , Yamazaki Yasuhiro H. , Allen Myles R.
Published: April 2013

A number of studies have set out to obtain a range of atmosphere and ocean model behavior by perturbing parameters in a single climate model (perturbed physics ensemble: PPE). Early studies used shallow layer slab ocean or flux‐adjusted coupled ocean‐atmosphere models to obtain a broad range of behavior as characterized by climate sensitivity. A recent study reports a relatively narrow range of sensitivities (2.2–3.2°C) in a PPE of 35 coupled models without flux adjustment, raising the question whether previous broad ranges were an artifact of the use of models that were not in top‐of‐atmosphere (TOA) energy balance. Moreover, no PPE experiment has reported a large spread of behavior of the ocean compared to that exhibited in a multi‐model ensemble (MME) such as Coupled Model Intercomparison Project phase 3 (CMIP3). In this work, we randomly perturb model parameters of a coupled ocean‐atmosphere general circulation model using a space‐filling design containing 10,000 combinations. The ensemble is run over the distributed computing platform of climateprediction.net under fixed pre‐industrial forcing without flux adjustment. We resample a second, 20,000‐member, ensemble with perturbations conditioned on the TOA fluxes from the first ensemble to not drift significantly from a realistic base state while targeting a range of behavior. Models within the targeted ensemble show realistic regional control climates when compared to the CMIP3 ensemble, although there is a bias in global mean surface temperature. The range of predicted equilibrium climate sensitivities of the targeted ensemble is substantially smaller than that obtained with flux adjustment, but larger than the range in the CMIP3 ensemble or in the 35‐model un‐flux‐adjusted PPE in a recent study mentioned above. The Atlantic meridional overturning circulation in the targeted ensemble exhibits a spread in strength as wide as that found in the CMIP3 ensemble. We conclude that flux adjustment is not a pre‐requisite for obtaining a broad spread of behavior in a perturbed physics ensemble.
Application of cloud vertical structure from CloudSat to investigate MODIS‐derived cloud properties of cirriform, anvil, and deep convective clouds
Authors: Young Alisa H. , Bates John J. , Curry Judith A.
Published: April 2013

CloudSat cloud vertical structure is combined with the CALIPSO Lidar and Collection‐5 Level 2 cloud data from Aqua's Moderate Resolution Imaging Spectroradiometer (MODIS) to investigate the mean properties of high/cirriform, anvil, and deep convective (DC) clouds. Cloud properties are sampled over 30°S–30°N for 1 year and compared to existing results of Collection‐4 Aqua MODIS high‐level cloud observations where cloud types were categorized using the International Satellite Cloud Climatology Project (ISCCP) cloud classification scheme. Results show high/cirriform sampled in this study have high biases in cloud top pressure and temperature due to CloudSat's sensitivity to thin high clouds. Mean cloud properties of DC show reasonable agreement with existing DC results notwithstanding mean cloud optical thickness which is ~23% higher due to the exclusion of thick cirrus and anvil clouds. Anvil cloud properties are a mix between high/cirriform and DC according to ISCCP cloud optical thickness thresholds whereby ~80% are associated with high/cirriform and the other 20% are associated with DC. The variability of cloud effective particle radii was also evaluated using DC with ≥5 dBZ echoes at and above 10 km. No evidence of larger cloud effective particle radii are given despite considering higher reaching echoes. Using ISCCP cloud optical thickness thresholds, ~25% of DC would be classified as cirrostratus clouds. These results provide a basis to evaluate the uncertainty of the ISCCP cloud classification scheme and MODIS‐derived cloud properties using active satellite observations.
Composition and hygroscopicity of the Los Angeles Aerosol: CalNex
Authors: Hersey Scott P. , Craven Jill S. , Metcalf Andrew R. , Lin Jack , Lathem Terry , Suski Kaitlyn J. , Cahill John F. , Duong Hanh T. , Sorooshian Armin , Jonsson Haflidi H. , Shiraiwa Manabu , Zuend Andreas , Nenes Athanasios , Prather Kimberly A. , Flagan Richard C. , Seinfeld John H.
Published: April 2013

Aircraft‐based measurements of aerosol composition, either bulk or single‐particle, and both subsaturated and supersaturated hygroscopicity were made in the Los Angeles Basin and its outflows during May 2010 during the CalNex field study. Aerosol composition evolves from source‐rich areas in the western Basin to downwind sites in the eastern Basin, evidenced by transition from an external to internal mixture, as well as enhancements in organic O : C ratio, the amount of organics and nitrate internally mixed on almost all particle types, and coating thickness on refractory black carbon (rBC). Transport into hot, dilute outflow regions leads to significant volatilization of semivolatile material, resulting in a unimodal aerosol comprising primarily oxygenated, low‐volatility, water‐soluble organics and sulfate. The fraction of particles with rBC or soot cores is between 27 and 51% based on data from a Single Particle Soot Photometer (SP2) and Aerosol Time of Flight Mass Spectrometer (ATOFMS). Secondary organics appear to inhibit subsaturated water uptake in aged particles, while CCN activity is enhanced with photochemical age. A biomass‐burning event resulted in suppression of subsaturated hygroscopicity but enhancement in CCN activity, suggesting that BB particles may be nonhygroscopic at subsaturated RH but are important sources of CCN. Aerosol aging and biomass burning can lead to discrepancies between subsaturated and supersaturated hygroscopicity that may be related to mixing state. In the cases of biomass burning aerosol and aged particles coated with secondary material, more than a single parameter representation of subsaturated hygroscopicity and CCN activity is needed.
Kinematic structure of a wildland fire plume observed by Doppler lidar
Authors: Charland A. M. , Clements C. B.
Published: April 2013

Wildland fires present a challenging environment to make meteorological measurements. Observations in the vicinity of wildland fires are needed to better understand fire‐atmosphere interactions and to provide data for the evaluation of coupled fire‐atmosphere models. An observational study was conducted during a low‐intensity prescribed fire in an area of complex terrain with grass fuels east of San José, California. A ground‐based scanning Doppler lidar acquired radial wind velocities and backscatter intensity in and around the fire plume from multiple horizontal and vertical scans. The development of a convergence zone was consistently observed to exist downwind of the plume and was indicated by a decrease in radial velocity of 3–5 m s−1. Divergence calculations made from the lidar radial velocities showed that the magnitude of convergence ranged between −0.06 and −0.08 s−1 downwind of the plumes, while a maximum of −0.14 s−1 occurred within the plume near the fire front. Increased radial velocities were observed at the plume boundary, indicating fire‐induced acceleration of the wind into the base of the convection column above the fire front. Thermodynamic measurements made with radiosondes showed the smoke plume had a potential temperature perturbation of 3.0 to 4.4 K and an increase in water vapor mixing ratio of 0.5 to 1.0 g kg−1. Plume heights determined from sequential range height indicator scans provided estimates of vertical velocity between 0.4 and 0.6 m s−1, representing the ambient background vertical velocity as the top of the plume likely reached equilibrium.
Enhanced vertical mixing associated with a nocturnal cold front passage and its impact on near‐surface temperature and ozone concentration
Authors: Hu Xiao‐Ming , Klein Petra M. , Xue Ming , Shapiro Alan , Nallapareddy Anita
Published: April 2013

A sudden rise in surface temperature is sometimes observed during the nighttime hours with the passage of cold fronts. The physics contributing to such nocturnal warming events and their potential impacts on atmospheric chemistry are not yet fully understood. In this study, a nocturnal warming event associated with a cold front passage in Oklahoma on 3 April 2006 is simulated with the Weather Research and Forecasting model with Chemistry (WRF/Chem). During the prefrontal period under clear‐sky and calm conditions, surface radiative cooling resulted in a decoupled shallow surface layer in which air temperature and wind speed decreased quickly and ozone was removed efficiently by chemical reactions. During the passage of the cold front, strong wind shear enhanced turbulent mixing, which weakened the temperature inversion near the surface. Warmer and ozone‐richer air from aloft was mixed downward to the surface. Thus, a sudden warming and nocturnal secondary ozone maxima were observed near the surface. Dry deposition of ozone at the surface was also enhanced in this warming event.
The solar chromospheric Ca and Mg indices from Aura OMI
Authors: DeLand Matthew , Marchenko Sergey
Published: April 2013

Direct observations of long‐term variations in solar UV irradiance from satellites, although important for monitoring the energetic forcing of the Earth's atmosphere, is challenging because of the difficulty of accurately tracking instrument response changes. Proxy indexes based on core‐to‐wing ratios of solar Fraunhofer lines, such as the Mg II feature at 280 nm, can minimize the impact of both time‐ and wavelength‐dependent instrument changes to better monitor solar UV activity. Here we present new data sets from the Ozone Monitoring Instrument (OMI) on the Aura satellite, which began operations in late 2004. The high quality of OMI irradiance measurements allows us to construct daily indexes for the Mg II, Ca II K (393.4 nm), Ca II H (396.8 nm), and Mg I (285.2 nm) lines. The OMI Ca II K index is well correlated with the Mg II index measured by multiple instruments, as well as the ground‐based National Solar Observatory (NSO) Ca II K 1.0 Å index data derived from Integrated Sunlight Spectrometer measurements. OMI Ca II K data can be used to evaluate anomalous short‐term features in the NSO data set. The OMI solar data products represent a valuable addition to current data sets.
Investigating enhanced Aqua MODIS aerosol optical depth retrievals over the mid‐to‐high latitude Southern Oceans through intercomparison with co‐located CALIOP, MAN, and AERONET data sets
Authors: Toth Travis D. , Zhang Jianglong , Campbell James R. , Reid Jeffrey S. , Shi Yingxi , Johnson Randall S. , Smirnov Alexander , Vaughan Mark A. , Winker David M.
Published: April 2013

A band of enhanced aerosol optical depth (AOD) over the mid‐to‐high latitude Southern Oceans exists in some passive satellite‐based aerosol data sets, including Moderate Resolution Imaging Spectroradiometer (MODIS) products. Past studies suggest several potential causes contributing to this phenomenon, including signal uncertainty, retrieval bias, and cloud contamination. In this paper, quality‐assured Aqua MODIS aerosol products in this zonal band are investigated to assess cloud contamination as a cause. Spatially and temporally collocated cloud and aerosol products produced by the Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) project relative to Aqua MODIS AOD in this region are considered. Maritime Aerosol Network (MAN) and Aerosol Robotic Network (AERONET) AOD data are also collocated with Aqua MODIS retrievals for surface context. The results of this study indicate that the high Aqua MODIS AOD are not seen in the CALIOP aerosol products, cannot be screened using active profiling of collocated observations for cloud presence, and are not detected by ground‐based observations such as MAN and AERONET. Enhanced AOD values are attributable primarily to stratocumulus and low broken cumulus cloud contamination, as identified with CALIOP products. But these clouds explain only about 30–40% of the total anomaly. Cirrus cloud contamination is also a factor. However, in contrast to the rest of the globe, they contribute less overall, relative to low‐level liquid water clouds, which are considered likely the result of misidentification of relatively warm cloud tops compared with surrounding open seas.