Mid-upper Troposphere Research Articles

Thermal contrast between the Tibetan Plateau and tropical Indian Ocean and its relationship to the South Asian summer monsoon: 青藏高原与印度洋热力对比及其与南亚夏季风的关系

The land–sea thermal contrast is an important driver for monsoon interannual variability and the monsoon onset. The thermal contrast between the Tibetan Plateau and the tropical Indian Ocean at the mid–upper troposphere is proposed as a thermal contrast index (TCI) for South Asian monsoon. The authors investigate the TCI associated with South Asian summer monsoon (SASM) intensity and SASM onset. It is observed that the TCI considering the Tibetan Plateau and tropical Indian Ocean demonstrates a stronger and closer correlation with SASM intensity (0.87) than either the Tibetan Plateau (0.42) or tropical Indian Ocean (−0.60) singly. It is implied that the TCI could preferably represent the impact of land–sea thermal condition on SASM activity. Further analysis reveals that the evolution of TCI is related to the SASM onset. The TCI is almost always larger in early onset years than it is in late onset years during the period before SASM onset. In addition, the change of the pentad-by-pentad increment of TCI leads the SASM variation. The correlation coefficient between the TCI increment and SASM index reaches a maximum when the TCI increment leads by 15 pentads. The results of this study show that the TCI plays an important role in SASM activities and is a potential indicator for SASM onset forecasting.摘要本文基于1979–2017年逐日再分析资料, 通过分析对流层中上层青藏高原和印度洋之间的热力差异, 提出了一个热力对比指数 (TCI) , 并分析了TCI与南亚夏季风的强度和爆发时间的关系.研究表明:相比单独的青藏高原或者印度洋的温度, TCI能更好地表示南亚夏季风强度的变化.TCI越大时, 南亚夏季风爆发时间越早;TCI逐候增量的变化超前南亚季风指数的变化, 两者相关系数在TCI逐候增量超前南亚季风指数15候时达到最大.TCI是预报南亚夏季风爆发的一个潜在指标.

Impact of the August Asian–Pacific Oscillation on Autumn Precipitation in Central Eastern China

The August Asian–Pacific Oscillation (APO) plays an important role in the variability of autumn (September ~ October mean) precipitation in central eastern China (CEC). Using observational and reanalysis data, the impact of the August APO on autumn CEC precipitation from 1960 to 2016 was studied. The statistical result showed that August APO is closely linked to the autumn precipitation anomalies in CEC with a significant positive correlation (r = 0.45). Further analysis revealed that when the APO is strong, the strengthened East Asian trough and the North Pacific high / vertical shear occur in the mid-lower / upper troposphere, resulting in anomalous southerly along the East Asia coast, which are favorable for strengthening the anomalous convergence and upward movement of moist warm air from the northwestern Pacific and arid cold air from the north China, introducing more precipitation, but that this configuration became much diminished during weak APO years. The possible mechanism can be explained as the thermal effect in the mid-upper troposphere can last from August until autumn, and the corresponding concurrent thermal effect would lead to anomalies in both atmospheric circulation and precipitation. Additionally, though an evidently negative relationship between preceding Nino indices and autumn CEC precipitation was revealed, the August APO induced changes in autumn CEC precipitation is greater than those of Nino indices, whether interannual or interdecadal changes, further indicating that the APO is an effective signal for precipitation prediction in CEC.

Projection of the East Asian westerly jet under six global warming targets

This study investigates the projected changes in the East Asian westerly jet (EAJ) under six global warming targets (1.5°C, 2.0°C, 2.5°C, 3.0°C, 3.5°C, and 4.0°C) relative to the present climate, using the outputs of CMIP5 models. The results show that the westerly tends to weaken slightly under the 1.5°C warming target. Under the 2.0°C target, it is projected to intensify south of the EAJ’s axis (approximately 40°N) and decay north of the axis. This change becomes increasingly evident under the 2.5°C and higher warming targets, which suggests that the EAJ’s axis will move farther and farther southward, but its intensity will change little with increasing global warming. Further analyses suggest that the change in the EAJ is closely related to the inhomogeneous rising rate of air temperature in the mid–upper troposphere. The relatively slow-rising air temperature in the mid–upper troposphere over the EAJ’s entrance and exit regions will lead to a negative (positive) meridional temperature gradient to the south (north), and will then accelerate (decelerate) the westerly on the EAJ’s south/north side.

Reexamining the relationship of La Niña and the East Asian Winter Monsoon

The northern and the southern modes are two distinct principle modes that dominate the winter mean surface air temperature (Ts) variations over East Asia (EA). The cold southern mode is represented by a significant cooling south of 45°N and is linked to La Nina events. An objective criterion, which could distinguish the spatial distributions and the maximum center of sea surface temperature anomaly (SSTA), is used to classify the La Nina events into two categories: mega-La Nina and equatorial La Nina. Their impacts are inspected onto the Ts southern mode. The mega-La Nina, featured by a significant K-shape warming in the western Pacific with the maximum SSTA cooling centered in the tropical central Pacific. As a response, an anomalous barotropic high is generated over North Pacific (NP) implying a weak zonal gradient between ocean and the EA continent, which induces a neutral Ts southern mode. The equatorial La Nina characterizes a significant cooling in the tropical eastern Pacific with convective descending motions shifting eastward to the east of the dateline. The resultant low-level circulation anomalies show an anomalous subtropical NP low and a gigantic abnormal EA continent high. The strong zonal gradient results in significant northerly anomalies over EA from 55°N to southeastern China. Over the mid-upper troposphere, the anomalous subtropical NP low extends westward to the Korean Peninsula, leading to a strengthened and southward shifted EA trough. Such abnormal circulation patterns favor the intrusion of cold air to southern EA and correspond to a strong Ts southern mode. The numerical results well validate the above processes and physical mechanisms.

Impact of cloud microphysical processes on the simulation of a hailstorm in East China

Two severe hailstorms that occurred during spring in east China were simulated by the Weather Research and Forecasting model, version 3.9, with four bulk microphysics schemes. Despite some general similarities in the simulation results, the microphysical schemes produced many differences in the radar structure, surface precipitation, wind field, and the mass of hydrometeors. In particular, the Morrison (MOR) scheme produced the best prediction of the radar structure, while the Milbrandt–Yau two-moment (MY2) scheme produced the worst. The mass of ice-phase particles differed greatly in different schemes. An accumulation zone of supercooled rainwater was found in the WDM6 scheme. Analysis of the mass and heat budgets of hydrometeors revealed that embryos of hail were mainly formed through the collection of ice and rain (MOR) or conversion from graupel (MY2), and developed by collecting cloud droplets (MOR) or rainwater (MY2). Microphysical processes generally cooled the air at lower levels while warming it in the mid-upper troposphere. Based on the 1-km resolution results simulated by MOR and MY2, the growth mechanism of hailstones was analyzed preliminarily. Hail embryos were first formed at the upper edge of the upright main updraft, which could be regarded as the “embryo curtain”, and developed through up–down cycling. To conclude, we present a conceptual model of the initial, developing, mature and dissipating stages of the two hailstorm cases.

Improving the simulation of East Asian summer monsoon with mesoscale enhancement in an AGCM

In this study, the parameterization of sea surface turbulent heat flux in National Center for Atmospheric Research Community Atmosphere Model version 5 (CAM5) is improved by including the contribution of mesoscale convective motion (i.e., mesoscale enhancement). The effect of mesoscale enhancement on the simulation of the East Asian summer monsoon circulation is further explored with SST-forced experiments that are conducted for the period of January 1979 to December 2015. Experiments suggest that the standard CAM5 exhibits evident biases in simulating the East Asian summer monsoon system. For example, the simulated westerlies at 850 hPa in low latitude of East Asia are too weak, the precipitations in the Bay of Bengal, the South China Sea, and the tropical western Pacific are underestimated, the simulated south Asia and western Pacific subtropical highs are too weak, and the upper and middle troposphere in the low latitudes of East Asia is too cold. By including the mesoscale enhancement in CAM5, these biases are clearly reduced. For example, the simulated summer mean precipitation in the Bay of Bengal is increased from 9.85 to 11.76 mm/day, which is closer to the observed value of 13.80 mm/day. Further analyses show that the improvements in the East Asian summer monsoon precipitations and circulations can be attributed to the enhancement of convections and the increase in water evaporation in the tropical western Pacific Ocean induced by mesoscale enhancement. Correspondingly, the latent heat due to vapor condensation in the atmosphere also increases, which reduces the cold bias in the mid-upper troposphere. Meanwhile, the geopotential height of the upper-middle-level atmosphere is elevated, which makes the simulated intensity of the south Asian and the western Pacific subtropical highs, two important components of the East Asian summer monsoon circulation, closer to the reality.

Interannual variation of the northward movement of the South Asian High towards the Tibetan Plateau and its relation to the Asian Summer Monsoon onset

The interannual variation of the northward movement of the upper-level South Asian High (SAH) towards the Tibetan Plateau (TP) in the early summer and its relation to the Asian summer monsoon (ASM) onset are still unclear. This study shows that when the ASM onset occurs earlier, there is an anomalous westerly/easterly in the lower/upper troposphere, accompanying an anomalous ascending motion over the southern part of Asia. Meanwhile, cyclonic circulation anomalies occur over the Arabian Sea, the Bay of Bengal, and the South China Sea and are conducive to the earlier development and spread of the deep convection and atmospheric heating. Consequently, the warm center in the mid-upper troposphere surrounding the TP generates earlier in May, resulting in an earlier reversal of the meridional temperature gradient over the southern slope of the TP. Based on the thermal wind relationship, the vertical shear of the zonal wind reverses earlier and finally the SAH shifts over the TP earlier. The opposite is true when the ASM onset occurs later. Compared with the evolution of the diabatic heating over South Asia from May to June, there is no significant difference in the diabatic heating over the TP between the earlier and later years, indicating that the ASM heating may play a more important role in the northward movement of the SAH than the TP heating. Therefore, the ASM heating is essential to the interannual variation of the northward shift of the SAH towards the TP in the early summer.

Does global warming amplify interannual climate variability?

Based on the outputs of 30 models from Coupled Model Intercomparison Project Phase 5 (CMIP5), the fractional changes in the amplitude interannual variability (σ) for precipitation (P′) and vertical velocity (ω′) are assessed, and simple theoretical models are constructed to quantitatively understand the changes in σ(P′) and σ(ω′). Both RCP8.5 and RCP4.5 scenarios show similar results in term of the fractional change per degree of warming, with slightly lower inter-model uncertainty under RCP8.5. Based on the multi-model median, σ(P′) generally increases but σ(ω′) generally decreases under global warming but both are characterized by non-uniform spatial patterns. The σ(P′) decrease over subtropical subsidence regions but increase elsewhere, with a regional averaged value of 1.4% K− 1 over 20°S–50°N under RCP8.5. Diagnoses show that the mechanisms for the change in σ(P′) are different for climatological ascending and descending regions. Over ascending regions, the increase of mean state specific humidity contributes to a general increase of σ(P′) but the change of σ(ω′) dominates its spatial pattern and inter-model uncertainty. But over descending regions, the change of σ(P′) and its inter-model uncertainty are constrained by the change of mean state precipitation. The σ(ω′) is projected to be weakened almost everywhere except over equatorial Pacific, with a regional averaged fractional change of − 3.4% K− 1 at 500 hPa. The overall reduction of σ(ω′) results from the increased mean state static stability, while the substantially increased σ(ω′) at the mid-upper troposphere over equatorial Pacific and the inter-model uncertainty of the changes in σ(ω′) are dominated by the change in the interannual variability of diabatic heating.

Origin of Indian summer monsoon rainfall biases in CMIP5 multimodel ensemble

Significant biases of coupled general circulation models (CGCMs) lead to considerable uncertainty in climate predictions and projections. Based on the historical simulations in the phase 5 of the Coupled Model Intercomparison Project (CMIP5), we identify that state-of-the-art CGCMs commonly simulate insufficient Indian summer monsoon (ISM) rainfall along with too weak monsoon circulations. Such ISM rainfall/circulation biases, however, are absent in the Atmospheric Model Intercomparison Project simulations forced by observed sea surface temperature (SST), suggesting that the common ISM biases are not intrinsic errors of the atmospheric models but arise from the interaction between the monsoon and the oceans. A multimodel statistical analysis further shows that the ISM rainfall/circulation biases in CMIP5 models can be traced back to the excessively cold SST over the northern Indian Ocean (NIO). The systemic cold SST biases in the NIO suppress local convective activity and reduce air temperature, resulting in a weak north–south thermal contrast in the mid-upper troposphere. This would induce an excessively weak ISM circulation and resultant insufficient monsoon rainfall. Furthermore, the dynamic effect of cold NIO SST biases on the ISM rainfall/circulation simulations is also confirmed through several sensitivity experiments by using the widely-applied Weather Research and Forecasting model. To the extent that the cold SST biases over the NIO may originate from an excessively strong Indian winter monsoon, improving the winter monsoon simulation is an important prerequisite for better summer climate simulations and predictions/projections over the broad ISM region.

Satellite observation of atmospheric methane: intercomparison between AIRS and GOSAT TANSO-FTS retrievals

Abstract. Space-borne observations of atmospheric methane (CH4) have been made using the Atmospheric Infrared Sounder (AIRS) on the EOS/Aqua satellite since August 2002 and the Thermal and Near-infrared Sensor for Carbon Observation Fourier Transform Spectrometer (TANSO-FTS) on the Greenhouse Gases Observing Satellite (GOSAT) since April 2009. This study compared the GOSAT TANSO-FTS thermal infrared (TIR) version 1.0 CH4 product with the collocated AIRS version 6 CH4 product using data from 1 August 2010 to 30 June 2012, including the CH4 mixing ratios and the total column amounts. The results show that at 300–600 hPa, where both AIRS and GOSAT-TIR CH4 have peak sensitivities, they agree very well, but GOSAT-TIR retrievals tend to be higher than AIRS in layer 200–300 hPa. At 300 hPa the CH4 mixing ratio from GOSAT-TIR is, on average, 10.3 ± 31.8 ppbv higher than that from AIRS, and at 600 hPa GOSAT-TIR retrieved CH4 is −16.2 ± 25.7 ppbv lower than AIRS CH4. Comparison of the total column amount of CH4 shows that GOSAT-TIR agrees with AIRS to within 1 % in the mid-latitude regions of the Southern Hemisphere and in the tropics. In the mid to high latitudes in the Northern Hemisphere, comparison shows that GOSAT-TIR is ∼ 1–2 % lower than AIRS, and in the high-latitude regions of the Southern Hemisphere the difference of GOSAT from AIRS varies from −3 % in October to +2 % in July. The difference between AIRS and GOSAT TANSO-FTS retrievals is mainly due to the difference in retrieval algorithms and instruments themselves, and the larger difference in the high-latitude regions is associated with the low information content and small degrees of freedom of the retrieval. The degrees of freedom of GOSAT-TIR retrievals are lower than that of AIRS, which also indicates that the constraint in GOSAT-TIR retrievals may be too strong. From the good correlation between AIRS and GOSAT-TIR retrievals and the seasonal variation they observed, we are confident that the thermal infrared measurements from AIRS and GOSAT-TIR can provide valuable information to capture the spatial and temporal variation of CH4, especially in the mid-upper troposphere, in most periods and regions.

Comparison of Atmospheric Methane Retrievals From AIRS and IASI

Atmospheric methane (CH4) is a standard product of the atmospheric infrared sounder (AIRS) aboard NASA's Aqua satellite, generated at the NASA Goddard Earth Sciences Data and Information Services Center (NASA/GES/DISC), and a product of the infrared atmospheric sounding interferometer (IASI) aboard METOP-A,-B, generated at National Oceanic and Atmospheric Administration's Comprehensive Large Array-data Stewardship System. In order to understand the capability of these two sensors in observing the spatial and temporal distribution of CH4 , this paper compares the CH4 products from AIRS and IASI with aircraft measurements, as well as the corresponding time series in tropics and high northern latitude regions. It is found that the mean degree of freedom from AIRS is smaller than IASI by –0.049 ± 0.152, and in their peak sensitive altitude between 350 and 650 hPa their difference (AIRS – IASI) is about 2.8 ± 17.2 ppb. Both AIRS and IASI can capture the latitudinal gradient, but there is a large scattering in the high northern latitude regions. They agree well in observing the summer enhancement of CH4 during the Monsoon season over South Asia, and the seasonal cycles over Siberia (except for a relatively larger difference in the cold season). These results highlight that AIRS and IASI can provide valuable information to capture the spatiotemporal variation of CH4 in the mid-upper troposphere in most periods and regions, but it is needed to further improve the data quality to make a consistent product using both sensors.

Consistent response of Indian summer monsoon to Middle East dust in observations and simulations

Abstract. The response of the Indian summer monsoon (ISM) circulation and precipitation to Middle East dust aerosols on sub-seasonal timescales is studied using observations and the Weather Research and Forecasting model coupled with online chemistry (WRF-Chem). Satellite data show that the ISM rainfall in coastal southwest India, central and northern India, and Pakistan is closely associated with the Middle East dust aerosols. The physical mechanism behind this dust–ISM rainfall connection is examined through ensemble simulations with and without dust emissions. Each ensemble includes 16 members with various physical and chemical schemes to consider the model uncertainties in parameterizing short-wave radiation, the planetary boundary layer, and aerosol chemical mixing rules. Experiments show that dust aerosols increase rainfall by about 0.44 mm day−1 (~10 % of the climatology) in coastal southwest India, central and northern India, and north Pakistan, a pattern consistent with the observed relationship. The ensemble mean rainfall response over India shows a much stronger spatial correlation with the observed rainfall response than any other ensemble members. The largest modeling uncertainties are from the boundary layer schemes, followed by short-wave radiation schemes. In WRF-Chem, the dust aerosol optical depth (AOD) over the Middle East shows the strongest correlation with the ISM rainfall response when dust AOD leads rainfall response by about 11 days. Further analyses show that increased ISM rainfall is related to enhanced southwesterly monsoon flow and moisture transport from the Arabian Sea to the Indian subcontinent, which are associated with the development of an anomalous low-pressure system over the Arabian Sea, the southern Arabian Peninsula, and the Iranian Plateau due to dust-induced heating in the troposphere. The dust-induced heating in the mid-upper troposphere is mainly located in the Iranian Plateau rather than the Tibetan Plateau. This study demonstrates a thermodynamic mechanism that links remote desert dust emissions in the Middle East to ISM circulation and precipitation variability on sub-seasonal timescales, which may have implications for ISM rainfall forecasts.

Local and remote climatic impacts due to land use degradation in the Amazon “Arc of Deforestation”

Many numerical studies, among them, global and regional models, have been used to simulate climatic impact due to Amazon deforestation. Most of them did not consider deforestation as usually observed and the induced dynamic changes. The present study explores the physical impacts due to Amazon deforestation by considering local and remote changes in the circulation and thermodynamics. For this, numerical experiments were conducted with RegCM3 using a relatively fine horizontal grid spacing (50 km), more realistic deforested areas (similar to the highway-network-shaped), and an updated land use map. The studied period was 2001–2006 October–March. As in most previous studies focusing on Amazon deforestation, the RegCM3-simulated air temperature increases over degraded areas, ranging from 1.0 to 2.5 °C, and precipitation decreases of around 10 %. This result is mainly related to depletion in evapotranspiration rates provided by lesser soil water extraction by the degraded vegetation. The weakening of upward motion in the mid-upper troposphere is an associated mechanism that explains the precipitation decrease after Amazon deforestation. A new result is the simulated precipitation increase, about 10 %, over the eastern South America and the adjacent South Atlantic Ocean. In these areas, the precipitation increase during October–March is associated with intensification of upper-level high pressure (the Bolivian high) coupled with negative geopotential height anomalies southeastward of the center of the high.

Impact of Mongolian Plateau versus Tibetan Plateau on the westerly jet over North Pacific Ocean

Mountains have long been considered to play an important role in the formation of modern climate. Particularly in the Asia-Pacific sector, the existence of Tibetan Plateau (TP) is believed to maintain the stationary planetary wave patterns and to intensify the westerly jet over North Pacific. However, the potential role of Mongolian Plateau (MP) has often been neglected in these studies. With an atmospheric general circulation model, we show that the MP, despite its smaller size, exerts a great influence on the planetary-scale circulation and the subtropical westerly jet. The MP amplifies the meridional thermal gradient at the mid-upper troposphere and thus strengthens the jet, which is primarily facilitated by its high-latitude location. By blocking westerly winds, the MP forces their northern branch further northward, which allows the southward penetration of cold air at the lee side, and intensifies the East Asian trough. In contrast, the impact of the TP itself is not as large as expected. Hence, one should not simply ascribe all the mountain-induced climate change to the TP uplift, because other smaller topography might be also very important.

Very short-lived bromomethanes measured by the CARIBIC observatory over the North Atlantic, Africa and Southeast Asia during 2009–2013

Abstract. Short-lived organic brominated compounds make up a significant part of the organic bromine budget in the atmosphere. Emissions of these compounds are highly variable and there are limited measurements, particularly in the extra-tropical upper troposphere/lower stratosphere and tropical troposphere. Measurements of five very short-lived bromomethanes (VSLB) were made in air samples collected on the CARIBIC project aircraft over three flight routes; Germany to Venezuela/Columbia during 2009–2011, Germany to South Africa during 2010 and 2011 and Germany to Thailand/Kuala Lumpur, Malaysia during 2012 and 2013. In the tropical troposphere, as the most important entrance region to the stratosphere, we observe a total mean organic bromine derived from these compounds across all flights at 10–12 km altitude of 3.4 ± 1.5 ppt. Individual mean tropical tropospheric mixing ratios across all flights were 0.43, 0.74, 0.14, 0.23 and 0.11 ppt for CHBr3, CH2Br2, CHBr2Cl, CHBrCl2 and CH2BrCl respectively. The highest levels of VSLB-derived bromine (4.20 ± 0.56 ppt) were observed in flights between Bangkok and Kuala Lumpur indicating that the South China Sea is an important source region for these compounds. Across all routes, CHBr3 and CH2Br2 accounted for 34% (4.7–71) and 48% (14–73) respectively of total bromine derived from the analysed VSLB in the tropical mid-upper troposphere totalling 82% (54–89). In samples collected between Germany and Venezuela/Columbia, we find decreasing mean mixing ratios with increasing potential temperature in the extra-tropics. Tropical mean mixing ratios are higher than extra-tropical values between 340–350 K indicating that rapid uplift is important in determining mixing ratios in the lower tropical tropopause layer in the West Atlantic tropics. O3 was used as a tracer for stratospherically influenced air and we detect rapidly decreasing mixing ratios for all VSLB above ∼100 ppb O3 corresponding to the extra-tropical tropopause layer.

Vertical structure of predictability and information transport over the Northern Hemisphere

Based on nonlinear prediction and information theory, vertical heterogeneity of predictability and information loss rate in geopotential height field are obtained over the Northern Hemisphere. On a seasonal-to-interannual time scale, the predictability is low in the lower troposphere and high in the mid-upper troposphere. However, within mid-upper troposphere over the subtropics ocean area, there is a relatively poor predictability. These conclusions also fit the seasonal time scale. Moving to the interannual time scale, the predictability becomes high in the lower troposphere and low in the mid-upper troposphere, contrary to the former case. On the whole the interannual trend is more predictable than the seasonal trend. The average information loss rate is low over the mid-east Pacific, west of North America, Atlantic and Eurasia, and the atmosphere over other places has a relatively high information loss rate on all-time scales. Two channels are found steadily over the Pacific Ocean and Atlantic Ocean in subtropics. There are also unstable channels. The four-season influence on predictability and information communication are studied. The predictability is low, no matter which season data are removed and each season plays an important role in the existence of the channels, except for the winter. The predictability and teleconnections are paramount issues in atmospheric science, and the teleconnections may be established by communication channels. So, this work is interesting since it reveals the vertical structure of predictability distribution, channel locations, and the contributions of different time scales to them and their variations under different seasons.

Midlatitude cirrus classification at Rome Tor Vergata through a multichannel Raman–Mie–Rayleigh lidar

Abstract. A methodology to identify and characterize cirrus clouds has been developed and applied to the multichannel-multiwavelength Rayleigh–Mie–Raman (RMR) lidar in Rome Tor Vergata (RTV). A set of 167 cirrus cases, defined on the basis of quasi-stationary temporal period conditions, has been selected in a data set consisting of about 500 h of nighttime lidar sessions acquired between February 2007 and April 2010. The derived lidar parameters (effective height, geometrical and optical thickness and mean back-scattering ratio) and the cirrus mid-height temperature (estimated from the radiosonde data of Pratica di Mare, WMO, World Meteorological Organization, site no. 16245) of this sample have been analyzed by the means of a clustering multivariate analysis. This approach identified four cirrus classes above the RTV site: two thin cirrus clusters in mid- and upper troposphere and two thick cirrus clusters in mid-upper troposphere. These results, which are very similar to those derived through the same approach at the lidar site of the Observatoire de Haute-Provence (OHP), allows characterization of cirrus clouds over the RTV site and attests to the robustness of such classification. To acquire some indications about the cirrus generation methods for the different classes, analyses of the extinction-to-backscatter ratio (lidar ratio, LReff, in terms of frequency distribution functions and dependencies on the mid-height cirrus temperature, have been performed. A preliminary study relating some meteorological parameters (e.g., relative humidity, wind components) to cirrus clusters has also been conducted. The RTV cirrus results, recomputed through the cirrus classification by Sassen and Cho (1992), show good agreement with other midlatitude lidar cirrus observations for the relative occurrence of subvisible (SVC), thin and opaque cirrus classes (10%, 49% and 41%, respectively). The overall mean value of cirrus optical depth is 0.37 ± 0.18, while most retrieved LReff values range between 10–60 sr, and the estimated mean value is 31 ± 15 sr, similar to LR values of lower latitude cirrus measurements. The obtained results are consistent with previous studies conducted with different systems and confirm that cirrus classification based on a statistical approach seems to be a good tool both to validate the height-resolved cirrus fields calculated by models and to investigate the key processes governing cirrus formation and evolution. However, the lidar ratio and optical depth analyses are affected by some uncertainties (e.g., lidar error noise, multiple scattering effects, supercooled water clouds) that reduce the confidence of the results. Future studies are needed to improve the characterization of the cirrus optical properties and, thus, the determination of their radiative impact.

Spatiotemporal variations in mid-upper tropospheric methane over China from satellite observations

Spaceborne measurements by the Atmospheric Infrared Sounder (AIRS) on the EOS/Aqua satellite provide a global view of methane (CH4) distribution in the mid-upper troposphere (MUT-CH4). The focus of this study is to analyze the spatiotemporal variations in MUT-CH4 over China from 2003 to 2008. Validation of AIRS CH4 products versus Fourier transform infrared profiles demonstrates that its RMS error is mostly less than 1.5%. A typical atmospheric methane profile is found that shows how concentrations decrease as height increases because of surface emissions. We found that an important feature in the seasonal variation in CH4 is the two peaks that exist in summer and winter in most parts of China, which is also observed in in-situ measurements at Mt. Waliguan, Qinghai Province, China (36.2879°N 100.8964°E, 3810 m). Also, in the summer, only one peak existed in western and southern China since there are no more significant anthropogenic sources in winter than at any other time of the year. Further analysis of the deseasonalized time-series of AIRS CH4 in three fixed pressure layers of AIRS from 2003 to 2008 indicates that CH4 in the Northern Hemisphere has increased abruptly since 2007, with no significant increase occurring before 2007. The increase in China is generally more significant than in other areas around the world, which again correlates with in-situ measurements at Mt. Waliguan.

Thermodynamic characteristics of tropical cyclones with rapid intensity change over the coastal waters of China

In order to investigate the different thermodynamic mechanisms between rapid intensifying (RI) and rapid weakening (RW) tropical cyclones (TCs), the thermodynamic structures of two sets of composite TCs are analyzed based on the complete-form vertical vorticity tendency equation and the NCEP/NCAR reanalysis data. Each composite is composed of five TCs, whose intensities change rapidly over the coastal waters of China. The results show that the maximum apparent heating source Q 1 exists in both the upper and lower troposphere near the RI TC center, and Q 1 gets stronger at the lower level during the TC intensification period. But for the RW TC, the maximum Q 1 exists at the middle level near the TC center, and Q 1 gets weaker while the TC weakens. The maximum apparent moisture sink Q 2 lies in the mid troposphere. Q 2 becomes stronger and its peak-value height rises while TC intensifies, and vice versa. The increase of diabatic heating with height near the TC center in the mid-upper troposphere and the increase of vertical inhomogeneous heating near the TC center in the lower troposphere are both favorable to the TCs’ rapid intensification; otherwise, the intensity of the TC decreases rapidly.

Seven Years’ Observation of Mid-Upper Tropospheric Methane from Atmospheric Infrared Sounder

The Atmospheric Infrared Sounder (AIRS) on EOS/Aqua platform provides a measurement of global methane (CH4) in the mid-upper troposphere since September, 2002. As a thermal infrared sounder, the most sensitivity of AIRS to atmospheric CH4 is in the mid-upper troposphere with the degree of freedom of ~1.0. Validation of AIRS CH4 product versus thousands of aircraft profiles (convolved using the AIRS averaging kernels) demonstrates that its RMS error (RMSE) is mostly less than 1.5%, and its quality is pretty stable from 2003 to 2009. For scientific analysis of the spatial and temporal variation of mid-upper tropospheric CH4 (MUT-CH4) in the High Northern Hemisphere (HNH), it is more valuable to use the AIRS retrieved CH4 in a layer of about 100 hPa below tropopause (“Representative Layer”) than in a fixed pressure layer. Further analysis of deseasonalized time-series of AIRS CH4 in both a fixed pressure layer and the “Representative Layer” of AIRS (only for the HNH) from 2003 to 2009 indicates that, similar to the CH4 in the marine boundary layer (MBL) that was found to increase in 2007–2008, MUT-CH4 was also observed to have a recent increase but the most significant increase occurred in 2008. MUT-CH4 continued to increase in 2009, especially in the HNH. Moreover, the trend of MUT-CH4 from 2006 to 2008 is lower than the trend of CH4 in the MBL by 30–40% in both the southern hemisphere and HNH. This delay for the MUT-CH4 increase of about one year than CH4 in the MBL as well as the smaller increase trend for MUT-CH4 suggest that surface emission is likely a major driver for the recent CH4 increase. It is also found that the seasonal cycle of MUT-CH4 is different from CH4 in the MBL due to the impact of transport, in addition to the surface emission and the photochemical loss.