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The accurate monitoring of climate change im- poses strict requirements upon observing systems, in partic- ular regarding measurement accuracy and long-term stability. Currently available data records of the essential climate vari- ables (temperature- T , geopotential- p , humidity-RH, wind, and cloud properties) in the upper-air generally fail to fulfil such requirements. This raises serious issues about the abil- ity to detect, quantify and understand recent climate changes and their causes. GCOS is currently implementing a Ref- erence Upper-Air Network (GRUAN) in order to fill this major void within the global observing system. As part of the GRUAN implementation plan we provide herein funda- mental guidelines for establishing and maintaining reference quality atmospheric observations which are based on prin- cipal concepts of metrology, in particular traceability. It is argued that the detailed analysis of the uncertainty budget of a measurement technique is the critical step for achieving this goal. As we will demonstrate with an example, detailed knowledge of the calibration procedures and data process- ing algorithms are required for determining the uncertainty of each individual data point. Of particular importance is the careful assessment of the uncertainties introduced by correc- tion schemes adjusting for systematic effects.

The GCOS (Global Climate Observing System) Reference Upper-Air Network (GRUAN) data processing for the Vaisala RS92 radiosonde was developed to meet the criteria for reference measurements. These criteria stipulate the collection of metadata, the use of well-documented correction algorithms, and estimates of the measurement uncertainty. An important and novel aspect of the GRUAN processing is that the uncertainty estimates are vertically resolved. This paper describes the algorithms that are applied in version 2 of the GRUAN processing to correct for systematic errors in radiosonde measurements of pressure, temperature, humidity, and wind, as well as how the uncertainties related to these error sources are derived. Currently, the RS92 is launched on a regular basis at 13 out of 15 GRUAN sites. An additional GRUAN requirement for performing reference measurements with the RS92 is that the manufacturer-prescribed procedure for the radiosonde's preparation, i.e. heated reconditioning of the sensors and recalibration during ground check, is followed. In the GRUAN processing however, the recalibration of the humidity sensors that is applied during ground check is removed. For the dominant error source, solar radiation, laboratory experiments were performed to investigate and model its effect on the RS92's temperature and humidity measurements. GRUAN uncertainty estimates are 0.15 K for night-time temperature measurements and approximately 0.6 K at 25 km during daytime. The other uncertainty estimates are up to 6% relative humidity for humidity, 10–50 m for geopotential height, 0.6 hPa for pressure, 0.4–1 m s−1 for wind speed, and 1° for wind direction. Daytime temperature profiles for GRUAN and Vaisala processing are comparable and consistent within the estimated uncertainty. GRUAN daytime humidity profiles are up to 15% moister than Vaisala processed profiles, of which two-thirds is due to the radiation dry bias correction and one-third is due to an additional calibration correction. Redundant measurements with frost point hygrometers (CFH and NOAA FPH) show that GRUAN-processed RS92 humidity profiles and frost point data agree within 15% in the troposphere. No systematic biases occur, apart from a 5% dry bias for GRUAN data around −40 °C at night.

The Vaisala RS92 radiosonde is the most widely used type of sonde in the current global radiosonde network. One of the largest biases in the RS92 humidity data is its daytime solar radiation dry bias (SRDB). An algorithm [referred to as NCAR radiation bias correction (NRBC)] was developed to correct the SRDB based on a more complicated algorithm developed by the Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN). The NRBC to relative humidity (RH) is a function of the measured RH and temperature, and the temperature solar radiation correction. The latter varies with pressure, season, and time of the day. The RH correction has a mean magnitude of about 2%–4% and 6%–8% in the lower–midtroposphere and upper troposphere, respectively. The NRBC is evaluated against the GRUAN-corrected RS92 data and the ground-based GPS-estimated precipitable water (PW). The corrected RH agrees with the GRUAN data within ±0.5% on average, with standard deviations of about 1%–2% and 2%–6% in the lower–midtroposphere and upper troposphere, respectively. The NRBC leads to reduced mean biases, and better agreement with the GPS PW and its diurnal cycle. The NRBC has been applied to historical radiosonde data at 65 stations. The radiosonde humidity data, both with and without the NRBC, are homogenized using the method of Dai et al. (2011). The NRBC results in consistently elevated RHs throughout the whole record in the homogenized data. This could have a significant impact on global reanalysis products when they are assimilated into the reanalysis models. However, the NRBC has insignificant effects on the long-term trends as the correction is primarily for mean biases.

While the global upper-air observing network has provided useful observations for operational weather forecasting for decades, its measurements lack the accuracy and long-term continuity needed for understanding climate change. Consequently, the scientific community faces uncertainty on key climate issues, such as the nature of temperature trends in the troposphere and stratosphere; the climatology, radiative effects, and hydrological role of water vapor in the upper troposphere and stratosphere; and the vertical profile of changes in atmospheric ozone, aerosols, and other trace constituents. Radiosonde data provide adequate vertical resolution to address these issues, but they have questionable accuracy and time-varying biases due to changing instrumentation and techniques. Although satellite systems provide global coverage, their vertical resolution is sometimes inadequate and they require independent reference observations for sensor and data product validation, and for merging observations from different platforms into homogeneous climate records. To address these shortcomings, and to ensure that future climate records will be more useful than the records to date, the Global Climate Observing System (GCOS) program is initiating a GCOS Reference Upper-Air Network (GRUAN) to provide high-quality observations using specialized radiosondes and complementary remote sensing profiling instrumentation that can be used for validation. This paper outlines the scientific rationale for GRUAN, its role in the Global Earth Observation System of Systems, network requirements and likely instrumentation, management structure, current status, and future plans. It also illustrates the value of prototype reference upper-air observations in constructing climate records and their potential contribution to the Global Space-Based Inter-Calibration System. We invite constructive feedback on the GRUAN concept and the engagement of the scientific community.

Polar stratospheric clouds (PSC) were observed with the multi-wavelength lidar of the MOANA project (Modelling and Observations of Aerosols in the Northern Atmosphere) during SESAME (Second European Stratospheric Arctic and Mid-latitude Experiment). The physical state, liquid or solid, of the cloud particles can be inferred from the lidar data. Using isentropic back-trajectories to obtain the thermal history of the sampled air masses, it is possible to reconcile most of the observations with current ideas on PSC formation and evolution. When the cloud particles were identified as liquid, changes in the size distribution of the droplets along the trajectory were calculated using a micro-physical box model. Backscatter ratios calculated from the size distributions are in broad agreement with the lidar data, giving confidence in current understanding of the evolution of ternary solution (H2SO4, HNO3 and H2O) droplets. Results from two soundings are shown which bear on the problem of the formation of solid particles. In the first, solid particles were detected. The air mass had cooled to the frost point 12 hours earlier. In the second no solid particles were detected although the air temperature was below the nitric acid trihydrate existence point, and had decreased by 12K in the previous 14 hours.

Advanced measurement and modelling techniques are employed to determine the partitioning of atmospheric water between the gas phase and the condensed phase in and around cirrus clouds, and thus to identify in-cloud and out-of-cloud supersaturations with respect to ice. In November 2008 the newly developed balloon-borne backscatter sonde COBALD (Compact Optical Backscatter and AerosoL Detector) was flown 14 times together with a CFH (Cryogenic Frost point Hygrometer) from Lindenberg, Germany (52° N, 14° E). The case discussed here in detail shows two cirrus layers with in-cloud relative humidities with respect to ice between 50% and 130%. Global operational analysis data of ECMWF (roughly 1° × 1° horizontal and 1 km vertical resolution, 6-hourly stored fields) fail to represent ice water contents and relative humidities. Conversely, regional COSMO-7 forecasts (6.6 km × 6.6 km, 5-min stored fields) capture the measured humidities and cloud positions remarkably well. The main difference between ECMWF and COSMO data is the resolution of small-scale vertical features responsible for cirrus formation. Nevertheless, ice water contents in COSMO-7 are still off by factors 2–10, likely reflecting limitations in COSMO's ice phase bulk scheme. Significant improvements can be achieved by comprehensive size-resolved microphysical and optical modelling along backward trajectories based on COSMO-7 wind and temperature fields, which allow accurate computation of humidities, ice particle size distributions and backscatter ratios at the COBALD wavelengths. However, only by superimposing small-scale temperature fluctuations, which remain unresolved by the NWP models, can we obtain a satisfying agreement with the observations and reconcile the measured in-cloud non-equilibrium humidities with conventional ice cloud microphysics.

Advanced measurement and modelling techniques are employed to estimate the partitioning of atmospheric water between the gas phase and the condensed phase in and around cirrus clouds, and thus to identify in-cloud and out-of-cloud supersaturations with respect to ice. In November 2008 the newly developed balloon-borne backscatter sonde COBALD (Compact Optical Backscatter and AerosoL Detector) was flown 14 times together with a CFH (Cryogenic Frost point Hygrometer) from Lindenberg, Germany (52° N, 14° E). The case discussed here in detail shows two cirrus layers with in-cloud relative humidities with respect to ice between 50% and 130%. Global operational analysis data of ECMWF (roughly 1° × 1° horizontal and 1 km vertical resolution, 6-hourly stored fields) fail to represent ice water contents and relative humidities. Conversely, regional COSMO-7 forecasts (6.6 km × 6.6 km, 5-min stored fields) capture the measured humidities and cloud positions remarkably well. The main difference between ECMWF and COSMO data is the resolution of small-scale vertical features responsible for cirrus formation. Nevertheless, ice water contents in COSMO-7 are still off by factors 2–10, likely reflecting limitations in COSMO's ice phase bulk scheme. Significant improvements can be achieved by comprehensive size-resolved microphysical and optical modelling along backward trajectories based on COSMO-7 wind and temperature fields, which allow accurate computation of humidities, homogeneous ice nucleation, resulting ice particle size distributions and backscatter ratios at the COBALD wavelengths. However, only by superimposing small-scale temperature fluctuations, which remain unresolved by the numerical weather prediction models, can we obtain a satisfying agreement with the observations and reconcile the measured in-cloud non-equilibrium humidities with conventional ice cloud microphysics. Conversely, the model-data comparison provides no evidence that additional changes to ice-cloud microphysics – such as heterogeneous nucleation or changing the water vapour accommodation coefficient on ice – are required., Atmospheric Chemistry and Physics, 12 (19), ISSN:1680-7375, ISSN:1680-7367

The accurate monitoring of climate change imposes strict requirements upon observing systems, in particular regarding measurement accuracy and long-term stability. Currently available data records of the essential climate variables (temperature-T, geopotential-p, humidity-RH, wind, and cloud properties) in the upper-air generally fail to fulfill such requirements. This raises serious issues about the ability to detect, quantify and understand recent climate changes and their causes. GCOS is currently implementing a Reference Upper-Air Network (GRUAN) in order to fill this major void within the global observing system. As part of the GRUAN implementation plan we provide herein fundamental guidelines for establishing and maintaining reference quality atmospheric observations which are based on principal concepts of metrology, in particular traceability. It is argued that the detailed analysis of the uncertainty budget of a measurement technique is the critical step for achieving this goal. As we will demonstrate with an example, detailed knowledge of the calibration procedures and data processing algorithms are required for determining the uncertainty of each individual data point. Of particular importance is the careful assessment of the uncertainties introduced by correction schemes adjusting for systematic effects.

A number of field-campaigns in the tropics have been conducted in the recent years with two different LIDAR systems at Paramaribo in Suriname (5.8° N, 55.2° W). The lidars detect particles in the atmosphere with high vertical and temporal resolution and are capable of detecting extremely thin cloud layers which frequently occur in the tropical tropopause layer (TTL). Radiosonde as well as operational ECMWF analysis show that temperature anomalies caused by equatorial Kelvin waves propagate downward, well below the cold point tropopause (CPT). We find a clear correlation between the temperature anomalies introduced by these waves and the occurrence of thin cirrus in the TTL. In particular we found that extremely thin ice clouds form regularly where cold anomalies shift the tropopause to high altitudes. This finding suggests an influence of Kelvin wave activity on the dehydration in the TTL and thus on the global stratospheric water vapour concentration.

In the framework of the European Project STAR the mobile Aerosol Raman Lidar (MARL) of the Alfred Wegener Institute (AWI) was operated in Paramaribo/Suriname (5.8°, 55.2°W) and carried out extensive observations of tropical cirrus clouds during the fall dry season from September 28 to November 15 2004. With 81% the coverage with ice clouds was very high in the upper troposphere (above 12 km). The frequency of occurrence of subvisible clouds was found to be clearly enhanced compared to the midlatitudes. The extinction-to-backscatter ratio of thin tropical cirrus is with 26 ± 7 sr significantly higher than that of mid-latitude cirrus (16 ± 9 sr).Subvisible cirrus occur mostly in the tropical tropopause layer (TTL) abovean upper tropospheric inversion. The formation conditions of these cloudsand the dehydration of the TTL was investigated by means of a newly developed trajectory model. Clouds were assumed to occur when the relativehumidity above ice (RHI) exceeds 100% due to adiabatic cooling and subsequently the humidity is reduced to the saturation vapor pressure. This simple dehydration scheme yields an amazingly good agreement with cloud andhumidity observations at Paramaribo. However, it challenges the way cirrusformation is generally conceived, since high supersaturation were thoughtto be required. Our results suggest, that the inclination of the TTL to contain solid particles is rather high, even under modestly moist conditions. Wealso detected extremely thin layers of solid particles slightly above the temperature minimum in sub-saturated air which are possibly stabilized by HNO3.

During the European heat wave summer 2003 with predominant high pressure conditions we performed a detailed study of upper tropospheric humidity and ice particles which yielded striking results concerning the occurrence of ice supersaturated regions (ISSR), cirrus, and contrails. Our study is based on lidar observations and meteorological data obtained at Lindenberg/Germany (52.2° N, 14.1° E) as well as the analysis of the European centre for medium range weather forecast (ECMWF). Cirrus clouds were detected in 55% of the lidar profiles and a large fraction of them were subvisible (optical depth

Airborne microphysical and optical properties of cirrus clouds, jet-stream and wave clouds were measured at temperatures ranging from −25° C to −62° C in the southern hemisphere from Punta Arenas (53° S) in March and April 2000 during the INCA experiment (INterhemispheric differences in Cirrus properties from Anthropogenic emissions). The observations related to cirrus clouds show general decreases of the ice water content (18 mg m−3 to 0.05 mg m−3), extinction coefficient (0.70 km−1 to 0.08 km−1), ice particle concentration (2.2 cm−3 to 0.5 cm−3), and the effective diameter (80 μm to 17 μm) linked to the variation of ambient temperature (−25° C to −60° C). The lowest temperature at which supercooled water droplets were detected was −33° C. The asymmetry parameter shows relatively small variations, with the smallest values (0.758) observed at the lowest temperatures. High-altitude clouds which form with a rapid vertical transport, i.e. jet-stream cirrus and orographic-wave ice-cloud, are characterized by very high values of ice particle concentrations (up to 100 cm−3) compared to mean values as a whole (1.45 cm−3). These two kinds of high clouds are the subjects of detailed case-studies. Although formed at a similar range of temperatures and with similar aerosol properties, the strong differences in cloud properties and humidity fields highlight the key role of the dynamical structure in controlling the formation, evolution and subsequent radiative properties of such high-altitude clouds.

During a measuring campaign in Lindenberg/Germany (14.5°E, 52.5°N) in August 2003, we observed an extended aerosol layer in the upper troposphere with our mobile Aerosol Raman Lidar (MARL). Backward trajectories indicated that this was a plume originating from forest fires. Water vapor Raman measurements performed with the same lidar showed a large discrepancy with co-located radiosonde measurements, which were not observed in undisturbed conditions. We interpret the unexpected properties of these aerosols as fluorescence induced by the laser beam at organic components of the aerosol particles. The detection of fluorescence from ambient aerosol with lidar systems has not yet been reported before. However, organic compounds such as polycyclic aromatic hydrocarbons sticking to the aerosol particles, or bioaerosol such as bacteria, spores or pollen fluoresce when excited with UV-radiation in a way that is detectable by our lidar system. Therefore, we conclude that fluorescence from organic material released by biomass burning induces inelastic backscatter signals which could be observed with the lidar. It provides for a new method to characterize atmospheric aerosols and should be taken into account when performing water vapor measurements with a Raman lidar.

With a lidar system that was installed in Lindenberg/Germany, we observed in June 2003 an extended aerosol layer at 13km altitude in the lowermost stratosphere. This layer created an inelastic backscatter signal that we detected with a water vapour Raman channel, but that was not produced by Raman scattering. Also, we find evidence for inelastic scattering from a smoke plume from a forest fire that we observed in the troposphere. We interpret the unexpected properties of these aerosols as fluorescence induced by the laser beam at organic components of the aerosol particles. Fluorescence from ambient aerosol had not yet been considered detectable by lidar systems. However, organic compounds such as polycyclic aromatic hydrocarbons sticking to the aerosol particles, or bioaerosol such as bacteria, spores or pollen fluoresce when excited with UV-radiation in a way that is detectable by our lidar system. Therefore, we conclude that fluorescence from organic material released by biomass burning creates, inelastic backscatter signals that we measured with our instrument and thus demonstrate a new and powerful way to characterize aerosols by a remote sensing technique. The stratospheric aerosol layer that we have observed in Lindenberg for three consecutive days is likely to be a remnant from Siberian forest fire plumes lifted across the tropopause and transported around the globe.

A unique data set of ship-borne lidar measurements of Saharan dust layers above the Atlantic ocean has been collected aboard the research vessel Polarstern with a mobile Aerosol Raman Lidar (MARL) during the LIMPIDO-campaign in June 2000. Extended Saharan dust layers have been observed in the region between 8.5º N and 34º N in an altitude range between 2 and 6 km. The continental, North African origin of the probed air masses is confirmed by 8-day backward trajectories. The Saharan dust is characterized by an optical depth in the range of 0.1 and 0.3, a depolarization around 10% and high lidar ratios of 45 sr at 532 nm and 75 sr at 355 nm. The backscattering by the dust particles at the UV-wavelength is relatively weak, resulting in a negative color index. From the measured optical properties the effective radius and the refractive index of the dust particles are derived using a new approach based on Mie Theory and non-spherical scattering calculations. The low backscatter coefficient observed at 355 nm is due to significant absorption which increases with decreasing wavelength. This finding agrees very well with results from satellite and sun photometer measurements. The effective radii decrease from about 3 mm at the base to 0.6 mm at the top of the dust plumes. The non-spherical shapes of the dust particles are responsible for the high values of the lidar ratios.

Sum frequency mixing of two solid state lasers was used to set up a DIAL system for stratospheric ozone measurements. By frequency mixing of a Ti:sapphire laser (720–960 nm) with the Nd:YAG second harmonic (532 nm), the wavelengths between 306 and 342 nm can be reached. The mixing efficiency of LBO, KDP and KD*P crystals was compared at different laser beam configurations. Tunable lidar measurements in the UV-B wavelength region and stratospheric ozone measurements were performed. The output pulse energy of 20 mJ reached here is sufficient to perform daylight measurements in the UV-B region.

Lidar measurements at 4 wavelengths and two polarizations were performed during the SESAME campaign in Sodankyla, Finland (67.37N, 26.65E). Using the wavelength dependence of the particle scattering the aerosol size distribution and the refractive index of the PSC particles were retrieved. A liquid PSC with a refractive index of 1.36 could be observed. We assume that this PSC consists of ternary solution particles in contradiction to the NAT-hypothesis.

Measurements of temporal and spatial distributions of sulfur dioxide, nitrogen dioxide, toluene, and aerosols have been made with two lidars in the industrial region of Cubatao, Brazl. Immissions, emissions and transport phenomena were investigated. The data provide a comprehensive picture of the actual situation and appear useful for future planning of new facilities in the area.

[1] The Intertropical Convergence Zone (ITCZ) passes twice a year over tropical Suriname, bringing two wet and two dry seasons. During a pilot study campaign in Suriname, cirrus clouds were observed with a mobile aerosol Raman lidar (MARL) and with balloon sondes containing a frost point hygrometer called Snow White, over the period October–November 2004. These observations are used to study the origin of cirrus clouds and the dynamical processes that determine their transport, using European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses. The height of cirrus occurrence is in phase with the height of the cold point tropopause, with maximum heights occurring during Northern Hemisphere winter that are about 2 km above the minimum values in summertime. The occurrence of cirrus often corresponds with a northerly meridional wind flow (in a layer underneath the tropopause), also when the ITCZ lies to the south in the period January–May. ECMWF analyses point out that inertial instability flow, in the form of vertically stacked meridional circulation cells in the upper troposphere (UT), can explain the transport of these cirrus events. Also evident is that radiative cooling of a moist layer transported in the UT leads to a thermal wind in the form of an easterly/westerly jet associated with the southward/northward transport of moist air. An interactive play between the inertial instability and thermal wind mechanisms explains many of the observed features of cirrus occurrence over Suriname. The observed cirrus mostly originates from the ITCZ or from deep convective centers to the south that form during the early summer monsoon.

[1] High-altitude tropical cirrus (TC) have been observed by lidar during a cruise of the German research vessel ‘Polarstern’ above the Atlantic between 8°S and 12°. The backscatter lidar data give a detailed picture of the vertical structure of the clouds with a high time resolution. Using the data of radiosondes, which were launched twice a day aboard ‘Polarstern,’ the temperature of the clouds as well as the structure of the tropical tropopause is characterized. The clouds are found in an altitude between 14 km and 17 km, with a mean optical depth of τ = 0.02 and a mean temperature of 198 K. Tropical Cirrus appear in two distinct layers which are separated by a change in wind direction. This structure was observed south as well as north of the ITCZ. Cloud tops are often found at the thermal tropopause, in some case these clouds are of the ultra-thin type.

[1] Cirrus clouds have been probed at two locations in the midlatitudes of the southern (SH) and northern hemisphere (NH) with a Mobile Raman Aerosol Lidar (MARL) as part of the European INCA field experiments in 2000. MARL offers the opportunity to observe visible and subvisible clouds and to determine the optical depth τ in a range from below 10−3 up to 3. Despite different synoptic conditions we find at both locations similar macrophysical cloud properties, like mean base and top temperature among others. The frequencies of the optical depth show a characteristic distribution with a high share of subvisible clouds at both locations. While thin haze layers are frequently detected in the NH alone, we find highly depolarizing cirrus clouds only in the SH. These differences indicate that different cirrus properties and formation mechanisms are present at the two sites.

Multispectral lidar measurements of polar stratospheric clouds (PSCs) from two winter campaigns in 1994/1995 and 1996/1997 at Sodankylä, Finland, have been evaluated together with temperature data from local radiosondes to find optical parameters for a PSC classification of different particle types and their existence temperatures. Precise depolarization measurements show that both solid and liquid particles exist below the NAT (nitric acid trihydrate) temperature. A comparison of temperatures at the PSC base and at the cloud top shows a good agreement with the NAT-existence temperature for solid type Ia clouds and a 3-4 K lower temperature for liquid type Ib clouds. The two particle families are therefore consistent with solid NAT particle formation and condensational growth of HNO3, H2O and H2SO4 liquid ternary solutions. The coexistence of solid and liquid particles has been observed by means of the temporal development of parallel and perpendicular polarized lidar signals. These time series of subsequent lidar measurements show stronger and faster fluctuations in the liquid particle mode compared to the solid particles and thus indicate a higher sensitivity toward temperature fluctuations for the liquid PSCs. While the optical properties of most observations are consistent with the definition of PSC type Ia (solid) and type Ib (liquid) clouds, a third type has been observed which does not fit into the current type Ia/Ib optical classification. This cloud type consists of solid particles but has a higher backscatter than type Ia PSC. Copyright 1999 by the American Geophysical Union.