Tropospheric water-vapour and ozone cross-sections in a zonal plane over the Central Equatorial Pacific Ocean

Tropospheric water-vapour and ozone measurements, using calibrated balloon-borne sensors, are reported from the Central Equatorial Pacific Experiment (CEPEX). The sensors were launched from the Research Vessel Vickers along 2 degrees S latitude between 156 degrees E (west of the international date line) and 155 degrees W (east of the date line). These measurements are combined with those from water-vapour sondes launched over the western Pacific warm pool, during the Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE). Taking the two experiments CEPEX and TOGA-COARE together, the sensors included frost-point hygrometers, Humicap-A Vaisala sondes, Humicap-H Vaisala sondes and electrochemical ozone-sondes. Taken together, the CEPEX and TOGA-COARE data provide over 150 vertical profiles of water vapour within the troposphere in varied conditions of convective activity ranging from disturbed to suppressed. The primary motivation behind the present analyses is to understand the role of tropical deep convection in the vertical distribution of water-vapour. With this in mind, the profiles have been analysed in relation to occasions of recent deep convection and occasions when convection was suppressed. We employ three different criteria to identify the profiles influenced by deep convection brightness temperature in the infrared-window channel of the Japanese Geostationary Meteorological Satellite (GMS); ozone as a quasi-conservative tracer for deep convection; and using water vapour itself, that is the wettest versus the driest soundings. Irrespective of the criteria used, we report here that the atmosphere, while under the influence of active deep convection, was found to have relative humidities in excess of 75% over most of the troposphere between the surface and about 14 km. The sondes were launched routinely over a period of 45 days (between CEPEX and TOGA-COARE), without biasing the sample towards convectively disturbed conditions. A feature of the convectively disturbed profile is a distinct minimum in relative humidity at about 700 hPa, where it was as low as 65%. The low relative humidity was accompanied by relatively high ozone mixing ratios, which raises the possibility of long-range transport of dry sub-tropical air into the warm, convectively disturbed, regions of the equatorial Pacific Ocean. Inspection of the analysed fields, and the wind fields from the sondes, supports this assertion. It then follows that the omnipresent minimum of moist static energy and minimum relative humidity at 700 hPa in the inner tropics may be the result of long-range, inclined, transport of dry air from non-convective regions. This detection suggests a linkage between the large-scale circulation, deep convection and the thermodynamic structure within the equatorial troposphere. The results presented here demonstrate the applicability of ozone as a quasi-conservative tracer of transport in the context of deep convection. The ozone-based criterion is used to diagnose recent deep convection, independent of the GMS satellite observations, and allows one to follow the evolution of relative humidity and of water-vapour mixing ratio after the dissipation of the cloud anvil to optically thin conditions. We show that the troposphere dries to low humidity soon after anvil dissipation. This observation leads to the hypothesis that moistening of the atmosphere, away from the core of Cb convection, occurs by evaporation of precipitation falling out of the anvils. After anvil dissipation, the ensuing subsidence in clear air causes the relative humidity and the water mixing ratio to decrease

Analysis of Raman Lidar and radiosonde measurements from the AWEX-G field campaign and its relation to Aqua validation

Early work within the Aqua validation activity revealed there to be large differences in water vapor measurement accuracy among the various technologies in use for providing validation data. The validation measurements were made at globally distributed sites making it difficult to isolate the sources of the apparent measurement differences among the various sensors, which included both Raman lidar and radiosonde. Because of this, the AIRS Water Vapor Experiment-Ground (AWEX-G) was held in October - November, 2003 with the goal of bringing validation technologies to a common site for intercomparison and resolution of the measurement discrepancies. Using the University of Colorado Cryogenic Frostpoint Hygrometer (CFH) as the water vapor reference, the AWEX-G field campaign resulted in new correction techniques for both Raman lidar, Vaisala RS80-H and RS90/92 measurements that significantly improve the absolute accuracy of those measurement systems particularly in the upper troposphere. Mean comparisons of radiosondes and lidar are performed demonstrating agreement between corrected sensors and the CFH to generally within 5% thereby providing data of sufficient accuracy for Aqua validation purposes. Examples of the use of the correction techniques in radiance and retrieval comparisons are provided and discussed

Observations of near-zero ozone concentrations over the convective Pacific: Effects on air chemistry

A series of measurements over the equatorial Pacific in March 1993 showed thai the volume mixing ratios of ozone were frequently well below 10 nanomoles per mole both in the marine boundary layer (MEL) and between 10 kilometers and the tropopause. These latter unexpected results emphasize the enormous variability of tropical tropospheric ozone and hydroxyl concentrations, which determine the oxidizing efficiency of the trophosphere. They also imply a convective short circuit of marine gaseous emissions, such as dimethyl sulfide, between the MBL and the uppermost troposphere, leading, for instance, to sulfate particle formation

Reference Quality Upper-Air Measurements: guidance for developing GRUAN data products

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

Measurements of Humidity in the Atmosphere: Validation Experiments (MOHAVE I and MOHAVE II). Results Overview and Implication for the Long-Term Lidar Monitoring of Water Vapor in the UT/LS

1. MOHAVE+MOHAVE II = very successful. 2. MOHAVE -> Fluorescence was found to be inherent to all three participating lidars. 3. MOHAVE II -> Fluorescence was removed and agreement with CFH was extremely good up to 16-18 km altitude. 4. MOHAVE II -> Calibration tests revealed unsuspected shortfalls of widely used techniques, with important implications for their applicability to longterm measurements. 5. A factor of 5 in future lidar signal-to-noise ratio is reasonably achievable. When this level is achieved water vapor Raman lidar will become a key instrument for the long-term monitoring of water vapor in the UT/L

Radiation Dry Bias of the Vaisala RS92 Humidity Sensor

The comparison of simultaneous humidity measurements by the Vaisala RS92 radiosonde and by the Cryogenic Frostpoint Hygrometer (CFH) launched at Alajuela, Cosla Rica, during July 2005 reveals a large solar radiation dry bias of the Vaisala RS92 humidity sensor and a minor temperature-dependent calibration error. For soundings launched at solar zenith angles between 10" and 30 , the average dry bias is on the order of 9% at the surface and increases to 50% at 15 km. A simple pressure- and temperature-dependent correction based on the comparison with the CFH can reduce this error to less than 7% at all altitudes up to 15.2 km, which is 700 m below the tropical tropopause. The correction does not depend on relative humidity, but is able to reproduce the relative humidity distribution observed by the CFH

Reference Upper-Air Observations for Climate: Rationale, Progress, and Plans

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