A large variety of water vapour data records which are constructed from either satellite measurements or outputs of reanalyses is available to date. Without proper background information and understanding of the limitations of these available data records, they may be incorrectly utilised or misinterpreted. Thus, assessments are conducted to objectively and independently evaluate and inter-compare available climate data records (CDRs) in order to point out strengths, differences and limitations and, if possible, to provide reasons for them. This is why the GEWEX Data and Assessments Panel (GDAP) has initiated the GEWEX water vapor assessment (G-VAP) which has the major purpose to quantify the current state-of-the-art in water vapour products (upper tropospheric humidity, specific humidity and temperature profiles as well as total column water vapor) being constructed for climate applications. G-VAP intends to answer, among others, the following questions:
a) How large are the differences in observed temporal changes in long-term data records of water vapor on global and regional scales?
b) Are the observed differences in temporal changes within uncertainty limits?
c) What is the degree of homogeneity (presence of break points) of each long-term data record?
A general overview of G-VAP will be given which includes an inventory of available water vapor data records (see also http://gewex-vap.org/?page_id=13) and an introduction to the G-VAP data archive (doi: 10.5676/EUM_SAF_CM/GVAP/V001). The focus of the presentation will be on observed inconsistencies among long-term CDRs (eleven TCWV CDRs and seven specific humidity and temperature profile CDRs). These inconsistencies are studied by inter-comparisons, comparisons to in-situ observations and a stability analysis. On basis of consistently applied tools major differences in CDRs have been identified, documented and to a large extend explained. We will focus mainly on TCWV, for which the results and the answers can be summarized as follows: On global ice-free ocean scale the trend estimates among long-term data records were generally found to be significantly different. Maxima in standard deviation among the data records are found over, e.g., tropical rain forests. These and other noticeable regions coincide with maxima in mean absolute differences among trend estimates. These distinct features can be explained with break points which manifest on regional scale and which typically do not appear in stability analysis relative to ground-based observations. Results from profile inter-comparisons will also been shown and exhibit, among others, that the observed break points are not only a function of region but also of parameter, i.e., break points observed in specific humidity data are typically not present in corresponding temperature data and vice versa.
With the publication of the WCRP report on G-VAP (16/2017, available at https://www.wcrp-climate.org/resources/wcrp-publications) G-VAP’s first phase ends. As agreed upon at the 7th G-VAP workshop G-VAP will continue with unchanged scope, objectives, and governance under the umbrella of GDAP. Also, core activities such as probability density function, consistency and stability analysis will be continued to provide a sustained service to the community. Finally, new science activities have been discussed and defined. Among others, these include the analysis of the quality of profile data records over stratus regions and the assessment and characterization of the differences between observations in all sky, cloudy sky and clear sky conditions. First results of this analysis will be shown.

In the Earth Climate system, water vapour is a key greenhouse gas influencing (directly and indirectly) the radiative balance of the Earth as well as surface and soil moisture fluxes. It is sufficiently abundant and short-lived that it is considered under natural control. With a predominant capacity for positive feedback, water vapour acts as the largest amplification mechanism for anthropogenic climate change compared to radiative forcing from greenhouse gases. The Planetary Boundary Layer (PBL) refers to the lowest region of the atmosphere (between 100 m to 3000 m) which is directly influenced by surface processes over both the land and ocean. Containing approximately 80% of the total mass of atmospheric water vapour, the PBL regulates the exchange of heat, moisture, momentum, trace gases and aerosols between the Earths surface and the free troposphere. The exchange of water vapour through evapotranspiration with the free troposphere is a key component in the water and energy cycle as without the subsequent phase changes most processes would be temperature driven. Therefore accurate characterisation of heat and water vapour transport is needed to fully describe the coupling between surface hydrology, clouds and precipitation.

Global observations of water vapour fields in this golden age of satellite remote sensing are made from numerous instruments measuring in the ultraviolet/visible, through the infrared bands, to the microwave regions of the electromagnetic spectrum. While these observations provide numerous observations of columnar, free-tropospheric and upper troposphere/lower stratosphere water vapour amounts, there is still an observational gap when considering resolved bulk planetary boundary layer (PBL) concentrations. In this study, we demonstrate the ability of the Greenhouse Gases Observing SATellite (GOSAT) too bridge this gap from highly resolved measurements in the shortwave infrared (SWIR). These new measurements of near-surface columnar water vapour are free of topographic artefacts and are interpreted as a proxy for bulk PBL water vapour. We first describe the retrieval process of PBL water vapour from GOSAT SWIR bands, including details on the retrieval uncertainty. Next, we describe validation results from inter-comparisons with radiosondes (over land surfaces only). This validation exercise uses collocations with the Analyzed Radiosounding Archive (ARSA) collected between June 2009 and May 2017 to assess the performance of GOSAT PBL water vapour. The relationship between these new PBL measurements to 2m surface humidity is examined with some initial comparisons to HadISDH. Finally, we discuss the potential for PBL water vapour Climate Data Records (CDRs) from GOSAT with the recent launch of GOSAT-2.

Accurate water vapour measurements are crucial for a better understanding of Arctic climate change and the global hydrological cycle [1]. Passive microwave satellite-borne sensors allow obtaining the vertically integrated water vapour content of the atmosphere (Total Water Vapour, TWV), which is especially important for the sparsely observed polar regions. However, the available retrievals work only over certain surface types. Over open ocean, microwave imagers like AMSR-2 or AMSR-E provide daily integrated water vapour [2]. Over ice and land surfaces, another method based on data of the microwave humidity sounder AMSU-B allows TWV retrieval, or values below 15 kg/m², a condition frequently met in the polar regions [3].
The main advantages of this latter retrieval are its full coverage of the sea ice and land areas and the higher accuracy over dry open water areas compared to other methods. However, this method retrieves erroneously low TWV over areas of convective clouds with high ice content, as studied in [4] for AMSU-B frequencies. An automatic filter to remove the affected spurious total water vapour values has been developed. Our approach is to perform a morphological analysis of the data to find the affected water vapour values, characterized for being small areas of low TWV (<4 kg/m²) surrounded by higher values. This additional screening is independent of other measurements and greatly improves the AMSU-B retrieval quality. In addition, in order to provide a long-term data series from 1998 until present day, the algorithm has been adapted to also use MHS sensor data, the successor of AMSU-B on the METOP and recent NOAA satellites. An assessment of the differences between both sensors TWV retrieval is presented.
To merge the two complementary retrievals, a study of the overlap area between the AMSU-B/MHS and AMSR-E retrieval has been done. The goal is to provide an Arctic-wide daily TWV dataset of 50 km resolution with seamless coverage from the high Arctic to mid latitudes from 2002 to date.

References
[1] B. Stevens and S. Bony, What are climate models missing?, Science 340 1053-1054 (2013)
[2] F.J. Wentz and T. Meissner, AMSR Ocean Algorithm, Version 2. Report number 121599A-1, 66 pp, Remote Sensing Systems, Santa Rosa, CA, 2006
[3] C. Melsheimer and G. Heygster, Improved Retrieval of Total Water Vapor Over Polar Regions from AMSU-B Microwave Radiometer Data, IEEE Trans. Geosci. Rem. Sens. 46 (2008)
[4] T. Sreerekha, Impact of clouds on microwave remote sensing, PhD. Dissertation, University of Bremen (2005)

The metropolitan area of Milan is a high-risk territory due to the presence of four torrential rivers (Lambro, Seveso, Groane and Olona rivers) in which severe and short thunderstorms can frequently trigger flash floods. In 2014, considering only the Milan municipality, Seveso floods produced damages for several million euros.
Funded by the Cariplo Foundation, the LAMPO project (Lombardy-based Advanced Meteorological Predictions and Observations) aims at mitigating the impact of severe thunderstorm in the Milan area by developing a nowcasting system based on the water vapor observations derived by an experimental dense network of low-cost GNSS receivers. The project is led by the Politecnico di Milano and ARPA della Lombardia, in cooperation with GReD srl and Università degli studi di Padova.
With the objective of building a nowcasting algorithm based on neural networks, we started by analyzing time series of atmospheric parameters from 2001 to 2017 in the Lombardy Region (specifically the data collected by 183 weather stations homogenously distributed in the region and by the existent GNSS permanent networks) and creating a monthly and seasonal climatology of pressure, temperature, relative humidity, integrated water vapor and wind speed. Additional information about extreme weather events has been collected from radar measurements and rain gauges with one-minute and five-minute temporal resolution.
Merging the information of convective cells database retrieved by radar data, rain gauge sensors and hydrometers on the Seveso area, we have first chosen two case studies of extreme events causing floods, and then we have systematically selected all the convective events generating a rain rate higher than the 95th percentile.
We then started investigating the anomaly of each parameter during the selected events with respect to the climatological value of the same area, in order to characterize the atmospheric structure during the extreme convective events. The difference between the current atmospheric values with the climatology represents the perturbation due to the presence of the extreme event and thus provides fundamental information to understand the convective and pre-convective environment.
With this work, we characterize the atmospheric structure for extreme convective events with particular emphasis on the water vapor gradient before the convection.
A further step will be the densification of the existent GNSS permanent networks (station inter-distances in the order of 50km) by installing a set of low cost receivers (interdistance less than 10 km) in the Seveso area to evaluate the impact that a finer resolution monitoring of water vapor have on the nowcasting of locally developed convective cells.

The water vapour in the lower stratosphere strongly influences the outgoing longwave radiation and thus is critical for the Earth radiation budget and climate. It is concerning that the distribution of the water vapour in this region remains poorly observed. Current satellite observations have limited ability to monitor the water vapor variations in this region because the nadir-view measurements usually do not have sufficient sensitivity to its low concentration (several ppm) and limb-view measurements have too large sampling footprints (several hundred km) to detect the small-scale variability, e.g., the hydration or dehydration due to deep convection.

In this work, we integrate high-resolution modeling with an innovative retrieval technique to investigate how deep convection influences the lower stratospheric water vapour distribution. First, strong overshooting convective events are simulated using the Global Environmental Multiscale model (GEM) with 4 self-nested domains over the North America continent with areas of 2,500, 1,280, 512 and 250 km2 and corresponding horizontal grid-spacing of 10.0, 2.5, 1.0 and 0.25 km, respectively. Our analysis is focused on the water vapour distribution above the deep convective clouds, which is validated against collocated satellite and aircraft measurements. The simulation identifies key processes that affect the water vapor budget in the lower stratosphere through the deep convective transport. The simulation also provides a synthetical database for describing the temporal and spatial variability of water vapour and for assessing the necessary instrumentation requirements for the observing the water vapour variation.

On the other hand, we develop a “cloud-assisted” retrieval technique to retrieve the water vapour distribution in the lower stratosphere. The innovative aspect of this method is that it takes advantage of the cloud layer that exists in the tropopause region to eliminate two difficulties that fundamentally impede the retrieval of water vapor in this region using satellite spectrometers. The first arises from the much larger concentration and variability in water vapor in the lower atmosphere, which due to the finite vertical resolution of the retrieval technique masks the signal of upper level water vapour. The second arises from the non-monotonic temperature change across the tropopause, which makes the retrieval lack thermal contrast to locate water vapour anomaly. Using both theoretical assessments based on synthetic data and experimental retrievals compared to aircraft in-situ water vapour measurements, we find that the new method greatly improves the detection of water vapour concentration anomalies caused by deep convection. Applying the method to the radiance measurements of the Atmospheric Infrared Sounder (AIRS), the distribution of water vapour above the deep convection is characterized.

The Spatial Heterodyne Observations of Water instrument (SHOW) is a limb-sounding satellite prototype that is being developed in Canada to provide high spatial resolution profiling of water vapour in the upper troposphere and lower stratosphere region. The instrument approach provides rapid imaging of limb scattered sunlight spectra in a near infrared vibrational absorption band of water vapour. In 2017, SHOW flew several flights on NASA’s ER-2 airplane, allowing for sub-orbital demonstration of the measurement technique. Here, we present the results from the measurement campaign and examine the sampling capabilities of the instrument.