Vapor Distribution Research Articles

Experimental and numerical investigation of low sulfur heavy fuel oil spray characteristics under high temperature and pressure conditions

The spray characteristics of low sulfur heavy fuel oil (HFO) are firstly investigated by the experimental and numerical methods, and some new findings are obtained. First, the experiments have been conducted to evaluate the influence of low-sulfurization on the spray characteristics for HFOs under varied conditions, while little effect is found on the spray macroscopic features. The tip penetration of HFO spray behaves like a non-evaporating spray, but it is like an evaporation spray in terms of the cone angle under high temperature and pressure conditions. The large increment in ambient temperature is found to show little effect on its tip penetration but lead to a visible reduction in cone angle. This is because the spray tip penetration and cone angle are separately predominated by non-volatile and volatile components. The evaporation and pyrolysis penetration lengths are proposed for low sulfur HFO sprays as criteria for discriminating the dominant periods by volatile and non-volatile components. Then, a multicomponent evaporation model combined with real-gas vapor–liquid equilibrium has been implemented into OpenFOAM code. The numerical simulations have been carried out and extensively validated against the experimental data. The simulated HFO spray exhibits a distinct liquid penetration length, which agrees with the proposed evaporation penetration length in experiment. This is contributed to confirming the occurrence of pyrolysis and polymerization in low sulfur HFO spray. Finally, the vapor distribution and stratification in the low sulfur HFO spray are presented, and the heavy-end multi-ring aromatics exhibit low evaporation rate and large effects on the mixture generations.

Analysis of diurnal to seasonal variability of Integrated Water Vapour in the South Indian Ocean basin using ground‐based GNSS and fifth‐generation ECMWF reanalysis (ERA5) data

AbstractThe spatial and temporal distribution of tropospheric water vapour in the South Indian Ocean (SIO) basin is investigated using observations collected from twelve GNSS stations spanning the basin. The comparison of GNSS‐derived integrated water vapour (IWV) content against radiosoundings and satellite‐borne microwave radiometer data shows good agreement, with global uncertainties ranging from 0.76 to 1.17 kg·m−2, depending on GNSS station locations. GNSS‐derived IWV contents show a strong seasonal cycle, characterized by higher water vapour content during the austral summer, when the InterTropical Convergence Zone (ITCZ) is located in the Southern Hemisphere. At the seasonal time‐scale, the observed annual IWV amplitude varies from 10 to 15 kg·m−2 near the Equator to 20 to 30 kg·m−2 in the Subtropics. The GNSS IWV signature of the Madden–Julian Oscillation (MJO) is hardly noticeable during the Austral winter, but varies from 1–2 to 4 kg·m−2 between the active and suppressed phases of the MJO during austral summer. At diurnal time‐scales, GNSS IWV shows larger diurnal amplitude over land (2–3 kg·m−2) than over open ocean (1–2 kg·m−2), with highest amplitudes (up to 7 kg·m−2) observed over large and mountainous islands. The phase analysis of the IWV diurnal cycle indicates that the diurnal maximum (minimum) is reached in the late afternoon/evening (morning) over land, at night (mid‐day) over ocean and in the early morning (late afternoon) at coastal locations. A comparison of GNSS‐derived IWV contents against fifth‐generation European Centre for Medium‐range Weather Forecasts (ECMWF) Reanalysis (ERA5) data shows that ERA5 generally correctly reproduces the IWV content at both seasonal, intra‐seasonal and diurnal time‐scales, although some discrepancies can be noticed over small islands characterised by steep orography. The signature of the MJO in ERA5 also shows good agreement with GNSS observations at most studied locations.

Effect of in-cylinder air-water interaction on water evaporation and performance characteristics of a direct water injected GDI engine

Water injection is one of the promising strategies for modern downsized GDI engines to meet performance and emission requirements under wide operating conditions. However, the benefits of water injection strongly depend upon water evaporation and distribution of water vapours inside the combustion chamber. In this regard, water injector location and orientation are two significant geometrical parameters which decide the in-cylinder positioning of the water spray, evaporation, and vapour distribution. Therefore, this CFD study presents the effects of water injector location, and orientation on the in-cylinder air-water interaction, water evaporation, vapour distribution, performance and emission characteristics of a wall-guided GDI engine. It also presents the effects of water injector location and orientations on water evaporation rate, water droplet distribution, heat release rate, IMEP, and exhaust emissions. In this study, the spark-plug is considered as located in the middle of the cylinder head, and the water injector is mounted in between the exhaust valves. Four locations of the water injector (21 mm, 25 mm, 29 mm, and 33 mm from the spark-plug) are considered for the investigation. Also, the injector orientation is varied from 12° to 28°, in the step of 4°. The engine is operated under naturally aspirated and full-load conditions at an engine speed of 2000 rev/min. Analysis of results shows that the water evaporation rate increases when more amount of water is sprayed in the region where the velocities of water droplets are in the opposite direction of that of the in-cylinder air. It is found that the water injector located at 29 mm with the orientation of 24° is found to be optimum configuration, where the IMEP improved by about 9.2%, while NOx and soot emissions reduced by about 48.2% and 22.6% respectively compared to those of the case of no_water.

An optical diagnostic technique based on ultraviolet absorption and schlieren for components stratification in a binary-component fuel–air mixture

The evaporation characteristics of multi-component fuel have significant effects on the fuel–air mixing process and subsequent processes, such as ignition, combustion, and harmful pollutants emission formation. When a multi-component fuel is directly injected into the combustion chamber, spatial and temporal equivalence ratio stratification may happen due to the preferential evaporation of components with different volatilities. However, there is a lack of effective optical diagnostics of the spatial stratification in a multi-component fuel spray. In this study, new optical technique based on ultraviolet absorption and visible light schlieren (UAVS) is for the first time proposed to measure the stratification of vapor distributions stratification in a binary-component fuel spray. UAVS technique was applied to investigate the effects of mixing ratios in a binary-component fuel spray (p-xylene and n-hexane) and ambient temperature on the stratification of components with different volatilities. The results denote the UAVS technique as the effective method for the observation of stratification of a multi-component fuel spray. In this study, the stratification phenomenon was the most evident at the ratio of 1:1. At the same time, when the ambient temperature is much higher than the dew point temperature of mixture, the stratification rarely occurs, because of the rapid evaporation thus providing time not enough for preferential evaporation. Graphic abstract

Towards tropospheric delay estimation using GNSS smartphone receiver network

Abstract Information on the water vapor distribution of the troposphere is useful for weather monitoring and forecast. Water vapor distribution can be estimated from tropospheric delays produced by high-grade Global Navigation Satellite System (GNSS) receivers. This type of techniques is currently used in the data assimilation process of Numerical Weather Prediction (NWP) models, especially, for the limited areas covered by these high-grade GNSS networks. We consider a new collaborative crowdsourcing-based alternative for obtaining these GNSS meteorology measurements. It relies on a GNSS smartphone receiver network, and hence promises to expand the use of GNSS meteorology techniques into areas not covered by high-end receiver networks. In order to assess the feasibility of estimating the troposphere water vapor distribution using such receiver networks, it is proposed a system architecture that supports the troposphere water vapor distribution estimation using a smartphone network. Next, it is presented the simulator test-bed that has been developed to emulate the proposed system in a representative way and to assess the system performances. The main motivation behind the simulator is that it provides a controlled environment for testing our method.

Quantifying Convective Aggregation Using the Tropical Moist Margin's Length

AbstractOn small scales, the tropical atmosphere tends to be either moist or very dry. This defines two states that, on large scales, are separated by a sharp margin, well identified by the antimode of the bimodal tropical column water vapor distribution. Despite recent progress in understanding physical processes governing the spatiotemporal variability of tropical water vapor, the behavior of this margin remains elusive, and we lack a simple framework to understand the bimodality of tropical water vapor in observations. Motivated by the success of coarsening theory in explaining bimodal distributions, we leverage its methodology to relate the moisture field's spatial organization to its time evolution. This results in a new diagnostic framework for the bimodality of tropical water vapor, from which we argue that the length of the margin separating moist from dry regions should evolve toward a minimum in equilibrium. As the spatial organization of moisture is closely related to the organization of tropical convection, we hereby introduce a new convective organization index (BLW) measuring the ratio of the margin's length to the circumference of a well‐defined equilibrium shape. Using BLW, we assess the evolution of self‐aggregation in idealized cloud‐resolving simulations of radiative‐convective equilibrium and contrast it to the time evolution of the Atlantic Intertropical Convergence Zone (ITCZ) in the ERA5 meteorological reanalysis product. We find that BLW successfully captures aspects of convective organization ignored by more traditional metrics, while offering a new perspective on the seasonal cycle of convective organization in the Atlantic ITCZ.

Effect of local hot spots on flow boiling in a parallel strip fin heat sink

The present study experimentally explored the flow boiling phenomena in mini-scale heat sinks under local heating conditions. A parallel strip fin heat sink (PSFHS) and a mini-channel heat sink (MCHS) are comparatively tested using R141b as working fluid. Nine separate spot-heaters are uniformly attached to the backside of the heat sinks to simulate the non-uniform heating condition. Thermocouples integrated in the nine heaters are used to gain the temperature profile. It can be found that the two-dimensional expansion of vapor in PSFHS improves the distribution of vapor and reduces the local vapor quality. Besides, the additional nucleation sites produced by the interrupted configuration of PSFHS generate more bubbles enhancing the local flow boiling. However, the bubbles coalesce and form slugs continuously, which would weaken the inflow of the working fluid. It is concluded that the temperature rise at the hot-spot location is significantly influenced by the structural configuration of the heat sinks. It is noted that for mini-scale heat sinks designed for hot-spot cooling, the interrupted channel configuration has a difference with the continuous channel configuration, but perhaps not significant.

Impact of preferential vaporization on combustion chamber deposit generation from a gasoline surrogate: A parametric study

The effect of injection pressure and impingement-surface temperature on combustion chamber deposit (CCD) inside a constant volume combustion chamber (CVCC) was studied. The CVCC was modified to capture the main characteristics of the spray-wall and film–flame interaction observed in gasoline direct injection (GDI) engines. The measurements were performed at three different injection pressures (30, 100, and 200 bar) and four wall temperatures (353, 393, 433, and 473 K) using a gasoline surrogate (S01) with four components (hexane, isooctane, toluene, 1-methylnaphthalene), under a global equivalence ratio of one. High speed Schlieren measurements and Mie scattering were used to characterize the spray–wall interaction. Moreover, the influence of the vapor distribution of the heavy and light fractions of a second non-fluorescent surrogate (S02, with similar vaporization behavior and composition to S01) doped with p-difluorobenzene (pDFB) and 1-Methylnaphtalene (1-MN) was analyzed around the impingement region. The fluorescent signal of the traces made it possible to study indirectly the effect of preferential vaporization on the CCD generation. Finally, the CCD build-up rate was determined by a gravimetric method. It was found that regardless of the injection pressure, the maximum production of CCD took place at a wall temperature of 393 K, and that an additional increase in the temperature reduced the build-up rate of CCD. The higher retention of heavy fraction on the impingement region at 353 and 393 K, identified by fluorescence, could not explain by itself the higher production of CCD outside the impingement region.

Reconstruction of Wet Refractivity Field Using an Improved Parameterized Tropospheric Tomographic Technique

In most previous studies of tropospheric tomography, water vapor is assumed to have a homogeneous distribution within each voxel. The parameterization of voxels can mitigate the negative effects of the improper assumption to the tomographic solution. An improved parameterized algorithm is proposed for determining the water vapor distribution by Global Navigation Satellite System (GNSS) tomography. Within a voxel, a generic point is determined via horizontal inverse distance weighted (IDW) interpolation and vertical exponential interpolation from the wet refractivities at the eight surrounding voxel nodes. The parameters involved in exponential and IDW interpolation are dynamically estimated for each tomography by using the refractivity field of the last process. By considering the quasi-exponential behavior of the wet refractivity profile, an optimal algorithm is proposed to discretize the vertical layers of the tomographic model. The improved parameterization algorithm is validated with the observational data collected over a 1-month period from 124 Global Positioning System (GPS) stations of Hunan Province, China. Assessments by GPS, radiosonde, and European Centre for Medium-Range Weather Forecasts (ECMWF) ReAnalysis 5 (ERA5) data, demonstrate that the improved model outperforms the traditional nonparametric model and the parameterized model using trilinear interpolation. In the assessment by GPS data, the improved model performs better than the traditional model and the trilinear parameterized model by 54% and 10%, respectively. Such improvements are 31% and 10% in the validation by radiosonde profiles. In comparison with the ERA5 reanalysis, the improved model yields a minimum overall root mean square (RMS) error of 8.94 mm/km, while those of the traditional and trilinear parametrized models are 10.79 and 9.73 mm/km, respectively. The RMS errors vertically decrease from ~20 mm/km at the bottom to ~5 mm/km at the top layer.

Effects of the Thickness of Boundary Layer on Droplet’s Evaporation Rate

Abstract The evaporation rate of a droplet was explained in relation to the thickness of the boundary layer and the condition near the droplet’s surface. However, the number of results obtained from experiments is very limited. This study aims to investigate the thickness of the boundary layer of an ethanol‐water mixture droplet and its effect on the evaporation rate by Z-type Schlieren visualization. Single and double droplets are tested and compared to identify the effect of the second droplet on the average and instantaneous evaporation rate. The double droplet’s lifetime is found to be longer than the single droplet’s lifetime. The formation of a larger vapor region on the top of the droplet indicates a higher instantaneous evaporation rate. The thickness of the boundary layer is found to increase with increase in ethanol concentration. Furthermore, a larger vapor distribution area is found in the case of higher ethanol concentration, which explains the faster evaporation rate at higher ethanol concentration.

Assessment of Cavitation Models for Compressible Flows Inside a Nozzle

This study assessed two cavitation models for compressible cavitating flows within a single hole nozzle. The models evaluated were SS (Schnerr and Sauer) and ZGB (Zwart-Gerber-Belamri) using realizable k-epsilon turbulent model, which was found to be the most appropriate model to use for this flow. The liquid compressibility was modeled using the Tait equation, and the vapor compressibility was modeled using the ideal gas law. Compressible flow simulation results showed that the SS model failed to capture the flow physics with a weak agreement with experimental data, while the ZGB model predicted the flow much better. Modeling vapor compressibility improved the distribution of the cavitating vapor across the nozzle with an increase in vapor volume compared to that of the incompressible assumption, particularly in the core region which resulted in a much better quantitative and qualitative agreement with the experimental data. The results also showed the prediction of a normal shockwave downstream of the cavitation region where the local flow transforms from supersonic to subsonic because of an increase in the local pressure.

Total column water vapor retrieval for Global Ozone Monitoring Experience-2 (GOME-2) visible blue observations

Abstract. We present a new total column water vapor (TCWV) retrieval algorithm in the visible blue spectral band for the Global Ozone Monitoring Experience 2 (GOME-2) instruments on board the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Metop satellites. The blue band algorithm allows the retrieval of water vapor from sensors which do not cover longer wavelengths, such as the Ozone Monitoring Instrument (OMI) and the Copernicus atmospheric composition missions Sentinel-5 Precursor (S5P), Sentinel-4 (S4) and Sentinel-5 (S5). The blue band algorithm uses the differential optical absorption spectroscopic (DOAS) technique to retrieve water vapor slant columns. The measured water vapor slant columns are converted to vertical columns using air mass factors (AMFs). The new algorithm has an iterative optimization module to dynamically find the optimal a priori water vapor profile. This makes it better suited for climate studies than usual satellite retrievals with static a priori or vertical profile information from the chemistry transport model (CTM). The dynamic a priori algorithm makes use of the fact that the vertical distribution of water vapor is strongly correlated to the total column. The new algorithm is applied to GOME-2A and GOME-2B observations to retrieve TCWV. The data set is validated by comparing it to the operational product retrieved in the red spectral band, sun photometer and radiosonde measurements. Water vapor columns retrieved in the blue band are in good agreement with the other data sets, indicating that the new algorithm derives precise results and can be used for the current and forthcoming Copernicus Sentinel missions S4 and S5.

Studies of Freon mixture separation using a large-scale model of distillation column

The operation efficiency of distillation columns with structured packing is maximal if the distribution of countercurrent vapor and liquid film flows over the column cross-section on the mass transfer surface is most uniform. Various types of structured packing are widely used in distillation columns. Formation of the temperature field maldistribution in the column cross-section is observed in large-scale distillation columns. The sizes of large-scale maldistributions on zones are commensurate with the column diameter. The aim of this work is to obtain experimental data on the separation efficiency in large-scale distillation columns and dynamics of formation of large-scale maldistributions of local parameters of vapor and liquid in a countercurrent flow over a structured packing during mixture separation. Separation of the R114/R21 freon mixture was carried out on a structured Mellapack 350.Y packing with a diameter of 0.9 m and height of 2100 mm. The experiments were carried out under conditions of complete reflux in the range of reduced vapor velocity of 0.017 < K v < 0.035 m/s. Experimental data were obtained on the efficiency of mixture separation, pressure drop over the structured packing and parameters determining the formation dynamics of the large-scale temperature field maldistribution in the column cross-section. The presented experimental data will be used for the construction and verification of a new model of mass transfer and efficiency of mixture separation in large-scale distillation packed columns.

A New GNSS-Derived Water Vapor Tomography Method Based on Optimized Voxel for Large GNSS Network

The Global Navigation Satellite System (GNSS) tomographic technique can be used for remote sensing of the three-dimensional water vapor (WV) distribution in the troposphere, which has attracted considerable interest. However, a significant problem in this technique is the excessive reliance on constraints (particularly in large GNSS networks). In this paper, we propose an improved tomographic method based on optimized voxel, which only considers the voxels passed by GNSS rays. The proposed method can completely prevent the tomographic algorithm interference of constraints that originated from empirical functions. Experiments in Nanjing in the periods of day-of-year (DOY) 182–184, 2019, and 244–246, 2019, show that the mean absolute error (MAE) and root mean square error (RMSE) of the WV density profile obtained using the proposed method are 0.9 and 1.3 g/m3, while those obtained using the conventional method are 1.3 and 1.8 g/m3, respectively, with respect to the radiosonde (RS) method. The numerical results show that the proposed method is reliable and has a superior accuracy to that of the conventional method.

An Approach for Assimilating FY4 Lightning and Cloud Top Height Data Using 3DVAR

The vertical distribution of water vapor affects the intensity of the updraft, downdraft and cold pool in convection, so how to adjust lightning proxy-humidity in the vertical direction is very important for convective scale numerical weather prediction (NWP). In this study, a data assimilation approach is presented that uses information from FengYun4 (FY4) lightning data with cloud top height (CTH) data. Specifically, the FY4 CTH is used to locate the upper boundary of the relative humidity adjustment. This method can effectively determine the vertical distribution of water vapor and obtain accurate pseudo-observations. Two severe convection events with different characteristics were studied to evaluate the data assimilation approach for short-term precipitation forecast. For comparison, two other relative humidity adjustment schemes with different vertical ranges were performed. One scheme adjusted the relative humidity between two isothermal layers and introduced the smallest water vapor increments compared with the other two data assimilation experiments and showed a slight improvement on precipitation forecast. The other scheme adjusted the relative humidity between the lifting condensation level (LCL) and a fixed height and introduced the maximum water vapor increments and exhibited better precipitation forecast based on Equitable Threat Scores (ETSs). The adjustment between LCL and CTH introduced appropriate amounts of water vapor and was adaptable for various convection developments and effectively avoided producing spurious convection in the particular case. Assimilation of FY4 lightning and CTH data improves the short-term precipitation forecast and provides the best forecast skill in a particular forecast period.

Regional Assimilation System for Transformed Retrievals from Satellite High-Resolution Infrared Data

AbstractSatellite retrievals strive to exploit the information contained in thousands of channels provided by hyperspectral sensors and show promise in providing a gain in computational efficiency over current radiance assimilation methods by transferring computationally expensive radiative transfer calculations to retrieval providers. This paper describes the implementation of a new approach based on the transformation proposed in 2008 by Migliorini et al., which reduces the impact of the a priori information in the retrievals and generates transformed retrievals (TRs) whose assimilation does not require knowledge of the hyperspectral instruments characteristics. Significantly, the results confirm both the viability of Migliorini’s approach and the possibility of assimilating data from different hyperspectral satellite sensors regardless of the instrument characteristics. The Weather Research and Forecasting (WRF) Model’s Data Assimilation (WRFDA) 3-h cycling system was tested over the central North Pacific Ocean, and the results show that the assimilation of TRs has a greater impact in the characterization of the water vapor distribution than on the temperature field. These results are consistent with the knowledge that temperature field is well constrained by the initial and boundary conditions of the Global Forecast System (GFS), whereas the water vapor distribution is less well constrained in the GFS. While some preliminary results on the comparison between the assimilation with and without TRs in the forecasting system are presented in this paper, additional work remains to explore the impact of the new assimilation approach on forecasts and will be provided in a follow-up publication.

Study on dual fuel combustion in an optical research engine by infrared diagnostics varying methane quantity and engine speed

Dual fuel combustion represents a possible solution for decreasing the emission produced by diesel engine, while maintaining the efficiency rate at comparable values. In this activity, methane and diesel are used for dual fuel combustion. Methane is injected in the intake manifold, when the intake valves are closed; meanwhile the diesel is delivered directly into the cylinder. The aim of this work is to study the effect of increasing methane concentration at two engine speeds (1500 and 2000 rpm, respectively), on the start and evolution of dual fuel combustion in a compression ignition research engine using infrared diagnostics.Infrared diagnostic has the advantage of being not intrusive, it gives a high spatial and temporal resolution of combustion phenomenon and it is able to qualitatively identify the local concentration of chemical species. Infrared camera detects the energy that the target emits as electromagnetic waves from 3 to 5.5 μm wavelength. Hence, no light sources or multiple optical accesses are needed; and commercial fuels can be used because they emit in the infrared as well.In the experiments, two filters are adopted to follow the whole combustion phenomenon from injection of diesel fuel to late combustion phase with high spatial and temporal resolution. Using the band-pass 3.3 μm, qualitative information about fuel vapour distribution and ignition location during low and high temperature heat release has been provided. Moreover, the combustion evolution is investigated by means of the band-pass filter at 4.2 μm, which is the characteristic of the asymmetric stretch of the CO2 molecule. In particular, autoignition and combustion evolution of the DF combustion are investigated increasing premixed ratio of methane. Differences with the conventional diesel combustion are highlighted. Diesel is the driven fuel that manages the autoignition and combustion of the DF mode in the investigated test cases.

The Infrared Absolute Radiance Interferometer (ARI) for CLARREO

The Absolute Radiance Interferometer (ARI) is an infrared spectrometer designed to serve as an on-orbit radiometric reference with the ultra-high accuracy (better than 0.1 K 3‑σ or k = 3 brightness temperature at scene brightness temperature) needed to optimize measurement of the long-term changes of Earth’s atmosphere and surface. If flown in an orbit that frequently crosses sun-synchronous orbits, ARI could be used to inter-calibrate the international fleet of infrared (IR) hyperspectral sounders to similar measurement accuracy, thereby establishing an observing system capable of achieving sampling biases on high-information-content spectral radiance products that are also &lt; 0.1 K 3‑σ. It has been shown that such a climate observing system with &lt;0.1 K 2‑σ overall accuracy would make it possible to realize times to detect subtle trends of temperature and water vapor distributions that closely match those of an ideal system, given the limit set by the natural variability of the atmosphere. This paper presents the ARI sensor's overall design, the new technologies developed to allow on-orbit verification and test of its accuracy, and the laboratory results that demonstrate its capability. In addition, we describe the techniques and uncertainty estimates for transferring ARI accuracy to operational sounders, providing economical global coverage. Societal challenges posed by climate change suggest that a Pathfinder ARI should be deployed as soon as possible.

Total column water vapour retrieval from S-5P/TROPOMI in the visible blue spectral range

Abstract. Total column water vapour has been retrieved from TROPOMI measurements in the visible blue spectral range and compared to a variety of different reference data sets for clear-sky conditions during boreal summer and winter. The retrieval consists of the common two-step DOAS approach: first the spectral analysis is performed within a linearized scheme and then the retrieved slant column densities are converted to vertical columns using an iterative scheme for the water vapour a priori profile shape, which is based on an empirical parameterization of the water vapour scale height. Moreover, a modified albedo map was used combining the OMI LER albedo and scaled MODIS albedo map. The use of the alternative albedo is especially important over regions with very low albedo and high probability of clouds like the Amazon region. The errors of the total column water vapour (TCWV) retrieval have been theoretically estimated considering the contribution of a variety of different uncertainty sources. For observations during clear-sky conditions, over ocean surface, and at low solar zenith angles the error typically is around values of 10 %–20 %, and during cloudy-sky conditions, over land surface, and at high solar zenith angles it reaches values around 20 %–50 %. In the framework of a validation study the retrieval demonstrates that it can well capture the global water vapour distribution: the retrieved H2O vertical column densities (VCDs) show very good agreement with the reference data sets over ocean for boreal summer and winter whereby the modified albedo map substantially improves the retrieval's consistency to the reference data sets, in particular over tropical land masses. However, over land the retrieval underestimates the VCD by about 10 %, particularly during summertime. Our investigations show that this underestimation is likely caused by uncertainties within the surface albedo and the cloud input data: low-level clouds cause an underestimation, but for mid- to high-level clouds good agreement is found. In addition, our investigations indicate that these biases can probably be further reduced by the use of improved cloud input data. For the general purpose we recommend only using VCDs with cloud fraction &lt;20 % and AMF &gt;0.1, which represents a good compromise between spatial coverage and retrieval accuracy. The TCWV retrieval can be easily applied to further satellite sensors (e.g. GOME-2 or OMI) for creating uniform, long-term measurement data sets, which is particularly interesting for climate and trend studies of water vapour.

The vertical structure and spatial variability of lower-tropospheric water vapor and clouds in the trades

Abstract. Horizontal and vertical variability of water vapor is omnipresent in the tropics, but its interaction with cloudiness poses challenges for weather and climate models. In this study we compare airborne lidar measurements from a summer and a winter field campaign in the tropical Atlantic with high-resolution simulations to analyze the water vapor distributions in the trade wind regime, its covariation with cloudiness, and their representation in simulations. Across model grid spacing from 300 m to 2.5 km, the simulations show good skill in reproducing the water vapor distribution in the trades as measured by the lidar. An exception to this is a pronounced moist model bias at the top of the shallow cumulus layer in the dry winter season which is accompanied by a humidity gradient that is too weak at the inversion near the cloud top. The model's underestimation of water vapor variability in the cloud and subcloud layer occurs in both seasons but is less pronounced than the moist model bias at the inversion. Despite the model's insensitivity to resolution from hecto- to kilometer scale for the distribution of water vapor, cloud fraction decreases strongly with increasing model resolution and is not converged at hectometer grid spacing. The observed cloud deepening with increasing water vapor path is captured well across model resolution, but the concurrent transition from cloud-free to low cloud fraction is better represented at hectometer resolution. In particular, in the wet summer season the simulations with kilometer-scale resolution overestimate the observed cloud fraction near the inversion but lack condensate near the observed cloud base. This illustrates how a model's ability to properly capture the water vapor distribution does not necessarily translate into an adequate representation of shallow cumulus clouds that live at the tail of the water vapor distribution.