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In the Atacama Desert, one of the driest places on Earth, the persistent absence of water preserves the record of environmental change, making it an invaluable proxy for studying the evolution of life on Earth. Due to the scarcity of in situ measurements and difficulties in satellite remote sensing, information on precipitation characteristics is limited even for the present climate. Guided by a case study of extreme precipitation in late January 2019, we derive a conceptual framework to explain how moisture transport combined with the diurnal circulation produces rainfall. We found a synoptic‐scale weather pattern that we named "moist northerlies" (MNs) based on surface observations, reanalysis, and high‐resolution simulation. During an MN event, moisture transport from the tropical Pacific is observed in the lower free‐troposphere in the forefront of an 850 hPa low‐pressure offshore Atacama. The diurnal circulation along the western Andean slope transports the moist free tropospheric air above the marine boundary layer inland, triggering clouds and storms. A trough over the southeast Pacific and a southward displaced Bolivian High seem to drive the MNs dynamically. Long‐term observations (1960–2020) show that most of the rainy days in the hyperarid core (75%) are triggered by MNs, occurring more frequently during neutral/La Niña conditions and phases 7‐8‐1 of the Madden‐Julian oscillation (MJO). A trend analysis (1991–2020) reveals that summer water vapor along the west coast of South America has increased rapidly due to the MNs, enhancing summer rainfall in Atacama. The implications of climate change and other climate variability modes are discussed. Plain Language Summary: The Atacama Desert, known as one of the driest places on Earth, holds vital information about how life has evolved in an extremely dry environment. Despite the dryness, rainfall is observed with some recurrence without being completely understood. This study focuses on understanding the mechanisms behind summer rain in the driest part of the Atacama Desert. We started investigating a specific extreme rainfall event in January 2019, discovering a weather pattern called "moist northerlies" (MNs) that transport moisture hundreds and thousands of kilometers from the tropical Pacific to the desert. The moisture is transported inland during the day, triggering rain in the Atacama. Over several decades, we found that MNs cause most rainy days in the desert. These MN events seem to be influenced by larger weather patterns such as the El Niño–Southern oscillation and the Madden‐Julian oscillation. Additionally, a rapid increase in humidity has been observed along the west coast of South America in recent decades, leading to more summer rainfall in the Atacama and greening in the Andean Precordillera. This study also discusses how anthropogenic global warming and natural climate variability might affect the MNs. Key Points: Summer moisture transport, named moist northerlies (MNs), brings free troposphere humidity from the tropical Pacific to the Atacama75% of austral summer rainfall days in the hyperarid core of the Atacama result from MNsThe Hadley cell expansion and the poleward shift of the Bolivian High are likely driving an increase of the MN frequency in recent decades [ABSTRACT FROM AUTHOR]

This paper illustrates measurements carried out by the Raman lidar BASIL in the frame of HOPE, revealing the presence of a clear-air dark band phenomenon (i.e. the appearance of a minimum in lidar backscatter echoes) in the upper portion of the convective boundary layer. The phenomenon is clearly distinguishable in the lidar backscatter echoes at 1064 nm. This phenomenon is attributed to the presence of lignite aerosol particles advected from the surrounding open pit mines in the vicinity of the measuring site.

Marine stratocumulus clouds of the eastern Pacific play an essential role in the earth's energy and radiation budget. Parts of these clouds off the western coast of South America form the major source of water to the hyperarid Atacama Desert coastal region at the northern coast of Chile. For the first time, a full year of vertical structure observations of the coastal stratocumulus and their environment is presented and analyzed. Installed at Iquique Airport in northern Chile in 2018/2019, three state-of-the-art remote sensing instruments provide vertical profiles of cloud macro- and micro-physical properties, wind, turbulence, and temperature as well as integrated values of water vapor and liquid water. Distinct diurnal and seasonal patterns of the stratocumulus life cycle are observed. Embedded in a land–sea circulation with a superimposed southerly wind component, maximum cloud occurrence and vertical extent occur at night but minima at local noon. Nighttime clouds are maintained by cloud-top cooling, whereas afternoon clouds reappear within a convective boundary layer driven through local moisture advection from the Pacific. During the night, these clouds finally re-connect to the maritime clouds in the upper branch of the land–sea circulation. The diurnal cycle is much more pronounced in austral winter, with lower, thicker, and more abundant (5×) clouds than in summer. This can be associated with different sea surface temperature (SST) gradients in summer and winter, leading to a stable or neutral stratification of the maritime boundary layer at the coast of the Atacama Desert in Iquique. [ABSTRACT FROM AUTHOR]

Measurements carried out by the Raman lidar system BASIL and the University of Cologne wind lidar are reported to demonstrate the capability of these instruments to characterize water vapour fluxes within the Convective Boundary Layer (CBL). In order to determine the water vapour flux vertical profiles, high resolution water vapour and vertical wind speed measurements, with a temporal resolution of 1 sec and a vertical resolution of 15-90, are considered. Measurements of water vapour flux profiles are based on the application of covariance approach to the water vapour mixing ratio and vertical wind speed time series. The algorithms are applied to a case study (IOP 11, 04 May 2013) from the HD(CP)2 Observational Prototype Experiment (HOPE), held in Central Germany in the spring 2013. For this case study, the water vapour flux profile is characterized by increasing values throughout the CBL with lager values (around 0.1 g/kg m/s) in the entrainment region. The noise errors are demonstrated to be small enough to allow the derivation of water vapour flux profiles with sufficient accuracy.

As part of the EUREC 4 A field campaign, the research vessel Maria S. Merian probed an oceanic region between 6 to 13.8 ∘ N and 51 to 60 ∘ W for approximately 32 d. Trade wind cumulus clouds were sampled in the trade wind alley region east of Barbados as well as in the transition region between the trades and the intertropical convergence zone, where the ship crossed some mesoscale oceanic eddies. We collected continuous observations of cloud and precipitation profiles at unprecedented vertical resolution (7–10 m in the first 3000 m) and high temporal resolution (1–3 s) using a W-band radar and micro rain radar (MRR), installed on an active stabilization platform to reduce the impact of ship motions on the observations. The paper describes the ship motion correction algorithm applied to the Doppler observations to extract corrected hydrometeor vertical velocities and the algorithm created to filter interference patterns in the MRR observations. Radar reflectivity, mean Doppler velocity, spectral width and skewness for W-band and reflectivity, mean Doppler velocity, and rain rate for MRR are shown for a case study to demonstrate the potential of the high resolution adopted. As non-standard analysis, we also retrieved and provided liquid water path (LWP) from the 89 GHz passive channel available on the W-band radar system. All datasets and hourly and daily quicklooks are publically available, and DOIs can be found in the data availability section of this publication. Data can be accessed and basic variables can be plotted online via the intake catalog of the online book "How to EUREC 4 A". [ABSTRACT FROM AUTHOR]

As part of the EUREC4A field campaign, the research vessel Maria S. Merian probed an oceanic region between 6° N and 13.8° N and 51° W to 60° W for approximately 32 days. Trade wind cumulus clouds were sampled in the trade-wind alley region east of Barbados as well as in the transition region between the trades and the intertropical convergence zone, where the ship crossed some mesoscale oceanic eddies. We collected continuous observations of cloud and precipitation profiles at unprecedented vertical resolution (7-10 m in the first 3000 m) and high temporal resolution (1-3 s) using a W-band radar and micro-rain radar (MRR-PRO), installed on an active stabilization platform to reduce the impact of ship motions on the observations. The paper describes the ship motion correction algorithm applied to the Doppler observations to extract corrected hydrometeors vertical velocities and the algorithm created to filter interference patterns in the MRR-PRO observations. Radar reflectivity, mean Doppler velocity, spectral width and skewness for W-band and attenuated reflectivity, mean Doppler velocity and rain rate for MRR-PRO are shown for a case study to demonstrate the potential of the high resolution adopted. As non-standard analysis, we also retrieved and provided liquid water path (LWP) from the 89 GHz passive channel available on the W-band radar system. All datasets and hourly and daily quicklooks are publically available. Data can be accessed and basic variables can be plotted online via the intake catalog of the online book "How to EUREC4A". [ABSTRACT FROM AUTHOR]

The existence of subsiding shells on the periphery of shallow cumulus clouds has major implications concerning the parameterization of shallow convection, with the mass exchange between the shell and cloudy air representing a significant deviation from the commonly used bulk‐plume parameterization. We examine the structure and frequency of subsiding shells in shallow cumulus convection using Doppler lidars at the Atmospheric Radiation Measurement Southern Great Plains facility in the central United States and at the Jülich ObservatorY for Cloud Evolution in western Germany. Doppler lidar indicates that the vertical subsiding shell extent is asymmetric, while shell width is typically ~100 m. Large‐eddy simulation can reasonably simulate the observed shell structure using a grid spacing of 10 m and suggests that much of the observed asymmetry is not a result of transient cloud evolution. Plain Language Summary: Doppler lidars allow for the inference of vertical air motion. On the edges of the shallow "popcorn" cumulus clouds, regions of sinking air (subsiding shells) are observed. If we wish to understand how these clouds interact with their environment, we must understand the structure of the subsiding shells that envelop them. As a cloud passes over the lidar, the front edge of the cloud is sampled first, and the back edge is sampled later. The back‐edge subsiding shell descends farther below cloud base than the front‐edge shell. High‐resolution models can resolve the observed shell structure and suggest that the differences between the front‐ and back‐edge shells do not arise from the evolution of the cloud during the tens of seconds it takes to pass over the lidar. Key Points: Doppler lidar indicates the presence of a vertically asymmetric subsiding shell in shallow cumulus that is on the order of 100 m wideLarge‐eddy simulations can reasonably reproduce observed shell structure and intensityMost of the asymmetry observed by lidar reflects fundamental cloud structures and is not simply a consequence of the sampling method [ABSTRACT FROM AUTHOR]

Finding observational evidence of land surface and atmosphere interactions is crucial for understanding the spatial and temporal evolution of the boundary layer, as well as for model evaluation, and in particular for large-eddy simulation (LES) models. In this study, the influence of a heterogeneous land surface on the spatial distribution of atmospheric water vapor is assessed. Ground-based remote sensing measurements from a scanning microwave radiometer (MWR) are used in a long-term study over 6 years to characterize spatial heterogeneities in integrated water vapor (IWV) during clear-sky conditions at the Jülich ObservatorY for Cloud Evolution (JOYCE). The resulting deviations from the mean of the scans reveal a season- and direction-dependent IWV that is visible throughout the day. Comparisons with a satellite-derived spatial IWV distribution show good agreement for a selection of satellite overpasses during convective situations but no clear seasonal signal. With the help of a land use type classification and information on the topography, the main types of regions with a positive IWV deviation were determined to be agricultural fields and nearby open pit mines. Negative deviations occurred mainly above elevated forests and urban areas. In addition, high-resolution large-eddy simulations (LESs) are used to investigate changes in the water vapor and cloud fields for an altered land use input. [ABSTRACT FROM AUTHOR]

Commercially available Doppler lidars have now been proven to be efficient tools for studying winds and turbulence in the planetary boundary layer. However, in many cases low signal-to-noise ratio is still a limiting factor for utilising measurements by these devices. Here, we present a novel post-processing algorithm for Halo Stream Line Doppler lidars, which enables an improvement in sensitivity of a factor of 5 or more. This algorithm is based on improving the accuracy of the instrumental noise floor and it enables longer integration times or averaging of high temporal resolution data to be used to obtain signals down to -32 dB. While this algorithm does not affect the measured radial velocity, it improves the accuracy of radial velocity uncertainty estimates and consequently the accuracy of retrieved turbulent properties. Field measurements using three different Halo Doppler lidars deployed in Finland, Greece and South Africa demonstrate how the new post-processing algorithm increases data availability for turbulent retrievals in the planetary boundary layer, improves detection of high-altitude cirrus clouds and enables the observation of elevated aerosol layers. [ABSTRACT FROM AUTHOR]

Commercially available Doppler lidars have now been proven to be efficient tools for studying winds and turbulence in the planetary boundary layer. However, in many cases low signal-to-noise ratio is still a limiting factor for utilising measurements by these devices. Here, we present a novel postprocessing algorithm for Halo Streamline Doppler lidars, which enables an improvement in sensitivity of a factor of five or more. This algorithm is based on improving the accuracy of the instrumental noise floor and it enables using longer integration times or averaging of high temporal resolution data to obtain signals down to -32 dB. While this algorithm does not affect the measured radial velocity, it improves the accuracy of radial velocity uncertainty estimates and consequently the accuracy of retrieved turbulent properties. Field measurements with three different Halo Doppler lidars deployed in Finland, Greece and South Africa demonstrate how the new post-processing algorithm increases data availability for turbulent retrievals in the planetary boundary layer, improves detection of high-altitude cirrus clouds, and enables the observation of elevated aerosol layers. [ABSTRACT FROM AUTHOR]

This paper illustrates measurements carried out by the Raman lidar BASIL in the frame of the HD(CP)2 Observational Prototype Experiment (HOPE), revealing the presence of a clear-air dark band phenomenon (i.e. a minimum in lidar backscatter echoes) in the upper portion of the convective boundary layer. The phenomenon is clearly distinguishable in the lidar backscatter echoes at 532 and 1064 nm, as well as in the particle depolarisation data. This phenomenon is attributed to the presence of lignite aerosol particles advected from the surrounding open pit mines in the vicinity of the measuring site. The paper provides evidence of the phenomenon and illustrates possible interpretations for its occurrence. [ABSTRACT FROM AUTHOR]

Blowing snow impacts Antarctic ice sheet surface mass balance by snow redistribution and sublimation. However, numerical models poorly represent blowing snow processes, while direct observations are limited in space and time. Satellite retrieval of blowing snow is hindered by clouds and only the strongest events are considered. Here, we develop a blowing snow detection (BSD) algorithm for ground-based remote-sensing ceilometers in polar regions and apply it to ceilometers at Neumayer III and Princess Elisabeth (PE) stations, East Antarctica. The algorithm is able to detect (heavy) blowing snow layers reaching 30m height. Results show that 78% of the detected events are in agreement with visual observations at Neumayer III station. The BSD algorithm detects heavy blowing snow 36% of the time at Neumayer (2011-2015) and 13% at PE station (2010-2016). Blowing snow occurrence peaks during the austral winter and shows around 5% interannual variability. The BSD algorithm is capable of detecting blowing snow both lifted from the ground and occurring during precipitation, which is an added value since results indicate that 92% of the blowing snow is during synoptic events, often combined with precipitation. Analysis of atmospheric meteorological variables shows that blowing snow occurrence strongly depends on fresh snow availability in addition to wind speed. This finding challenges the commonly used parametrizations, where the threshold for snow particles to be lifted is a function of wind speed only. Blowing snow occurs predominantly during storms and overcast conditions, shortly after precipitation events, and can reach up to 1300ma:g:l: in the case of heavy mixed events (precipitation and blowing snow together). These results suggest that synoptic conditions play an important role in generating blowing snow events and that fresh snow availability should be considered in determining the blowing snow onset. [ABSTRACT FROM AUTHOR]

This paper illustrates measurements carried out by the Raman lidar BASIL in the frame of the HD(CP)2) Observational Prototype Experiment (HOPE), revealing the presence of a clear-air dark band phenomenon (i.e. a minimum in lidar backscatter echoes) in the upper portion of the convective boundary layer. The phenomenon is clearly distinguishable in the lidar backscatter echoes at 532 and 1064 nm, as well as in the particle depolarization data. This phenomenon is attributed to the presence of lignite aerosol particles advected from the surrounding open pit mines in the vicinity of the measuring site. The paper provides evidence of the phenomenon and illustrates possible interpretations for its occurrence. [ABSTRACT FROM AUTHOR]

Sun-induced chlorophyll fluorescence (F) is a novel remote sensing parameter providing an estimate of actual photosynthetic rates. A combination of this new observable and Monteith's light use efficiency (LUE) concept was suggested for an advanced modeling of gross primary productivity (GPP). In this demonstration study, we evaluate the potential of both F and the more commonly used photochemical reflectance index (PRI) to approximate the LUE term in Monteith's equation and eventually improve the forward modeling of GPP diurnals. Both F and the PRI were derived from ground and airborne based spectrometer measurements over two different crops. We demonstrate that approximating dynamic changes of LUE using F and PRI significantly improves the forward modeling of GPP diurnals. Especially in sugar beet, a changing photosynthetic efficiency during the day was traceable with F and incorporating F in the forward modeling significantly improved the estimation of GPP. Airborne data were projected to produce F and PRI maps for winter wheat and sugar beet fields over the course of one day. We detected a significant variability of both, F and the PRI within one field and particularly between fields. The variability of F and PRI was higher in sugar beet, which also showed a physiological down-regulation of leaf photosynthesis. Our results underline the potential of F to serve as a superior indicator for the actual efficiency of the photosynthetic machinery, which is linked to physiological responses of vegetation. [ABSTRACT FROM AUTHOR]

We present a method for deriving horizontal-humidity variability from a single-scanning passive microwave radiometer (MWR). The MWR used has full scanning capabilities in azimuth and elevation and is sensitive to the path of integrated water vapor as well as cloud liquid water. Applying a simple linear-gradient model together with an assumed vertical profile derived from the closest radiosonde ascent, the strength and direction of the horizontal-humidity gradient can be determined with a temporal resolution on the order of 15–20 min. For the case of an approaching frontal system, the derived humidity field can explain up to 88% of the measured humidity variance—the missing variance can most probably be attributed to convective activity. [ABSTRACT FROM PUBLISHER]

The summertime thermal circulation in the region of an asymmetric valley exit is investigated by means of observations and high-resolution model simulations. The northeastward-oriented Alpine Lech Valley opening into the Bavarian Alpine foreland has an eastern slope exceeding the western slope by about 15 km. Northerly winds along the eastern slope are frequently observed, reaching substantial strength during fair weather conditions. A field experiment has been conducted to explore this phenomenon and to pinpoint the connection of the northeasterly flow to the Lech Valley wind circulation. Numerical simulations have also been carried out to support the interpretation of the observations. It is found that the northerlies owe their existence to the dominantly easterly flow along the foothills of the Alps, which is partly induced by the Alpine heat low but may be strengthened by favorable synoptic conditions. Examples for both situations will be discussed. The diurnal flow in the Lech Valley has little obvious impact on these northeasterlies. On days with moderate synoptic easterly flow, a wake is present on the lee of the eastern slope of the exit region, accompanied by a shear zone along the edge of the wake. This shear zone is forced southward during the daytime because of thermally initiated pressure gradients between the Alpine foreland and the Alps, leading to sudden wind changes in the exit area at the time of its passage. [ABSTRACT FROM AUTHOR]

The Atacama desert is one of the driest places on earth but climate data and climate models show large biases there with especially too high precipitation. Meteorological observations in the region are sparse and limited. Thus we built up a new network of 15 climate stations which is intended to work as a reference for climate models as well as ground truth for satellite missions. Goal of the network is also to understand how water is transported into the desert. Especially fog and dew play an important role as precipitation is with annual values around 0.1 mm/a extremely low.The network is build up by three transects from the coast of the pacific up to the slope of the Andes. The transects cover typical land forms of the region: the gypsum desert, result of the extreme dryness of the region; a wide valley running from west to east through the coastal mountain range allowing inflow of moist air from the pacific into the desert, and a narrow valley showing a strong gradient of decreasing vegetation from the coast to inland. Instrumentation of the network includes not only standard parameters like temperature, wind and precipitation but also solar and terrestrial radiation, soil temperatures and soil moisture. A 'leaf wetness' sensor gives information about deposition of liquid water by dew or fog. All data is available under DOI 10.5880/CRC1211DB.1.First results of the meteorological parameters show very regular patterns with some seasonal variation. Coastal stations show in general during daytime southerly winds, i.e. along the coast and easterly winds at night i.e. wind blowing from land to sea. This is a combination of the synoptic situation (high pressure system above the Pacific), a land/sea breeze system and a cold air drainage flow from the coastal cliff. All inland stations show a daily pattern different from this: starting at midday wind is coming from the west i.e. from the Pacific ocean whereas during night it is blowing from the east i.e. from the Andes. Wind speeds are during daytime usually higher than during night indicating a mountain wind system driven by solar heating.At the stations in the coastal mountain range the west wind during daytime is usually moister than the east wind during night. This indicates that the daytime west wind originates in the oceanic boundary layer. At the most eastward stations at the foot of the Andes this day-night difference is not present. This means that over the day moisture is transported into the desert. A rough maximum estimate assuming that the advected moisture is deposited as dew in the central valley of the desert gives 0.5mm/day which would exceed the annual average rainfall of the region. Possible alternative pathways of the water vapor will be discussed. [ABSTRACT FROM AUTHOR]