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Measurements of fog microphysics were conducted during the winter season of 2021 ‐2022 at the Barakah Nuclear Power Plant (BNPP), located in the Western coastal region of the United Arab Emirates. Twelve fog events were observed during this period. The primary objective of this study is to detail the microphysical characteristics of these events and refine current visibility parameterization schemes based on in-situ measurements of fog microphysical properties. All observed fog events are found to share a common feature of a bimodal distribution in droplet number concentration (N d), with modes at 4.5 μm and 23.2 μm. Despite the high proportion of smaller fog droplets associated with the fine mode, the greatest contribution to the liquid water content (LWC) comes essentially from medium to large droplets between 10 and 35 μm. The recalibration of existing visibility parameterization schemes revealed that the increase in horizontal visibility with increasing FI (fog index) tends to be more gradual for the studied cases compared to standard visibility parameterization schemes. Additionally, the fog droplet sedimentation velocity, estimated to be at a maximum of 1.85 cm s−1, occurs predominantly in the LWC range of 100–200 mg m−3, corresponding to a median volume diameter 24.8 μm. Our findings shed new light on the complexity of fog microphysics and its impact on visibility, underscoring their importance in refining weather models for accurate fog forecasting. • Observed consistent bimodal size distribution of fog droplets across 12 fog events, with a dominance of smaller droplets. • Smaller droplets (1-10 μm) formed initially and contributed majorly to Liquid Water Content (LWC). • Larger droplets (10-50 μm) developed later, contributing substantially to total condensed water. • Fog droplets sedimentation velocity can reach up to 1.85 cm s-1, correlated to a median volume diameter of 24.8 µm. [ABSTRACT FROM AUTHOR]

• Excellent agreement of IWV from MWR and SP against GNSS. • Linear dependence of MBE and SD on IWV; different instrument- and site-wise. • Weaker dependence of MBE and SD with SZA for MWR-GNSS and SP-GNSS IWV differences. • Seasonal dependence of MBE and SD mainly modulated by dependence on IWV. • Importance of the use of metadata to filter out low-quality data. This work presents a comparison of integrated water vapor (IWV) data recorded from microwave radiometer (MWR) and sun-photometer (SP) using global navigation satellite system (GNSS) IWV as reference in five mid-latitude sites of Portugal and Spain (2003–2021). A very high correlation is obtained for both instruments (R2 between 0.94 and 0.98), although, while MWR shows a wet bias, SP exhibits a dry one. In addition, a dependence of mean bias error (MBE) and standard deviation (SD) on IWV has been observed, increasing (larger discrepancies) both indices as IWV increases. The solar zenith angle (SZA) dependence is also studied, finding slightly larger discrepancies for the MWR than for SP in comparison with GNSS for very high values of SZA. Finally, a marked seasonal dependence is observed for SP-GNSS differences, modulated by the IWV dependence explained above. In contrast, the seasonal dependence for MWR is quite weaker than it is for SP. Therefore, in spite of the excellent agreement found among the different instruments, it is recommended that: i) the dependence with IWV be studied and corrected to further increase the performance of the instruments, and ii) metadata be used to filter out situations in which the instrument cannot operate properly. [ABSTRACT FROM AUTHOR]

Microphysical properties of low level liquid clouds at the Naqu site over the Tibetan Plateau (TP) are characterized using empirical regression algorithms based on ground-based millimeter cloud radar (MMCR) boundary mode observations in July 6-31, 2014. Monthly averaged temperature profiles measured over the Naqu site by radiosondes at Beijing local time 8:00 and 20:00 and diurnal variation of microwave radiometer (MWR) temperature profiles indicate a 0 °C layer above 1.2 km. Only clouds below 1.2 km are examined in this study and they are assumed as pure liquid in phase. The parameters used in the regression equations have been scaled based on MWR liquid water path when there are only low non-precipitating liquid clouds and MWR measurements are available. Statistically, the characteristic properties of liquid clouds show a single mode distribution for cloud droplet effective radius (re) with most frequent values around 5-7 μm, and for cloud liquid water content (LWC) with most frequent values below 0.2 g/m3. The diurnal distribution shows weak variation with slightly low values in the morning and evening time; and the vertical distribution shows increasing cloud droplet re and LWC with height. Especially, cloud droplet re increases from approximately 4-6 μm to 8-12 μm with height. The monthly mean cloud droplet re and LWC are approximately 5.7 μm and 0.07 g/m3 in July 2014. The liquid cloud properties characterized here have been shown comparable to those obtained from MODIS satellite observations, which has an average value of 5.1 μm for the same observation period. [ABSTRACT FROM AUTHOR]

A one degree beamwidth, multi-frequency (20 to 30 and 89GHz), dual-polarization radiometer with full azimuth and elevation scanning capabilities was built with the purpose of improving the detection of in-flight icing hazards to aircraft in the near airport environment. This goal was achieved by collocating the radiometer with Colorado State University's CHILL polarized Doppler radar and leveraging the similar beamwidth and volume scan regiments of the two instruments. The collocated instruments allowed for the liquid water path and water vapor measurements derived from the radiometer to be merged with the radar moment fields to determine microphysical and water phase characteristics aloft. The radiometer was field tested at Colorado State University's CHILL radar site near Greeley, Colorado during the summer of 2009. Instrument design, calibration, and initial field testing results are discussed in this paper. [ABSTRACT FROM AUTHOR]

We present the formulation of a variational-method-based retrieval algorithm for a ground-based microwave radiometer. Absorption by air, water vapor, cloud liquid water and rain water are incorporated into the formulation. The absorption of microwave radiation by air is calculated using a line-by-line method, while that by liquid water is calculated using an empirical formula that models the complex refractive index of liquid water. The root-mean-squared error of the retrieved temperature was no more than 1 °C in the lowest 2000 m of the profiles based on the 1-year verification data. The absorption of microwave radiation by rain water is important for improving the accuracy of the retrieval profiles during rainy conditions. [ABSTRACT FROM AUTHOR]

Based on the theory of thermal radiation and its transfer in the atmosphere, the response of a ground-based microwave radiometer to a lightning-superheated cylinder in the atmosphere is studied and the theoretical expressions are given for the relationship between brightness temperatures and parameters such as distance, size, duration and temperature of the lightning-superheated cylinder. The results from simulated calculations show that it is quite possible for a lightning-superheated cylinder to be observed by a microwave radiometer working in the 50–60 GHz band with a sensitivity of 0.3 K, typically used for atmospheric temperature profiling. The brightness temperature observed with any one of the channels in the band increases as the distance between the lightning and the radiometer decreases. Lightning at a short distance would make the brightness temperature observed by the channels near to 60 GHz increase more while distant lightning would make the brightness temperature observed by the channels near to 50 GHz increase more. This feature could be used to retrieve lightning distance and features of the lightning-heated air cylinder from brightness temperature observations. One lightning observation by a ground-based radiometer, the challenge of such observations, and a theoretical analysis are presented. Additional observations are needed for more thorough exploration of this unique remote sensing capability. [ABSTRACT FROM AUTHOR]

An improved version of the vibrating wire sensor, used to measure supercooled cloud liquid water content, was developed by Anasphere Inc. and tested during early 2012. The sensor works on the principle that supercooled liquid will freeze to the vibrating wire and reduce the frequency at a known rate proportional to the liquid water content as the sensor rises through the cloud attached to a weather balloon and radiosonde. The disposable Anasphere sensor interfaces with an InterMet Systems iMet radiosonde. This updated sensor reduces the weight of the instrument while updating the technology when compared to the preceding balloon-borne sensor that was developed in the 1980's by Hill and Woffinden. Balloon-borne test flights were performed from Boulder, Colorado during February and March of 2012. These flights provided comparisons to integrated liquid water and profiles of liquid water content derived from a collocated multichannel microwave radiometer, built and operated by Radiometrics Corporation. Inter-comparison data such as these are invaluable for calibration, verification and validation of remote-sensing instruments. The data gathered from this sensor are potentially important to detection of icing hazards to aircraft, validation of microphysical output from numerical models, and calibrating remote sensors measuring supercooled liquid water. [ABSTRACT FROM AUTHOR]

This work presents observations of water phase dynamics that demonstrate the theoretical Wegener-Bergeron-Findeisen concepts in mixed-phase winter storms. The work analyzes vertical profiles of air vapor pressure, and equilibrium vapor pressure over liquid water and ice. Based only on the magnitude ranking of these vapor pressures, we identified conditions where liquid droplets and ice particles grow or deplete simultaneously, as well as the conditions where droplets evaporate and ice particles grow by vapor diffusion. The method is applied to ground-based remote-sensing observations during two snowstorms, using two distinct microwave profiling radiometers operating in different climatic regions (North American Central High Plains and Great Lakes). The results are compared with independent microwave radiometer retrievals of vertically integrated liquid water, cloud-base estimates from a co-located ceilometer, reflectivity factor and Doppler velocity observations by nearby vertically pointing radars, and radiometer estimates of liquid water layers aloft. This work thus makes a positive contribution toward monitoring and nowcasting the evolution of supercooled droplets in winter clouds. [ABSTRACT FROM AUTHOR]

The planetary boundary layer (PBL) height plays a major role in air quality and weather forecast studies. However, it cannot be directly measured as it can only be determined from the measured profiles of atmospheric parameters such as., temperature, moisture, and aerosol backscatter based on different retrieval mechanisms. This paper presents the PBL features such as., stable boundary layer (SBL), convective boundary layer (CBL), and residual layer (RL) detected from the microwave radiometer (MWR), mini Micro Pulse Lidar (MPL), and radiosondes (RS) and an intercomparison was done between the instruments during the period of 16 February – 31 May 2019 in Mexico City. RS were launched thrice a day during this period at 06 LST, 12 LST, and 18 LST respectively. The PBL heights comparison was classified into 3 categories: MWR-RS, mini MPL-RS, and mini MPL-MWR. The daytime CBL heights for the MWR-RS comparison correlate well (r = 0.96 at 12 LST, r = 0.88 at 18 LST) as well as RL height (r = 0.94 at 06 LST). The CBL and RL heights for the mini MPL-RS and mini MPL-MWR comparison also agree very well (0.89 < r < 0.99). Overall, the corresponding SBL height comparisons at 06 LST yielded lower agreements (r < 0.66) with the lowest correlation values for the mini MPL-MWR comparison (r = 0.39), which is likely due to the inability of the mini MPL to detect aerosol backscatter below ~120 m above ground level (agl) as well as the stability of the atmosphere, which prevents uniform mixing of the aerosol particles. [Display omitted] • Intercomparison of microwave radiometer, micro-pulse lidar and radiosondes during 3 months. • Overall good agreement in the detection of the convective and residual boundary layer heights. • Lower agreement for the stable boundary layer height. [ABSTRACT FROM AUTHOR]

Abstract: Microwave radiometers (MWRs) can be useful for the detection of mesoscale phenomena because they provide thermodynamic profiles in a minute time scale. These profiles are mainly used in non-precipitation conditions due to degraded accuracy of the MWR measurements in precipitation. Recently, Radiometrics Corporation used proprietary neural network methods to retrieve temperature, humidity and liquid profiles from off-zenith (15° elevation) radiometer observations to provide higher accuracy during precipitation. In this paper, using the MWR-retrieved temperature and humidity profiles with collocated radiosondes from June 2010 to September 2013 in Wuhan, the impact of precipitation on the MWR measurement accuracy as well as the effect of off-zenith neural network methods on it is investigated. In precipitation, the correlation coefficients of the MWR temperature and vapor density profiles against radiosondes are smaller than those in non-precipitation, and the bias and RMS against radiosondes also increase, especially around 2km heights. For the MWR relative humidity profile, the correlation coefficient in precipitation is obvious smaller than that in non-precipitation below 4.5km, and the bias and RMS against radiosondes are clearly larger above 5.5km. Moreover, the differences between the precipitation and non-precipitation cases mostly are statistically significant. Compared with the results of the zenith observation, the off-zenith observation makes a positive effect on reducing the impact of precipitation on the accuracy of MWR temperature and vapor density retrievals. On the whole, the MWR temperature bias and RMS against radiosondes in precipitation are reduced from 3.6 and 4.2K to 1.3 and 3.1K, respectively, and the MWR vapor density bias is also reduced from 1.10g/m3 to 0.18g/m3 with the RMS decreasing from 2.90g/m3 to 1.91g/m3. The temperature correlation coefficient between the MWR and radiosonde in precipitation is clearly improved above 3km heights, and the temperature bias and RMS are significantly reduced at most heights. For the MWR vapor density retrievals in precipitation, the correlation coefficient, bias and RMS against radiosondes are clearly improved above 2km heights. Additionally, the off-zenith observations during non-precipitation cases are also better compared to zenith observations. Therefore, off-zenith observations generally are better than zenith observations. This could be due to the fact that the off-zenith observations are more representative of the conditions in which radiosonde observations are also taken. [Copyright &y& Elsevier]

The thermodynamic characteristics of the atmosphere during the pre-monsoon thunderstorm period and the transition phase from pre-monsoon to monsoon period over the gateway of Indian summer monsoon is investigated in this study. The study utilizes Stratosphere-Troposphere (ST) wind profiling Radar, Microwave Radiometer, and GPS radiosonde observations. A sharp increase in the boundary-layer and middle-layer moisture content, abrupt changes in the mixing height, lifting condensation level (LCL), convective available potential energy (CAPE), and other thermodynamic variables are noted. The pre-monsoon period is characterized by intermittent deep convection that moistens the middle layers, while stratiform clouds are observed in the monsoon period and deep moist layer is observed. The dry middle-layers observed before the monsoon onset got moistened consistently as the low-level jet strengthened. The LCL height was lower than the mixing height after the onset of monsoon. The wet-bulb zero level (WBZ) height was reduced to 2 km during the pre-monsoon and shifted gradually to higher altitudes as the onset of monsoon occurs. Variations in WBZ height may help in the predictability of monsoon onset. The study has also investigated the characteristics of a thunderstorm event before the onset and suitable indices for early indication of the storm. • Atmospheric thermodynamic characteristics during transition to monsoon. • Pre-monsoon season characterized by intermittent deep convection. • The pre-monsoon wet-bulb zero level height reduced to 2 km indicating hail occurrence. • Mid-layer moistening prior to monsoon onset, monsoon characterized by stratiform clouds. • LCL height lower than mixing height after onset. [ABSTRACT FROM AUTHOR]

Planetary boundary layer height (PBLH) is a key parameter for air quality prediction and boundary layer meteorology studies. The height of thermodynamic boundary layer (H θ) and material boundary layer (H C) in different seasons are compared by using microwave radiometer (MWR) and ceilometer. There are considerable differences between H θ and H C and the causes are further studied. We found that when the boundary layer is markedly stable at night, the MWR's algorithm used to estimate PBLH has a retrieval restriction and directly leads to a much higher H θ than H C (1 to 2.5 km higher). This unrealistic PBLH can be identified and eliminated effectively by using temperature differences from the surface to 1 km. During periods of transportation and diffusion of aerosols, strong southerly winds transporting pollutants northward above H θ or strong northerly winds throughout the boundary layer removing pollutants will cause a much higher H C than H θ (0.5 to 2 km higher). However, when transportation occurs in the lower layer within H θ , H C and H θ are consistent. The aerosol layers is able to characterize the multiple-layered structure of the nocturnal boundary layer (NBL) and the evolution of the boundary layer by using backscatter. The stable boundary layer (SBL) ranges from 0.3 to 0.6 km at night. Compared with the idealized thermodynamic boundary layer, there isn't a distinct boundary between the SBL and the growing mixing layer (ML) after sunrise. The residual layer (RL) ranges from 0.5 to 1.5 km, it lasts 4 or 5 h after sunrise and shows up again within 2 h after sunset. The maximum detectable range of Doppler Lidar has a large backscatter gradient, so is strongly consistent with the RL at night and the ML in the daytime during a pollution event. The combined observation of microwave radiometer and ceilometer can well reveal the completed characteristics of boundary layer structures. Unlabelled Image [ABSTRACT FROM AUTHOR]

The Planetary Boundary Layer (PBL) is an important part of the atmosphere that is relevant in different atmospheric fields like pollutant dispersion, and weather forecasting. In this study, we analyze four and five-year datasets of measurements gathered with a ceilometer and a microwave radiometer to study the PBL structure respectively, in the mid-latitude urban area of Granada (Spain). The methodologies applied for the PBL Height (PBLH) detection (gradient method for ceilometer and the combination of parcel method and temperature gradient method for microwave radiometer) provided a description in agreement with the literature about the PBL structure under simple scenarios. Then, the PBLH behavior is characterized by a statistical study of the convective and stable situations, so that the PBLH was obtained from microwave radiometer measurements. The analysis of the PBLH statistical study shows some agreement with other PBLH studies such as daily pattern and yearly cycle, and the discrepancies were explained in terms of distinct latitudes, topography and climate conditions. Finally, it was performed a joint long-term analysis of the residual layer (RL) provided by ceilometer and the stable and convective layer heights determined by microwave radiometer, offering a complete picture of the PBL evolution by synergetic combination of remote sensing techniques. The PBL behavior has been used for explaining the daily cycle of Black Carbon (BC) concentration, used as tracer of the pollutants emissions associated to traffic. • PBLH detection from the combination of ceilometer (PBLH Ceilometer) and MWR (PBLH MWR) measurements. • PBLH long-term analyses. • Statistical analysis of Black Carbon concentration and Ventilation Coefficient. • RL long-term analyses. [ABSTRACT FROM AUTHOR]

The goal of this study is twofold. First, we verify the retrievals of temperature and humidity profiles from a ground-based microwave radiometer (MWR) using radiosonde soundings. Second, we assess the potential of the MWR to nowcast fog formation and dissipation in a hyper-arid environment through the analysis of its profiles. The MWR is installed and operated at Masdar Institute in Abu Dhabi, United Arab Emirates (UAE), adjacent to Abu Dhabi's International Airport. MWR observations collected from 18 February 2017 to 31 March 2019. A comparison with airport radiosonde data reveals a good agreement in particular for temperature, with the two measurements generally within 1 K from the surface to 10 km. For the specific humidity, the biases can reach 5 g kg−1 in the PBL. Biases can be attributed to errors in the radiosonde measurements and a non-optimized retrieval algorithm for the MWR. A total of 14 fog events are identified during the study period. A strong near-surface inversion with lapse rates in the lowest 250 m of up to 30 K km−1 is needed for fog to form. Subsidence warming and drying associated with the subtropical anticyclone can descend down to 500 m, aiding the formation of fog at night. Cloud base height retrievals from the infrared (IR) camera capture fog onset and dissipation and complement the MWR observations. A synergy of different instruments could allow for more accurate fog detection. • A comparison of microwave radiometer (MWR) and radiosonde temperature profiles reveals a good agreement. • The average Convective Boundary Layer height derived from MWR at measurement location is around 1.6 km. • Strong surface-based inversion and near-surface water vapour mixing ratio of at least 6 gm kg-1 needed for fog to occur. [ABSTRACT FROM AUTHOR]