Comparison and Verification of Coherent Doppler Wind Lidar and Radiosonde Data in the Beijing Urban Area

As a new type of wind field detection equipment, coherent Doppler wind lidar(CDWL) still needs more relevant observation experiments to compare and verify whether it can achieve the accuracy and precision of traditional observation equipment in urban areas. In this experiment, a self-developed CDWL provided four months of observations in the southern Beijing area. After the data acquisition time and height match, the wind profile data obtained based on a Doppler beam swinging(DBS) five-beam inversion algorithm were compared with radiosonde data released from the same location. The standard deviation(SD) of wind speed is 0.8 m s^(–1), and the coefficient of determination R~2 is 0.95. The SD of the wind direction is 17.7° with an R~2 of 0.96. Below the height of the roughness sublayer(about 400 m), the error in wind speed and wind direction is significantly greater than the error above the height of the boundary layer(about 1500 m). For the case of wind speeds less than 4 m s^(–1), the error of wind direction is more significant and is affected by the distribution of surrounding buildings. Averaging at different height levels using suitable time windows can effectively reduce the effects of turbulence and thus reduce the error caused by the different measurement methods of the two devices.

A New Method for Deriving High-Vertical-Resolution Wind Vector Data from the L-Band Radar Sounding System in China

High-vertical-resolution radiosonde wind data are highly valuable for describing the dynamics of the meso-and microscale atmosphere. However, the current algorithm used in China's L-band radar sounding system for calculating highvertical-resolution wind vectors excessively smooths the data, resulting in significant underestimation of the calculated kinetic energy of gravity waves compared to similar products from other countries, which greatly limits the effective utilization of the data. To address this issue, this study proposes a novel method to calculate high-vertical-resolution wind vectors that utilizes the elevation angle, azimuth angle, and slant range from L-band radar. In order to obtain wind data with a stable quality, a two-step automatic quality control procedure, including the RMSE-F(root-mean-square error F) test and elemental consistency test are first applied to the slant range data, to eliminate continuous erroneous data caused by unstable signals or radar malfunctions. Then, a wind calculation scheme based on a sliding second-order polynomial fitting is utilized to derive the high-vertical-resolution radiosonde wind vectors. The evaluation results demonstrate that the wind data obtained through the proposed method show a high level of consistency with the high-resolution wind data observed using the Vaisala Global Positioning System and the data observed by the new Beidou Navigation Sounding System. The calculation of the kinetic energy of gravity waves in the recalculated wind data also reaches a level comparable to the Vaisala observations.

Dynamic and Thermodynamic Features of Low and Middle Clouds Derived from Atmospheric Radiation Measurement Program Mobile Facility Radiosonde Data at Shouxian, China

By using the radiosonde measurements collected at Shouxian,China,we examined the dynamics and thermodynamics of single- and two-layer clouds formed at low and middle levels.The analyses indicated that the horizontal wind speed above the cloud layers was higher than those within and below cloud layers.The maximum balloon ascent speed（5.3 m s^-1） was located in the vicinity of the layer with the maximum cloud occurrence frequency（24.4%）,indicating an upward motion（0.1-0.16 ms^-1）.The average thickness,magnitude and gradient of the temperature inversion layer above single-layer clouds were117±94 m,1.3±1.3℃ and 1.4±1.5℃（100 m）^-1,respectively.The average temperature inversion magnitude was the same（1.3℃） for single-low and single-middle clouds;however,a larger gradient[1.7±1.8℃（100 m）^-1]and smaller thickness（94±67 m） were detected above single-low clouds relative to those above single-middle clouds[0.9±0.7℃（100 m）^-1 and157±120 m].For the two-layer cloud,the temperature inversion parameters were 106±59 m,1.0±0.9℃ and 1.0±1.0℃（100 m）^-1 above the upper-layer cloud and 82 ± 60 m,0.6±0.9℃ and 0.7±0.6℃（100 m）^-1 above the low-layer cloud.Absolute differences between the cloud-base height（cloud-top height） and the lifting condensation level（equilibrium level）were less than 0.5 km for 66.4%（36.8%） of the cases analyzed in summer.

Climatology of Cloud-base Height from Long-term Radiosonde Measurements in China

Clouds are critical to the global radiation budget and hydrological cycle, but knowledge is still poor concerning the observed climatology of cloud-base height （CBH） in China. Based on fine-resolution sounding observations from the China Radiosonde Network （CRN）, the method used to estimate CBH was modified, and uncertainty analyses indicated that the CBH is good enough. The accuracy of CBH estimation is verified by the comparison between the sounding-derived CBHs and those estimated from the micro-pulse lidar and millimeter-wave cloud radar. As such, the CBH climatology was compiled for the period 2006-16. Overall, the CBH exhibits large geographic variability across China, at both 0800 Local Standard Time （LST） and 2000 LST, irrespective of season. In addition, the summertime cloud base tends to be elevated to higher altitudes in dry regions [i.e., Inner Mongolia and the North China Plain （NCP）]. By comparison, the Tibetan Plateau （TP）, Pearl River Delta （PRD） and Sichuan Basin （SCB） have relatively low CBHs （〈 2.4 km above ground level）. In terms of seasonality, the CBH reaches its maximum in summer and minimum in winter. A low cloud base tends to occur frequently （〉 70%） over the TP, PRD and SCB. In contrast, at most sites over the Yangtze River Delta （YRD） and the NCP, about half the cloud belongs to the high-cloud category. The CBH does not exhibit marked diurnal variation in summer, throughout all CRN sites, probably due to the persistent cloud coverage caused by the East Asia Summer Monsson. To the best of our knowledge, this is the first CBH climatology produced from sounding measurements in China, and provides a useful reference for obtaining observational cloud base information.

Comparison of COSMIC Radio Occultation Refractivity Profiles with Radiosonde Measurements

In recent years, radio occultation （RO） technology making use of global positioning system （GPS） signals has been exploited to obtain profiles of atmospheric parameters in the neutral atmosphere. In this paper, the RO refractivity profiles obtained from the Constellation Observing System for Meteorology, Ionosphere and Climate （COSMIC） mission are statistically compared with the observations of 38 radiosonde stations provided by the Australian Bureau of Meteorology during the period from 15 July 2006 through 15 July 2007. Different collocation criteria are compared at first, and COSMIC RO soundings that occur within 3 hours and 300 km of radiosonde measurements are used for the final statistical comparison. The overall results show that the agreements between the COSMIC refractivity profiles and the radiosonde soundings from the 38 stations are very good at 0-30 km altitude, with mean absolute relative refractivity deviations of less than 0.5%. Latitudinal comparisons indicate that there are negative refractivity deviations in the lower troposphere over the low latitude and middle latitude regions and large standard deviations exist in the lower troposphere of low latitude regions, which can reach up to ～6%. The comparisons of COSMIC RO refractivity profiles and radiosonde observations for 3 polar stations in four different seasons indicate that the accuracy of GPS RO profiles is better in the Austral summer and autumn than in the Austral spring and winter during the year from September 2006 to August 2007.

Comparison of Beijing MST Radar and Radiosonde Horizontal Wind Measurements

To determine the performance and data accuracy of the 50 MHz Beijing Mesosphere-Stratosphere-Troposphere （MST） radar, comparisons of radar measured horizontal winds in the height range 3-25 km with radiosonde observations were made during 2012. A total of 427 profiles and 15 210 data pairs were compared. There was very good agreement between the two types of measurement. Standard deviations of difference （mean difference） for wind direction, wind speed, zonal wind and meridional wind were 24.86° （0.77°）, 3.37 （-0.44）, 3.33 （-0.32） and 3.58 （-0.25） m s^-1, respectively. The annual standard deviations of differences for wind speed were within 2.5-3 m s^-1 at all heights apart from 10-15 km, the area of strong winds, where the values were 3-4 m s^-1. The relatively larger differences were mainly due to wind field variations in height regions with larger wind speeds, stronger wind shear and the quasi-zero wind layer. A lower MST radar SNR and a lower percentage of data pairs compared will also result in larger inconsistencies. Importantly, this study found that differences between the MST radar and radiosonde observations did not simply increase when balloon drift resulted in an increase in the real-time distance between the two instruments, but also depended on spatiotemporal structures and their respective positions in the contemporary synoptic systems. In this sense, the MST radar was shown to be a unique observation facility for atmospheric dynamics studies, as well as an operational meteorological observation system with a high temporal and vertical resolution.

Impacts of Reference Time Series on the Homogenization of Radiosonde Temperature

Using radiosonde temperatures of 92 selected stations in China,the uncertainties in homogenization processes caused by different reference series,including nighttime temperature,the NCEP （National Centers for Environmental Prediction） and ERA-40 （European Centre for Medium-Range Weather Forecasts） forecasting background,are examined via a two-phase regression approach.Although the results showed limited consistency in the temporal and spatial distribution of identified break points （BPs） in the context of metadata events of instrument model change and correction method,significant uncertainties still existed in BP identification,adjustment,and impact on the estimated trend.Reanalysis reference series generally led to more BP identification in homogenization.However,those differences were parts of global climatic shifts,which may have confused the BP calculations.Discontinuities also existed in the reanalysis series due to changes in the satellite input.The adjustment values deduced from the reanalysis series ranged widely and were larger than those from the nighttime series and,therefore,impacted the estimated temperature trend.

Intercomparison of GPS radiosonde soundings during the eastern tropical Indian Ocean experiment

Temperature and relative humidity profiles derived from two China-made global positioning system（GPS） radiosondes（GPS-TK and CF-06-A） during the east tropical Indian Ocean（ETIO） experiment were compared with Vaisala RS92-SGP to assess the performances of China-made radiosondes over the tropical ocean.The results show that there have relative large biases in temperature observations between the GPSTK and the RS92-SGP in the low troposphere,with a warm bias of greater than 2 K in the day and a cooling bias of 0.6 K at night.The temperature differences of the CF-06-A were small in the troposphere both in daytime and nighttime,and became large peak-to-peak fluctuations in the stratosphere.The intercomparison of the relative humidity showed that the CF-06-A had large random errors due to the limitation of sensors and the lack of correction scheme,and the GPS-TK had large systematic biases in the low troposphere which might be related to the temperature impact.GPS height measurements are clearly suitable for China-made radiosonde systems operation.At night,the CF-06-A and the GPS-TK could provide virtual potential temperature and atmospheric boundary layer height measurements of suitable quality for both weather and climate research.As a result of the intercomparison experiment,major errors in the Chinamade radiosonde systems were well indentified and subsequently rectified to ensure improving accuracy for historical and future radiosondes.

Statistical characteristics of the surface ducts over the South China Sea from GPS radiosonde data

Based on the global position system （GPS） radiosonde data near the sea surface, the surface duct characteristics over the South China Sea （SCS） were statistically analyzed. The annual surface duct occurrence over the SCS was about 64%. Of the observed surface ducts, duct heights mainly distributed between 18 and 42 m, with M slopes in the range of -0.3 to -0.2 M units/m. Those ducts accounted for about 80% of the ducting cases. For the total profiles, the duct occurrences in a day changed slowly and were more than 60% in all times. The surface ducts formed more easily in the daytime than in the nighttime and most of the duct height were at bellow about 32 m. Additionally, The seasonal variation of the SCS ducts appeared to be evident, except that the mean duct thickness was almost constant, about 33 m for all seasons. The highest occurrence was about 71% in the autumn, followed by in the summer, spring and winter. In spring, their top-height existed more often at a height of more than 48 m. Their mean duct strength became stronger trend from spring to winter, with the M-slope in the range between -0.26 and -0.18 M units/m. Those results agreed well with other studies, provided considering the data resolution. The statistical analysis was reliable and gave the duct estimation for the SCS. Such duct climatology not only has important implications for communication systems and the reliability of the radar observation, but also can provide useful information to improve the accuracy of the meteorological radar measurements.

Validation of the “HY-2” altimeter wet tropospheric path delay correction based on radiosonde data

Wet tropospheric path delay （PD） is a highly variable term for the altimeter measurement of a sea surface height, caused by the refraction effect of atmospheric water vapor and cloud liquid water. In order to esti- mate PD values, the ＂HY-2＂ system includes a calibration microwave radiometer （CMR） operating at 18.7, 23.8 and 37 GHz. The PD data of the CMR were compared and validated by coincident radiosonde profiles from ten globally distributed radiosonde stations during October 2011 to August 2012. The temporal interval was 1 h. In order to avoid land contamination, different spatial intervals between these two data sets were tested. The empirical fit function of PD uncertainty and spatial interval was found and extrapolated to the ideal situation that the data of CMR and radiosonde were totally coincident. The stability of the brightness temperature of the CMR and its impact on the PD correction was also studied. Consequently, the uncertainty of the PD algorithm of the CMR was estimated to be 2.1 cm.

A quality control method for high-resolution radiosonde temperature and wind data

This study proposes a method to derive the climatological limit thresholds that can be used in an operational/historical quality control procedure for Chinese high vertical resolution(5–10 m)radiosonde temperature and wind speed data.The whole atmosphere is divided into 64 vertical bins,and the profiles are constructed by the percentiles of the values in each vertical bin.Based on the percentile profiles(PPs),some objective criteria are developed to obtain the thresholds.Tibetan Plateau field data are used to validate the effectiveness of the method in the application of experimental data.The results show that the derived thresholds for 120 operational stations and 3 experimental stations are effective in detecting the gross errors,and those PPs can clearly and instantly illustrate the characteristics of a radiosonde variable and reveal the distribution of errors.

Atmospheric River Signatures in Radiosonde Profiles and Reanalyses at the Dronning Maud Land Coast,East Antarctica

Atmospheric rivers(ARs)are an important component of the hydrological cycle linking moisture sources in lower latitudes to the Antarctic surface mass balance.We investigate AR signatures in the atmospheric vertical profiles at the Dronning Maud Land coast,East Antarctica,using regular and extra radiosonde measurements conducted during the Year of Polar Prediction Special Observing Period November 2018 to February 2019.Prominent AR events affecting the locations of Neumayer and Syowa cause a strong increase in specific humidity extending through the mid-troposphere and a strong low-level jet(LLJ).At Neumayer,the peak in the moisture inversion(up to 4 g kg^?1)is observed between 800 and 900 hPa,while the LLJ(up to 32 m s^?1)is concentrated below 900 hPa.At Syowa the increase in humidity is less pronounced and peaks near the surface,while there is a substantial increase in wind speed(up to 40 m s?1)between 825 and 925 hPa.Moisture transport(MT)within the vertical profile during the ARs attains a maximum of 100 g kg?1 m s?1 at both locations,and is captured by both ERA-Interim and ERA5 reanalysis data at Neumayer,but is strongly underestimated at Syowa.Composites of the enhanced MT events during 2009?19 show that these events represent an extreme state of the lower-tropospheric profile compared to its median values with respect to temperature,humidity,wind speed and,consequently,MT.High temporal-and vertical-resolution radiosonde observations are important for understanding the contribution of these rare events to the total MT towards Antarctica and improving their representation in models.

GPS Water Vapor and Its Comparison with Radiosonde and ERA-Interim Data in Algeria

Remote sensing of atmospheric water vapor using global positioning system（GPS） data has become an effective tool in meteorology,weather forecasting and climate research. This paper presents the estimation of precipitable water（PW）from GPS observations and meteorological data in Algeria,over three stations located at Algiers,Bechar and Tamanrasset.The objective of this study is to analyze the sensitivity of the GPS PW estimates for the three sites to the weighted mean temperature（T;）,obtained separately from two types of T;–T;regression [one general,and one developed specifically for Algeria（T;stands for surface temperature）],and calculated directly from ERA-Interim data. The results show that the differences in T;are of the order of 18 K,producing differences of 2.01 mm in the final evaluation of PW. A good agreement is found between GPS-PW and PW calculated from radiosondes,with a small mean difference with Vaisala radiosondes.A comparison between GPS and ERA-Interim shows a large difference（4 mm） in the highlands region. This difference is possibly due to the topography. These first results are encouraging,in particular for meteorological applications in this region,with good hope to extend our dataset analysis to a more complete,nationwide coverage over Algeria.

The Significant Role of Radiosonde-measured Cloud-base Height in the Estimation of Cloud Radiative Forcing

The satellite-based quantification of cloud radiative forcing remains poorly understood,due largely to the limitation or uncertainties in characterizing cloud-base height(CBH).Here,we use the CBH data from radiosonde measurements over China in combination with the collocated cloud-top height(CTH) and cloud properties from MODIS/Aqua to quantify the impact of CBH on shortwave cloud radiative forcing(SWCRF).The climatological mean SWCRF at the surface(SWCRFSUR),at the top of the atmosphere(SWCRFTOA),and in the atmosphere(SWCRFATM) are estimated to be-97.14,-84.35,and 12.79 W m^(-2),respectively for the summers spanning 2010 to 2018 over China.To illustrate the role of the cloud base,we assume four scenarios according to vertical profile patterns of cloud optical depth(COD).Using the CTH and cloud properties from MODIS alone results in large uncertainties for the estimation of SWCRFATM,compared with those under scenarios that consider the CBH.Furthermore,the biases of the CERES estimation of SWCRFATM tend to increase in the presence of thick clouds with low CBH.Additionally,the discrepancy of SWCRFATM relative to that calculated without consideration of CBH varies according to the vertical profile of COD.When a uniform COD vertical profile is assumed,the largest SWCRF discrepancies occur during the early morning or late afternoon.By comparison,the two-point COD vertical distribution assumption has the largest uncertainties occurring at noon when the solar irradiation peaks.These findings justify the urgent need to consider the cloud vertical structures when calculating the SWCRF which is otherwise neglected.

Diurnal variability of the planetary boundary layer height estimated from radiosonde data

Diurnal variations in the planetary boundary layer height(PBLH)at different latitudes over different surface characteristics are described,based on 45 years(1973−2017)of radiosonde observations.The PBLH is determined from the radiosonde data by the bulk Richardson number(BRN)method and verified by the parcel method and the potential temperature gradient method.In general,the BRN method is able to represent the height of the convective boundary layer(BL)and neutral residual layer cases but has relatively large uncertainty in the stable BL cases.The diurnal cycle of the PBLH over land is quite different from the cycle over ocean,as are their seasonal variations.For stations over land,the PBLH shows an apparent diurnal cycle,with a distinct maximum around 15:00 LT,and seasonal variation,with higher values in summer.Compared with the PBLH over land,over oceans the PBLH diurnal cycles are quite mild,the PBLHs are much lower,and the seasonal changes are less pronounced.The seasonal variations in the median PBLH diurnal cycle are positively correlated with the near-surface temperature and negatively correlated with the near-surface relative humidity.Finally,although at most latitudes the daytime PBLH exhibits,over these 45 years,a statistically significant increasing trend at most hours between 12:00 LT and 18:00 LT over both land and ocean,there is no significant trend over either land or ocean in the nighttime PBLH for almost all the studied latitudes.

Antarctic Radiosonde Observations Reduce Uncertainties and Errors in Reanalyses and Forecasts over the Southern Ocean:An Extreme Cyclone Case

Cyclones with strong winds can make the Southern Ocean and the Antarctic a dangerous environment.Accurate weather forecasts are essential for safe shipping in the Southern Ocean and observational and logistical operations at Antarctic research stations.This study investigated the impact of additional radiosonde observations from Research Vessel"Shirase"over the Southern Ocean and Dome Fuji Station in Antarctica on reanalysis data and forecast experiments using an ensemble data assimilation system comprising the Atmospheric General Circulation Model for the Earth Simulator and the Local Ensemble Transform Kalman Filter Experimental Ensemble Reanalysis,version 2.A 63-member ensemble forecast experiment was conducted focusing on an unusually strong Antarctic cyclonic event.Reanalysis data with(observing system experiment)and without(control)additional radiosonde data were used as initial values.The observing system experiment correctly captured the central pressure of the cyclone,which led to the reliable prediction of the strong winds and moisture transport near the coast.Conversely,the control experiment predicted lower wind speeds because it failed to forecast the central pressure of the cyclone adequately.Differences were found in cyclone predictions of operational forecast systems with and without assimilation of radiosonde observations from Dome Fuji Station.

Macrophysical properties of specific cloud types from radiosonde and surface active remote sensing measurements over the ARM Southern Great Plains site

Accurate observation of clouds is challenging because of the high variability and complexity of cloud types and occurrences.By using the long-term cloud data collected during the ARM program at the Southern Great Plains central facility during 2001-2010,the consistencies and differences in the macrophysical properties of clouds between radiosonde and ground-based active remote sensing are quantitatively evaluated according to six cloud types：low;mid-low（ML）;high-midlow;mid;high-mid（HM）;and high.A similar variability trend is exhibited by the radiosonde and surface observations for the cloud fractions of the six cloud types.However,the magnitudes of the differences between the two methods are different among the six cloud types,with the largest difference seen in the high clouds.The distribution of the cloud-base height of the ML,mid,and HM clouds agrees in both methods,whereas large differences are seen in the cloud-top height for the ML and high clouds.The cloud thickness variations generally agree between the two datasets for the six cloud types.

Comparisons in the global planetary boundary layer height obtained from COSMIC radio occultation,radiosonde,and reanalysis data

The global planetary boundary layer height(PBLH)estimated from 11 years(2007–17)of Integrated Global Radiosonde Archive(IGRA)data,Constellation Observing System for Meteorology,Ionosphere and Climate(COSMIC)soundings,and European Center for Medium-Range Weather Forecasts(ECMWF)interim reanalysis(ERAInterim)data,are compared in this study.In general,the spatial distribution of global PBLH derived from ERAInterim is consistent with the one from IGRA,both at 1200 UTC and 0000 UTC.High PBLH occurs at noon local time,because of strong radiation energy and convective activity.There are larger differences between the results of COSMIC and the other two datasets.PBLHs derived from COSMIC are much higher than those from radiosonde and reanalysis data.However,PBLHs derived from the three datasets all exhibit higher values in the low latitudes and lower ones in the high latitudes.The latitudinal difference between IGRA and COSMIC ranges from−1700 m to−500 m,while it ranges from−500 m to 250 m for IGRA and ERA-Interim.It is found that the differences among the three datasets are larger in winter and smaller in summer for most studied latitudes.

Impact of Assimilation of Radiosonde and UAV Observations from the Southern Ocean in the Polar WRF Model

Weather forecasting in the Southern Ocean and Antarctica is a challenge above all due to the rarity of observations to be assimilated in numerical weather prediction(NWP)models.As observations are expensive and logistically challenging,it is important to evaluate the benefit that additional observations could bring to NWP.Atmospheric soundings applying unmanned aerial vehicles(UAVs)have a large potential to supplement conventional radiosonde sounding observations.Here,we applied UAV and radiosonde sounding observations from an RV Polarstern cruise in the ice-covered Weddell Sea in austral winter 2013 to evaluate the impact of their assimilation in the Polar version of the Weather Research and Forecasting(Polar WRF)model.Our experiments revealed small to moderate impacts of radiosonde and UAV data assimilation.In any case,the assimilation of sounding data from both radiosondes and UAVs improved the analyses of air temperature,wind speed,and humidity at the observation site for most of the time.Further,the impact on the results of 5-day-long Polar WRF experiments was often felt over distances of at least 300 km from the observation site.All experiments succeeded in capturing the main features of the evolution of near-surface variables,but the effects of data assimilation varied between different cases.Due to the limited vertical extent of the UAV observations,the impact of their assimilation was limited to the lowermost 1?2-km layer,and assimilation of radiosonde data was more beneficial for modeled sea level pressure and near-surface wind speed.

Water Vapor Variability in the Tropical Western Pacific from 20-year Radiosonde Data

The 20-year (1976-1995) daily radinsonde data at 17 stations in the tropical western Pacific was ana- lyzed. The analysls shows that the atmosphere is more humid in a warmer climate on seasonal,inter-annual and long-term (20-year) time scales, implying a positive water vapor feedback The vertlcal structure of the long-term trends in relative humidity (RH) is distinct from that on short-term (seasonal and inter-annual) time scales, suggesting that observed water vapor changes on short time scales could not be considered as a surrogate of long-term climate change. The increasing trend of RH (3%-5% / decade) in the upper troposphere is stronger than that in the lower troposphere (1 %-2% / decade). Such verthal structure would amplify positive water vapor feedback in comparison to the common assumption of constant RH changes vertically. The empicical orthogonal function (EOF) analysis of vertical structure of RH varmtions shows distinct features of the vert1cal structure of the first three EOFs. The first three EOFs are optimal for repre- sentation of water vapor profiles and provide some hints on physical mechanisms respons1ble for observed humidity variability. Vaisala radiosondes were used at nine stations, and VIZ radiosondes used at other eight stations The Vaisala data are corrected for temperature-dependence error using the correction siheme developed by NCAR / ATD and Vaisala. The comparison of Vaisala and VIZ data shows that the VIZ-measured RHs after October 1993 have a moist bias of - 10% at RHs < 20%. During 1976-1995, several changes in- cluding both instruments and reporting practice have been made at Vaisala stations and introduce errors to long-term RH variations.