Features of the Cloud Base Height and Determining the Threshold of Relative Humidity over Southeast China

Clouds play a critical role in adjusting the global radiation budget and hydrological cycle; however, obtaining accurate information on the cloud base height (CBH) is still challenging. In this study, based on Lidar and aircraft soundings, we investigated the features of the CBH and determined the thresholds of the environmental relative humidity (RH) corresponding to the observed CBHs over Southeast China from October 2017 to September 2018. During the observational period, the CBHs detected by Lidar/aircraft were commonly higher in cold months and lower in warm months; in the latter, 75.91% of the CBHs were below 2000 m. Overall, the RHs at the cloud base were mainly distributed between 70 and 90% for the clouds lower than 1000 m, in which the most concentrated RH was approximately 80%. In addition, for the clouds with a cloud base higher than 1000 m, the RH thresholds decreased dramatically with increasing CBH, where the RH thresholds at cloud bases higher than 2000 m could be lower than 60%. On average, the RH thresholds for determining the CBHs were the highest (72.39%) and lowest (63.56%) in the summer and winter, respectively, over Southeast China. Therefore, to determine the CBH, a specific threshold of RH is needed. Although the time period covered by the collected CBH data from Lidar/aircraft is short, the above analyses can provide some verification and evidence for using the RH threshold to determine the CBH.

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Climatology of Cloud Vertical Structures from Long-Term High-Resolution Radiosonde Measurements in Beijing

Atmosphere ◽

10.3390/atmos11040401 ◽

2020 ◽

Vol 11 (4) ◽

pp. 401

Author(s):

Qing Zhou ◽

Yong Zhang ◽

Shuze Jia ◽

Junli Jin ◽

Shanshan Lv ◽

...

Keyword(s):

High Resolution ◽

Atmospheric Circulation ◽

Frequency Distribution ◽

Hydrological Cycle ◽

Global Radiation ◽

Circulation Model ◽

Middle Level ◽

Radiation Budget ◽

Cloud Base

Clouds are significant in the global radiation budget, atmospheric circulation, and hydrological cycle. However, knowledge regarding the observed climatology of the cloud vertical structure (CVS) over Beijing is still poor. Based on high-resolution radiosonde observations at Beijing Nanjiao Weather Observatory (BNWO) during the period 2010–2017, the method for identifying CVS depending on height-resolved relative humidity thresholds is improved, and CVS estimation by radiosonde is compared with observations by millimeter-wave cloud radar and ceilometer at the same site. Good consistency is shown between the three instruments. Then, the CVS climatology, including the frequency distribution and seasonal variation, is investigated. Overall, the occurrence frequency (OF) of cloudy cases in Beijing is slightly higher than that of clear-sky cases, and the cloud OF is highest in summer and lowest in winter. Single-layer clouds and middle-level clouds are dominant in Beijing. In addition, the average cloud top height (CTH), cloud base height (CBH), and cloud thickness in Beijing are 6.2 km, 4.0 km, and 2.2 km, respectively, and show the trend of reaching peaks in spring and minimums in winter. In terms of frequency distribution, the CTH basically resides below an altitude of 16 km, and approximately 43% of the CBHs are located at altitudes of 0.5–1.5 km. The cloud OF has only one peak located at altitudes of 4–8 km in spring, whereas it shows a trimodal distribution in other seasons. The height at which the cloud OF reaches its peak is highest in summer and lowest in winter. To the best of our knowledge, the cloud properties analyzed here are the first to elucidate the distribution and temporal variation of the CVS in Beijing from a long-term sounding perspective, and these results will provide a scientific observation basis for improving the atmospheric circulation model, as well as comparisons and verifications for measurements by ground-based remote sensing equipment.

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Perturbations in relative humidity in the boundary layer represent a possible mechanism for the formation of small convective clouds

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-28729-2013 ◽

2013 ◽

Vol 13 (11) ◽

pp. 28729-28749 ◽

Cited By ~ 1

Author(s):

E. Hirsch ◽

I. Koren ◽

O. Altaratz ◽

Z. Levin ◽

E. Agassi

Keyword(s):

Relative Humidity ◽

Inversion Layer ◽

Cloud Base ◽

Atmospheric Conditions ◽

Thermal Inversion ◽

Convective Clouds ◽

Air Pockets ◽

Lifting Condensation Level ◽

Shed Light ◽

Cloud Base Height

Abstract. An air parcel model was developed to study the formation of small convective clouds that appear under conditions of weak updraft and a strong thermal inversion layer above the clouds. Observations suggest that these clouds are characterized by a cloud base height far lower than the lifting condensation level. Considering such atmospheric conditions, the air parcel model shows that these clouds cannot be the result of classical thermals or plumes that are caused by perturbations in the temperature near the surface. We suggest that such clouds are the result of perturbations in the relative humidity of elevated air pockets. These results explain the existence of small clouds that standard methods fail to predict and shed light on processes related to the formation of convective clouds from the lowest end of the size distribution.

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Low visibility regime at aerodromes in European Russia

Hydrometeorological research and forecasting ◽

10.37162/2618-9631-2020-4-63-77 ◽

2020 ◽

Vol 4 ◽

pp. 63-77

Author(s):

N.P. Shakina ◽

◽

E.N. Skriptunova ◽

A.A. Zavialova ◽

◽

...

Keyword(s):

Relative Humidity ◽

Surface Wind ◽

European Russia ◽

Occurrence Frequency ◽

Cloud Base ◽

Time Intervals ◽

Frequency Distributions ◽

Annual Cycles ◽

Low Visibility ◽

Cloud Base Height

The characteristics are presented for the visibility regime at 42 aerodromes in European Russia calculated from the data of aerodrome observations reported in METAR telegrams with 30 minute (more rarely, 1 hour) time intervals. The occurrence frequency distributions of horizontal visibility ≤ 300 m and ≤ 800 m are calculated and analyzed over the period of 2001-2019. The tables are presented and discussed for the annual cycles of the occurrence frequency, as well as for its distributions under different weather phenomena. The occurrence frequency distributions depending on cloud base height, relative humidity, speed and direction of surface wind are presented. The results are also presented for the duration of low visibility episodes: it is demonstrated that such episodes are short, as a rule (for example, the visibility ≤ 300 m is continually observed for not more than 2 hours in 65-85 % of cases). The results of quantification of the correspondence between the occurrence or absence of low visibility and other weather characteristics observed at the same aerodromes demonstrate certain perspectives for developing (at least for several aerodromes) statistical methods to forecast this rather rare phenomenon basing on outputs of numerical atmosphere models. Keywords: visibility, aerodrome observations, annual cycle of low visibility, relative humidity, cloud base height, weather phenomena

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Cloud Base Height Estimation from ISCCP Cloud-Type Classification Applied to A-Train Data

Advances in Meteorology ◽

10.1155/2017/3231719 ◽

2017 ◽

Vol 2017 ◽

pp. 1-14 ◽

Cited By ~ 2

Author(s):

Yao Liang ◽

Xuejin Sun ◽

Steven D. Miller ◽

Haoran Li ◽

Yongbo Zhou ◽

...

Keyword(s):

Global Radiation ◽

Estimation Method ◽

Separation Distance ◽

Cloud Base ◽

Radiation Balance ◽

Cloud Type ◽

Active Sensors ◽

Combined Algorithm ◽

Type Classification ◽

Cloud Base Height

Cloud base height (CBH) is an important cloud macro parameter that plays a key role in global radiation balance and aviation flight. Building on a previous algorithm, CBH is estimated by combining measurements from CloudSat/CALIPSO and MODIS based on the International Satellite Cloud Climatology Project (ISCCP) cloud-type classification and a weighted distance algorithm. Additional constraints on cloud water path (CWP) and cloud top height (CTH) are introduced. The combined algorithm takes advantage of active and passive remote sensing to effectively estimate CBH in a wide-swath imagery where the cloud vertical structure details are known only along the curtain slice of the nonscanning active sensors. Comparisons between the estimated and observed CBHs show high correlation. The coefficient of association (R2) is 0.8602 with separation distance between donor and recipient points in the range of 0 to 100 km and falls off to 0.5856 when the separation distance increases to the range of 401 to 600 km. Also, differences are mainly within 1 km when separation distance ranges from 0 km to 600 km. The CBH estimation method was applied to the 3D cloud structure of Tropical CycloneBill, and the method is further assessed by comparing CTH estimated by the algorithm with the MODIS CTH product.

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Clouds over Hyytiälä, Finland: an algorithm to classify clouds based on solar radiation and cloud base height measurements

10.5194/amt-2020-130 ◽

2020 ◽

Author(s):

Ilona Ylivinkka ◽

Santeri Kaupinmäki ◽

Meri Virman ◽

Maija Peltola ◽

Ditte Taipale ◽

...

Keyword(s):

Solar Radiation ◽

Global Radiation ◽

Simple Algorithm ◽

Cloud Base ◽

Aerosol Formation ◽

Detection Range ◽

Clear Sky ◽

Overall Performance ◽

Sky Conditions ◽

Cloud Base Height

Abstract. We developed a simple algorithm to classify clouds based on global radiation and cloud base height measured by pyranometer and ceilometer, respectively. We separated clouds into seven different classes (stratus, stratocumulus, cumulus, nimbostratus, altocumulus+altostratus, cirrus+cirrocumulus+cirrostratus and clear sky+cirrus). We also included classes for cumulus and cirrus clouds causing global radiation enhancement, and classified multilayered clouds, when captured by the ceilometer, based on their height and characteristics (transparency, patchiness and uniformity). The overall performance of the algorithm was nearly 70 % when compared with classification by an observer using total sky images. The performance was best for clouds having well-distinguishable effects on solar radiation: nimbostratus clouds were classified correctly in 100 % of the cases. The worst performance corresponds to cirriform clouds (50 %). Although the overall performance of the algorithm was good, it is likely to miss the occurrence of high and multilayered clouds. This is due to the technical limits of the instrumentation: the vertical detection range of the ceilometer and occultation of the laser pulse by the lowest cloud layer. We examined the use of brightness parameter, which is defined as a ratio between measured global radiation and modeled radiation at the top of the atmosphere, as an indicator of clear sky conditions. Our results show that cumulus, altocumulus, altostratus and cirriform clouds can be present when the parameter indicates clear sky conditions. Those conditions have previously been associated with enhanced aerosol formation under clear sky. This is an important finding especially in case of low clouds coupled to the surface which can influence aerosol population via aerosol-cloud interactions. Overall, caution is required when the parameter is used in the analysis of processes affected by partitioning of radiation by clouds.

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Reducing misclassified precipitation phase in conceptual models using cloud base heights and relative humidity to adjust air temperature thresholds

Hydrology Research ◽

10.2166/nh.2021.072 ◽

2021 ◽

Author(s):

Lic James M. Feiccabrino

Keyword(s):

Relative Humidity ◽

Air Temperature ◽

Conceptual Models ◽

Phase Changes ◽

Cloud Base ◽

Temperature Threshold ◽

Air Temperatures ◽

Low Cloud ◽

Precipitation Phase ◽

Cloud Base Height

Abstract In cold region, conceptual models assigned precipitation phase, liquid (rain) or solid (snow), cause vastly different atmospheric, hydrological, and ecological responses, along with significant differences in evaporation, runoff, and infiltration fates for measured precipitation mass. A set air temperature threshold (ATT) applied to the over 30% annual precipitation events occurring with surface air temperatures between −3 and 5 °C resulted in 11.0 and 9.8% misclassified precipitation in Norway and Sweden, respectively. Surface air temperatures do not account for atmospheric properties causing precipitation phase changes as snow falls toward the ground. However, cloud base height and relative humidity (RH) measured from the surface can adjust ATT for expected hydrometeor-atmosphere interactions. Applying calibrated cloud base height ATTs or a linear RH function for Norway (Sweden) reduced to 4.3% (2.8%) and 14.6% (8.9%) misclassified precipitation, respectively. Cloud base height ATTs had lower miss-rates with low cloud bases, 100 m in Norway and 300 m in Sweden. Combining the RH method with cloud base ATT in low cloud conditions resulted in 16.1 and 10.8% reduction in misclassified precipitation in Norway and Sweden, respectively. Therefore, the conceptual model output should improve through the addition of available surface data without coupling to an atmospheric model.

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Investigating long-term variations in cloud base height over Sofia, Bulgaria

10.5194/ems2021-197 ◽

2021 ◽

Author(s):

Viktor Levi ◽

Evgeni Vladimirov ◽

Ventsislav Danchovski

Keyword(s):

Traffic Safety ◽

Climate Models ◽

Global Radiation ◽

Cloud Base ◽

Advantages And Disadvantages ◽

Climate Forecasting ◽

Spatial Coverage ◽

Weather And Climate ◽

Surface Measurements ◽

Cloud Base Height

<p>&#160;</p><p>Clouds have a key role in weather and climate forecasting due to their effect on global radiation and water budget. Clouds change the radiation energy in the Earth-atmosphere system by reducing both incoming and outgoing parts, depending on their macro- and microphysical characteristics such as cloud base height (CBH), optical properties etc. These clouds properties are generally related to cloud types, so the effects in weather and climate caused by various cloud types differ greatly. It is known that high clouds cause the earth's surface to heat up, while low clouds cause cooling. Obviously, cloud radiation forcing is an important source of uncertainty in the numerical weather and climate models, so the registered and expected changes in the properties of clouds due to a warming climate need in-depth studies. But cloud base height is not only important for weather and climate forecasting, but also for airplane traffic safety.&#160; Nowadays, retrieving the CBH is mainly based on satellite and ground-based observations. Satellite-borne instruments provide tempting spatial coverage but uncertainty in CBH estimation should be considered. In contrast, many ground-based observations of the CBH are characterized by higher accuracy. Nowadays, ceilometers - lidars specifically designed to detect CBH, that operate continuously and unattended, providing high vertical and time-resolution data, are reference instrument in CBH measurement. In addition, rawinsondes provide in-situ measurements of temperature, humidity, and pressure, so that the CBH can be evaluated by the lifting condensation level or by threshold value in relative humidity. In areas where only surface measurements are available, a simple adiabatic model of a rising air parcel can be applied in the CBH assessment. In this work, based on ceilometer, rawinsonde and surface measurements, the characteristics of CBH over Sofia, Bulgaria are studied in detail. We start with an intercomparison between CBHs obtained from three types of ground-based observations, considering the individual advantages and disadvantages of the methods by using ceilometer as reference. Finally, the daily, seasonal and interannual variability of CBH over Sofia are interpreted.</p>

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Clouds over Hyytiälä, Finland: an algorithm to classify clouds based on solar radiation and cloud base height measurements

Atmospheric Measurement Techniques ◽

10.5194/amt-13-5595-2020 ◽

2020 ◽

Vol 13 (10) ◽

pp. 5595-5619

Author(s):

Ilona Ylivinkka ◽

Santeri Kaupinmäki ◽

Meri Virman ◽

Maija Peltola ◽

Ditte Taipale ◽

...

Keyword(s):

Solar Radiation ◽

Global Radiation ◽

Cloud Base ◽

Aerosol Formation ◽

Clearness Index ◽

Detection Range ◽

Clear Sky ◽

Overall Performance ◽

Sky Conditions ◽

Cloud Base Height

Abstract. We developed a simple algorithm to classify clouds based on global radiation and cloud base height measured by pyranometer and ceilometer, respectively. We separated clouds into seven different classes (stratus, stratocumulus, cumulus, nimbostratus, altocumulus + altostratus, cirrus + cirrocumulus + cirrostratus and clear sky + cirrus). We also included classes for cumulus and cirrus clouds causing global radiation enhancement, and we classified multilayered clouds, when captured by the ceilometer, based on their height and characteristics (transmittance, patchiness and uniformity). The overall performance of the algorithm was nearly 70 % when compared with classification by an observer using total-sky images. The performance was best for clouds having well-distinguishable effects on solar radiation: nimbostratus clouds were classified correctly in 100 % of the cases. The worst performance corresponds to cirriform clouds (50 %). Although the overall performance of the algorithm was good, it is likely to miss the occurrences of high and multilayered clouds. This is due to the technical limits of the instrumentation: the vertical detection range of the ceilometer and occultation of the laser pulse by the lowest cloud layer. We examined the use of clearness index, which is defined as a ratio between measured global radiation and modeled radiation at the top of the atmosphere, as an indicator of clear-sky conditions. Our results show that cumulus, altocumulus, altostratus and cirriform clouds can be present when the index indicates clear-sky conditions. Those conditions have previously been associated with enhanced aerosol formation under clear skies. This is an important finding especially in the case of low clouds coupled to the surface, which can influence aerosol population via aerosol–cloud interactions. Overall, caution is required when the clearness index is used in the analysis of processes affected by partitioning of radiation by clouds.

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