An improved hydrometeor detection method for millimeter-wavelength cloud radar

Ge, Jinming; Zhu, Zeen; Zheng, Chuang; Xie, Haichao; Zhou, Tian; Huang, Jianping; Fu, Qiang

doi:10.5194/acp-17-9035-2017

Cited by 18 publications

(17 citation statements)

References 44 publications

(53 reference statements)

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“…To accurately simulate the total cloud cover and its impact on the radiative energy budget, climate models need to reliably represent the cloud vertical overlap, which has received less attention than necessary because of the lim-ited availability of regional cloud observations. In view of the passive sensors only providing limited information about the cloud overlap (Chang and Li, 2005a, b;Huang, 2006;Huang et al, 2005Huang et al, , 2006 and the vertically resolved advantage of active sensors (Ge et al, 2017(Ge et al, , 2018Zhao et al, 2016Zhao et al, , 2017, this study utilizes the 4 years (2007)(2008)(2009)(2010) of data from the CloudSat cloud product and collocated ERA-Interim reanalysis data to analyze the cloud overlaps over the Tibetan Plateau and to build an empirical relationship between cloud overlap properties and large-scale atmospheric dynamics. It is confirmed that the contiguous cloud layers tend to have maximum overlap at small separation but gradually become randomly overlapped with an increase in the layer separation.…”

Section: Conclusion and Discussionmentioning

confidence: 99%

The impact of atmospheric stability and wind shear on vertical cloud overlap over the Tibetan Plateau

Jian

et al. 2018

Atmos. Chem. Phys.

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Abstract. Studies have shown that changes in cloud cover are responsible for the rapid climate warming over the Tibetan Plateau (TP) in the past 3 decades. To simulate the total cloud cover, atmospheric models have to reasonably represent the characteristics of vertical overlap between cloud layers. Until now, however, this subject has received little attention due to the limited availability of observations, especially over the TP. Based on the above information, the main aim of this study is to examine the properties of cloud overlaps over the TP region and to build an empirical relationship between cloud overlap properties and large-scale atmospheric dynamics using 4 years (2007–2010) of data from the CloudSat cloud product and collocated ERA-Interim reanalysis data. To do this, the cloud overlap parameter α, which is an inverse exponential function of the cloud layer separation D and decorrelation length scale L, is calculated using CloudSat and is discussed. The parameters α and L are both widely used to characterize the transition from the maximum to random overlap assumption with increasing layer separations. For those non-adjacent layers without clear sky between them (that is, contiguous cloud layers), it is found that the overlap parameter α is sensitive to the unique thermodynamic and dynamic environment over the TP, i.e., the unstable atmospheric stratification and corresponding weak wind shear, which leads to maximum overlap (that is, greater α values). This finding agrees well with the previous studies. Finally, we parameterize the decorrelation length scale L as a function of the wind shear and atmospheric stability based on a multiple linear regression. Compared with previous parameterizations, this new scheme can improve the simulation of total cloud cover over the TP when the separations between cloud layers are greater than 1 km. This study thus suggests that the effects of both wind shear and atmospheric stability on cloud overlap should be taken into account in the parameterization of decorrelation length scale L in order to further improve the calculation of the radiative budget and the prediction of climate change over the TP in the atmospheric models.

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Section: Conclusion and Discussionmentioning

confidence: 99%

The impact of atmospheric stability and wind shear on vertical cloud overlap over the Tibetan Plateau

Jian

et al. 2018

Atmos. Chem. Phys.

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“…The primary instruments that we used in this study is the KAZR which has been described by Ge et al (2017). KAZR is a dual-polarization Doppler radar operated with two modes.…”

Section: Data Setsmentioning

confidence: 99%

“…The longest system shutdown of 5 days from 26 to 31 July 2014 was caused by a high shelter temperature. The cloud mask (i.e., discrimination of signal from noise) was achieved by using an improved cloud mask algorithm for cloud radar proposed by Ge et al (2017). As shown in Figure 1, hydrometeors are well identified from the original observed data by this method.…”

Section: Data Setsmentioning

confidence: 99%

Midlatitude Cirrus Clouds at the SACOL Site: Macrophysical Properties and Large‐Scale Atmospheric States

Zheng

Xie

et al. 2018

JGR Atmospheres

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Two‐year observations of a Ka‐band Zenith Radar at the Semi‐Arid Climate and Environment Observatory of Lanzhou University (SACOL) are used to document the midlatitude cirrus cloud macroproperties. Generally, cirrus occurs 41.6% of the observation time and most frequently appear at about 7.2 km above ground level. The cirrus macroproperties are strongly coupled with large‐scale atmospheric states; thus, its occurrence and location over the SACOL have significant seasonal variations. A k‐mean clustering method is used to classify cirrus into four distinct regimes without a prior knowledge about the meteorological process. Contrasting to the different cirrus physical properties in each regime, the cirrus event of each regime has a distinct seasonal distribution and the synoptic conditions from the ERA‐Interim reanalysis responsible for each cirrus regime are also quite different. Since global climate models typically overestimate cirrus cloud thickness due to inadequate parameterization or coarse grid resolution, we examined the probability density functions of large‐scale vertical velocity associated with each cirrus regime and the relationship between cirrus thickness and vertical velocity. It is found that the differences of the vertical velocity probability density functions among the cirrus regimes are as distinct as their macroproperties and a significant correlation exists between cirrus thickness and the vertical velocity, although the large‐scale vertical motion is nearly as likely to be descending as ascending when cirrus clouds are observed. This may imply that large‐scale vertical velocity can be used to constrain the variations of cirrus thickness simulated by global climate models.

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“…However, as is shown below, this decrease in false detections comes at the cost of a large reduction in the identification of true signals, meaning the false detections were reduced but so was the amount of correctly identified clouds (details in section 3). Independently, Ge et al (2017), hereafter G17, proposed an improved hydrometeor detection algorithm by adopting a bilateral filter, which is initially used in images process (Tomasi & Manduchi, 1998) to improve the weak signal detection. It has demonstrated good performance when applied to ground-based Ka-band cloud radar data collected at Semi-Arid Climate and Environment Observatory of Lanzhou University (Ge et al, 2018(Ge et al, , 2019Huang et al, 2008;Zhu et al, 2017), which can reduce radar noise while preserving cloud edges.…”

Section: Introductionmentioning

confidence: 99%

Improved Hydrometeor Detection Method: An Application to CloudSat

et al. 2020

Earth and Space Science

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Clouds play an important role in the climate system and are a principal source of uncertainty in climate projections. CloudSat has provided an unprecedented opportunity to study the vertical structure of clouds, and its observations are being widely used in scientific studies. However, some clouds are not detected or are only weakly detected by CloudSat. In most studies, the weakest detections, specifically those detected by the so‐called along‐track integration scheme, are typically ignored due to the high rate of false detections, namely, a significant probability that a detected cloud is actually a region of increased measurement noise, rather than a true cloud signal. False detections have been reduced in the latest version (called R05 for release 5) of the CloudSat cloud mask product but at a cost of a significant loss in the true weak signals (i.e., a higher false omission rate). In this study, the CloudSat hydrometeor detection algorithm used in R05 is modified by adding a bilateral filter scheme to improve the detection of weak signals. By comparing with the CALIPSO lidar vertical feature mask, it is shown that the new scheme largely reduces the false detection rate compared to the R04 version, while retaining a large fraction of the true weak signals that have been lost in the R05 version. Implementing this scheme in future CloudSat data processing is expected to lead to a better detection of thin clouds.

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An improved hydrometeor detection method for millimeter-wavelength cloud radar

Cited by 18 publications

References 44 publications

The impact of atmospheric stability and wind shear on vertical cloud overlap over the Tibetan Plateau

The impact of atmospheric stability and wind shear on vertical cloud overlap over the Tibetan Plateau

Midlatitude Cirrus Clouds at the SACOL Site: Macrophysical Properties and Large‐Scale Atmospheric States

Improved Hydrometeor Detection Method: An Application to CloudSat

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