Jinming Ge

Characteristics of Taklimakan dust emission and distribution: A satellite and reanalysis field perspective

Dust particles from the Taklimakan Desert can be lofted vertically up to 10 km due to the unique topography and northeasterly winds associated with certain synoptic conditions. Then they can be transported horizontally to regions far downwind by westerlies. We combined data from the Multiangle Imaging Spectroradiometer (MISR) and the Cloud-Aerosol Lidar with Orthogonal Polarization to investigate the three-dimensional distribution of dust over the Taklimakan Desert and surrounding areas. During spring and summer, a dust belt with high aerosol optical depths (AOD) extends eastward from the Taklimakan Desert to the Loess Plateau along the Hexi Corridor and southward to the Tibetan Plateau. However, the dust extinction coefficients decrease rapidly from 0.340 km−1 near surface to 0.015 km−1 at 5 km in spring, while the extinction values vary within 0.100 ± 0.020 between the altitudes of 1.6 and 3.5 km and decrease to 0.023 km−1 at 5 km in summer, indicating that dust aerosol is relatively well mixed vertically. We further used MISR daily AOD to identify high- and low-dust days and then analyzed composite difference patterns of temperature, geopotential height, and wind between high- and low-dust days. It was found that although the synoptic situations of spring and summer are quite different, there are two common features: a strong anticyclonic wind anomaly over the Taklimakan at 500 hPa and an enhanced easterly wind over the Tarim Basin at 850 hPa for the two seasons. These conditions are favorable for dust entrainment from the dry desert surface, vertical lofting, and horizontal transport.

Shortwave radiative closure experiment and direct forcing of dust aerosol over northwestern China

Modeled and observed solar diffuse fluxes at the surface usually have unacceptably large discrepancies. It is necessary to address and resolve these discrepancies in order to accurately calculate a reliable aerosol direct radiative forcing (DRF). We present and compare two methods of deriving dust aerosol optical properties from the MFRSR (Multi-Filter Rotating Shadowband Radiometer) observations and the AERONET products. The single-scattering albedo (SSA) values from MFRSR are found to be 10% less than those from the AERONET. This difference is mainly due to the different imaginary part of refractive index retrieved by the MFRSR compared to AERONET. These two sets of dust aerosol optical properties are used in the SBDART model to simulate the shortwave fluxes that are compared with the surface observations to perform the radiative closure experiment. The diffuse simulations using the AERONET-derived aerosol SSA may have significant discrepancies compared with the observed diffuse irradiances. The DRFs at the top of atmosphere (TOA) simulated with the MFRSR-derived aerosol optical properties are positive while the DRFs with the AERONET are negative. The sign of the DRFs at the surface and in the atmosphere using the MFRSR is the same as those using the AERONET while the magnitudes from the MFRSR are much larger. It indicates that dust aerosols with higher absorption as derived from the MFRSR heat the aerosol layer but cool the surface much more than those based on the AERONET, which may have an important impact on the boundary layer processes. Citation: Ge, J. M., J. P. Huang, J. Su, J. R. Bi, and Q. Fu (2011), Shortwave radiative closure experiment and direct forcing of dust aerosol over northwestern China, Geophys. Res. Lett., 38, L24803, doi:10.1029/2011GL049571.

Automated detection of cloud and aerosol features with SACOL micro-pulse lidar in northwest China

The detection of cloud and aerosols using a modified retrieval algorithm solely for a ground-based micropulse lidar (MPL) is presented, based on one-year data at the Semi-Arid Climate Observatory and Laboratory (SACOL) site (35.57°N, 104.08°E, 1965.8 m), northwest of China, from March 2011 to February 2012. The work not only identifies atmosphere particle layers by means of the range-dependent thresholds based on elastic scattering ratio and depolarization ratio, but also discriminates the detected layers by combining empirical thresholds of the atmospheres thermodynamics states and scattering properties and continuous wavelet transform (CWT) analyses. Two cases were first presented in detail that demonstrated that the modified algorithm can capture atmosphere layers well. The cloud macro-physical properties including cloud base height (CBH), cloud geometrical thickness (CGT), and cloud fraction (CF) were then analyzed in terms of their monthly and seasonal variations. It is shown that the maximum/minimum CBHs were found in summer (4.66 ± 1.95km)/autumn (3.34 ± 1.84km). The CGT in winter (1.05 ± 0.43km) is slightly greater than in summer (0.99 ± 0.44km). CF varies significantly throughout year, with the maximum value in autumn (0.68), and a minimum (0.58) in winter, which is dominated by single-layered clouds (81%). The vertical distribution of CF shows a bimodal distribution, with a lower peak between 1 and 4km and a higher one between 6and 9km. The seasonal and vertical variations in CF are important for the local radiative energy budget.

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

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. Plain Language Summary Cirrus clouds are composed of large amount of ice crystals, most frequently distributed in the midlatitude storm track regions and the tropics, and cover about 30% of the Earth surface. They have significant impact on water cycle and radiative balance and thus play an important role in our climate system. These processes strongly depend on the cirrus properties such as top height and vertical distribution. However, cirrus clouds are still a great challenge to be accurately represented in climate models due to incomplete knowledge of their occurrence and physical and dynamical properties that can cause large uncertainties in climate prediction. We obtained the cirrus macroproperties from the cloud radar observations at the Semi-Arid Climate and Environment Observatory of Lanzhou University (a midlatitude site in western China) and found that the cirrus macroproperties are strongly coupled with large-scale atmospheric states. This may help us to better understand the connection between cirrus properties and dynamic processes.