Quantifying the aerosol/cloud-mediated radiative effect at a global scale requires simultaneous satellite retrievals of cloud condensation nuclei (CCN) concentrations and cloud base updraft ...velocities (Wb
). Hitherto, the inability to do so has been a major cause of high uncertainty regarding anthropogenic aerosol/cloud-mediated radiative forcing. This can be addressed by the emerging capability of estimating CCN and Wb
of boundary layer convective clouds from an operational polar orbiting weather satellite. Our methodology uses such clouds as an effective analog for CCN chambers. The cloud base supersaturation (S) is determined by Wb
and the satellite-retrieved cloud base drop concentrations (Ndb
), which is the same as CCN(S). Validation against ground-based CCN instruments at Oklahoma, at Manaus, and onboard a ship in the northeast Pacific showed a retrieval accuracy of ±25% to ±30% for individual satellite overpasses. The methodology is presently limited to boundary layer not raining convective clouds of at least 1 km depth that are not obscured by upper layer clouds, including semitransparent cirrus. The limitation for small solar backscattering angles of <25° restricts the satellite coverage to ∼25% of the world area in a single day.
Cold clouds are the main operation target of artificial precipitation enhancement, and its key is to find a supercooled cloud water area where the catalyst can be seeded to promote the formation of ...precipitation particles and increase precipitation to the ground. Based on the multi-spectral characteristics of the Fengyun-4A (FY-4A) satellite, a methodology for identifying supercooled cloud water is developed. Superimposed by a cloud top brightness temperature of 10.8 µm, a combination of 0.46 µm, 1.6 µm, and 2.2 µm red–green–blue (RGB) composites are used to identify the cloud phase and to obtain the real-time supercooled cloud water distribution every 5 min and in a 2 km resolution for the whole coverage of China. Based on the RGB composition, the supervised machine learning method K-mean clustering was applied to classify the cloud top phase. The results were validated extensively with Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP). It is worthwhile to highlight that the corresponding hit rate reached 87% over the full disk domain for both the summer and winter seasons. Furthermore, on 29 November 2019, microphysical properties were measured, and the data of supercooled cloud droplets and ice crystals were obtained using YUN-12 transport aircraft in Taiyuan. After simultaneously matching the satellite with an airborne track, the cloud particle image data were obtained near the cloud top and within the clouds during the climb and descending stages of the flight. The phase obtained from the microphysical properties of supercooled cloud droplets and ice crystals was compared with cloud phase results identified by FY-4A and Moderate Resolution Imaging Spectroradiometer (MODIS) cloud phase products. The case study and comparison show that (1) the supercooled water clouds and ice particles identified by FY-4A are in good agreement with those from the airborne measurement at the cloud top and within the cloud and (2) the positions and shapes of water clouds and ice clouds identified by FY-4A correspond well with MODIS cloud phase products. However, there is a small deviation in the extent of ice clouds, which is mainly located in the transition area between ice clouds and water clouds. The extent of ice clouds identified by FY-4A is slightly larger than that of MODIS products. Combined with airborne detection, the comparison shows that the ice clouds identified by the FY-4A satellite are consistent with aircraft detection. The supercooled cloud water identified by FY-4A can meet the needs of the operational precipitation enhancement of cold clouds, improve operational effectiveness, and promote the application of satellite technology for weather modification.
Heavy aerosol loads have been observed to suppress warm rain by reducing cloud drop size and slowing drop coalescence. The ice forming nuclei (IFN) activity of the same aerosols glaciate the clouds ...and create ice precipitation instead of the suppressed warm rain. Satellite observations show that desert dust and heavy air pollution over East Asia have similar ability to glaciate the tops of growing convective clouds at glaciation temperature of Tg < ∼ −20°C, whereas similarly heavy smoke from forest fires in Siberia without dust or industrial pollution glaciated clouds at Tg ≤ −33°C. The observation that both smoke and air pollution have same effect on reducing cloud drop size implies that the difference in Tg is due to the IFN activity. This dependence of Tg on aerosol types appears only for clouds with re‐5 < 12 μm (re‐5 is the cloud drop effective radius at the −5°C isotherm, above which ice rarely forms in cloud tops). For the rest of the clouds the glaciation temperature increases strongly with re‐5 with little relation to the aerosol types, reaching Tg> ∼ −15°C for the largest re‐5, which are typical to marine clouds in pristine atmosphere.
Key Points
Air pollution aerosols over East Asia are as potent ice nuclei as desert dust
Both heavily polluted and dusty convective clouds glaciate at temperatures above −20°C
The glaciation restores at least some of the suppressed rain by these aerosols
The entrainment rate (λ) is difficult to estimate, and its uncertainties cause a significant error in convection parameterization and precipitation simulation, especially over the Tibetan Plateau, ...where observations are scarce. The λ over the Tibetan Plateau, and its adjacent regions, is estimated for the first time using five-year satellite data and a reanalysis dataset. The λ and cloud base environmental relative humidity (RH) decrease with an increase in terrain height. Quantitatively, the correlation between λ and RH changes from positive at low terrain heights to negative at high terrain heights, and the underlying mechanisms are here interpreted. When the terrain height is below 1 km, large RH decreases the difference in moist static energy (MSE) between the clouds and the environment and increases λ. When the terrain height is above 1 km, the correlation between λ and RH is related to the difference between MSE turning point and cloud base, because of decreases in specific humidity near the surface with increasing terrain height. These results enhance the theoretical understanding of the factors affecting λ and pave the way for improving the parameterization of λ.
To accurately identify ideal operating conditions for scientific cloud seeding and to collect evidence of the effects of seeding, a set of three flights, with figure‐8‐like paths, was designed for ...supercooled liquid water detection. These flights were conducted near the cloud top before seeding, during seeding, and upon re‐entry. Using both in situ aircraft and satellite remote sensing observations, a comprehensive analysis was conducted to understand the response of cloud properties to seeding within supercooled cloud tops. Satellite observations showed clear icing cloud tracks after seeding with abundant supercooled liquid water near the cloud top. Radar observations confirmed enhanced radar reflectivity echoes in cloud‐seeding regions. For regions cloud seeded with supercooled liquid water, the liquid droplets quickly froze into ice crystals, which further grew into large ice crystals via vapor deposition, riming, and collision‐coalescence processes. With these processes, large ice crystals formed and rapidly fell downward, causing the sinking of cloud tops and precipitation to fall to the ground. By contrast, there were no such phenomena in the regions without supercooled liquid water or in those regions with supercooled liquid, but otherwise without seeding. In addition, radar reflectivity echoes quickly decreased in all regions without seeding. The evolutionary characteristics of radar echoes and in situ observed cloud and precipitation properties suggest that the response of cloud microphysical characteristics to seeding varies with time and location due to various seeding potentials, leading to different formation conditions of both cloud‐top icing and cloud tracks.
Key Points
Satellite observations showed significant icing cloud tracks after seeding near cloud tops abundant in supercooled liquid water
Radar observations confirmed the enhanced radar reflectivity echoes in the seeded regions
In regions seeded with supercooled water, droplets quickly transformed to ice crystals, causing cloud‐top sinking and precipitation
Lipids and lipid-metabolizing esterases/lipases are highly important for the mycobacterial life cycle and, possibly, for mycobacterial virulence. In this study, we expressed 10 members of the Lip ...family of Mycobacterium tuberculosis. Among the 10 proteins, LipL displayed a significantly high enzymatic activity for the hydrolysis of long-chain lipids. The optimal temperature for the lipase activity of LipL was demonstrated to be 37°C, and the optimal pH was 8.0. The lipase active center was not the conserved motif G-x-S-x-G, but rather the S-x-x-K and GGG motifs, and the key catalytic amino acid residues were identified as G50, S88, and K91, as demonstrated through site-directed mutagenesis experiments. A three-dimensional modeling structure of LipL was constructed, which showed that the GGG motif was located in the surface of a pocket structure. Furthermore, the subcellular localization of LipL was demonstrated to be on the mycobacterial surface by Western blot analysis. Our results revealed that the LipL protein could induce a strong humoral immune response in humans and activate a CD8+ T cell-mediated response in mice. Overall, our study identified and characterized a novel lipase denoted LipL from M. tuberculosis, and demonstrated that LipL functions as an immunogen that activates both humoral and cell-mediated responses.
The ice crystal habits, distributions and growth processes in two snowfall cloud cases on 29 November 2009 and 3 March 2012 in northern China were compared and analyzed with aircraft data. The ...results showed that ice crystal habits were affected by the height of ice clouds. Ice crystals in clouds with cloud top temperatures of −12.6 °C were predominantly needle, plate, dendrite and irregular. When the cloud top temperature was lower than −19.5 °C, plates, dendrites and irregular ice crystals were observed in addition to needles, capped-column crystals were observed in the lower and middle layers of clouds, and column crystals were observed in the upper layer of clouds. The liquid water content of the two snowfall processes was lower than 0.1 g·m−3. Ice particles grew mainly via deposition, riming and aggregation processes. On 29 November, the liquid water content of the stratospheric mixed snowfall cloud was distributed in the lower part of the cloud. The maximum values of particle concentration and ice water content detected by a cloud imaging probe were 187 L−1 and 1.05 g·m−3, which were at −8.7 °C, and the ice water content was higher. On 3 March, the liquid water content of snowfall in stratiform clouds was located in the middle layer, and the maximum ice water was low, which was only 0.052 g m−3. The ice water value on 29 November was higher, which was mainly due to the convective zone embedded in the cumulus mixed cloud containing a large number of riming and aggregated snow crystals. Using an exponential function to fit the crystal spectrum of the two snowfall processes, N0 and λ were 109−1011 m−4 and 108−1010 m−4 and 103−104 m−1 and 104 m−1, respectively. Compared with 3 March, N0 on 29 November was larger and the variation range of λ was one more order of magnitude. N0 and λ conformed to a power function distribution. By analyzing the scatter plot of the correlation coefficient and slope, it was found that the exponential function can accurately express the crystal spectrum of snow clouds.
The aerosol effects on clouds and precipitation in deep convective cloud systems are investigated using the Weather Research and Forecast (WRF) model with the Morrison two-moment bulk microphysics ...scheme. Considering positive or negative relationships between the cloud droplet number concentration (N
c
) and spectral dispersion (ɛ), a suite of sensitivity experiments are performed using an initial sounding data of the deep convective cloud system on 31 March 2005 in Beijing under either a maritime ('clean') or continental ('polluted') background. Numerical experiments in this study indicate that the sign of the surface precipitation response induced by aerosols is dependent on the ɛ−N
c
relationships, which can influence the autoconversion processes from cloud droplets to rain drops. When the spectral dispersion ɛ is an increasing function of N
c
, the domain-average cumulative precipitation increases with aerosol concentrations from maritime to continental background. That may be because the existence of large-sized rain drops can increase precipitation at high aerosol concentration. However, the surface precipitation is reduced with increasing concentrations of aerosol particles when ɛ is a decreasing function of N
c
. For the ɛ−N
c
negative relationships, smaller spectral dispersion suppresses the autoconversion processes, reduces the rain water content and eventually decreases the surface precipitation under polluted conditions. Although differences in the surface precipitation between polluted and clean backgrounds are small for all the ɛ−N
c
relationships, additional simulations show that our findings are robust to small perturbations in the initial thermal conditions.
The Tibetan Plateau (TP) plays an important role in formation and development of the East Asian atmospheric circulation, climate variability, and disastrous weathers in China. Among the many topics ...on TP meteorology, it is critical to understand the microphysical characteristics of clouds over the TP; however, observations of the cloud micro-physics in this area are insufficient mainly due to sparse stations and limited cloud physical data. The Visible Infrared Imaging Radiometer Suite (VIIRS), onboard the Suomi National Polar-orbiting Partnership (
SNPP
) satellite, has an improved imaging spectroradiometer with 17 channels of 750-m moderate resolution and 5 channels of 375-m image resolution. The high-resolution instrument has an advantage for observing the small or initial convective clouds. Based on the methodologies that we proposed before for retrieving cloud microphysical properties from
SNPP
, an automated mapping software package named Automatic Mapping of Convective Clouds (AMCC) has been developed at the scale of satellite swath. The properties of convective clouds are retrieved by AMCC and their values are averaged over 0.33° × 0.33° grids based on the
SNPP
/VIIRS satellite data over the TP during the summers of 2013–17. The results show that: (1) the temperature of lifting condensation level (T
LCL
) at Naqu meteorological station and the cloud base temperature (
T
b
) retrieved from VIIRS are linearly correlated, with a correlation coefficient of 0.87 and standard deviation (STD) of 3.0°C; (2) convective clouds over the TP have the following macro- and micro-physical properties. First, the cloud base temperature (
T
b
) is about −5°C, the cloud base height above the ground (
H
b
) ranges between 1800 and 2200 m, and the cloud water content is low. Second, the cloud condensation nuclei concentration (
N
CCN
) is between 200 and 400 mg
−1
with 0.7% in maximum supersaturation (
S
max
); consequently, the condensation growth of water cloud droplet with less
N
CCN
and higher
S
max
is fast. Third, because the precipitation initiation depth (
D
14
) varies within 1500–2000 m and 500–1000 m at the Yarlung Zangbo River basin and southern Tibet, respectively, the clouds over these areas are more prone to precipitation. Fourth, mean height of the cloud top above sea level (
H
top
) is between 10 and 13 km, but the cloud depth (
D
cld
) is rather small, which is about 5000 m in southern TP and gradually reduces to 2500 m in northern TP. Fifth, the glaciation temperature (
T
g
) ranges from −30°C in central and southern TP to −25°C in northern TP, which, combined with the warmer
T
g
and the
T
b
less than 0°C, leads to the domination of ice process in the clouds; (3) the macro- and microphysical properties of convective clouds over the TP explain why rainfall there is frequent and lasts over a short time with small amount and large rain drops.
The identification of raindrops and hailstones is of great significance to the study of precipitation characteristics from the aspect of microphysics and can provide important data support for ...weather modification. In this paper, an identification method of raindrops and hailstones based on digital holographic interference is proposed. The grayscale gradient variance method is used to obtain the focus position of the particles. By means of binarization and morphological processing, digital holograms are processed to obtain clear profiles of the particles. Then the contour parameters of the particles are used to obtain the equivalent volume diameter and roundness. Finally, according to the equivalent volume diameter, roundness and lens-like effect of the particles, the phase states of the raindrop and hailstone are identified by the algorithm. Experiments show that the method proposed in this paper has a good identification effect on raindrops and hailstones. The research results can provide reference for the research of the identification method of raindrops and hailstones and the acquisition of accurate characteristic parameters.
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