Research
Remote Sensing of Cloud and Aerosol Properties
Satellite-based remote sensing of cloud properties
The satellite image on the left is a composite color full-disk visible image of the Western Hemisphere was captured from NOAA GOES-16 satellite at 1:07 pm EST on Jan. 15, 2017 (the very first image of GOES-16). The "white stuff" in the image are clouds. In fact, at any given time, clouds over about 60% of the surface area of our planet. They play a pivotal role in determining the radiative energy budget of our climate system. On one hand clouds reflect the incoming solar energy back to space which tends to cool the surface. On the other hand, same as greenhouse gases clouds absorb the thermal infrared energy emitted by surface which keeps the surface warm.
An important branch of our research is to use space-borne and ground-based remote sensing instruments to understand, quantify and simulate the properties of clouds, including their spatial and temporal distributions, macro- and microphysical properties, as well as the corresponding radiative effects.
Our recent studies in the area include:
- Inter-comparison of satellite and ground based cloud observations at Azores (Zhang et al. 2017)
- A novel framework to explain how cloud heterogeneity influences cloud retrievals. (Zhang et al. 2016; Highlighted by JGR editor)
- Development of retrieval algorithm for retrieving cloud properties from high resolution (~30m) ASTER observations (Werner et al. 2016)
- Investigation of how cloud vertical microphysical structure influences the liquid water path retrieval (Miller et al. 2016).
Remote Sensing of Aerosols above Clouds
Although most tropospheric aerosols are emitted into the atmospheric boundary layer, they can be convectively lifted above low-level clouds, or in some cases they are emitted at altitudes higher than the boundary layer and are subsequently transported over low-level cloud decks. In fact, above-cloud aerosols (ACAs) have been observed in several regions of the globe . The Figure to the right shows multiyear seasonal mean occurrence frequency of ACA derived from 8 years of daytime CALIOP observations.
Recently, we have teamed up with NASA scientists to develop novel remote sensing techniques to infer the properties of above-cloud aerosols from satellite observations. As summarized in Yu & Zhang (2013), instruments onboard NASA’s A-train satellite constellation provide valuable observations of the aerosol layer and underlying clouds. In particular, the lidar on the space-borne mission CALIPSO provides unique observations of the vertical distribution of the aerosol layer that have been widely used to characterize the aerosol layer above cloud over SE Atlantic and assess its impacts on the radiation budget. The seasonally transported SE Atlantic aerosol layer can influence the regional radiative energy budget through the direct radiative effect (DRE), and the semi-direct effect. In addition, when the aerosol layer is in direct contact with the underlying cloud, the aerosol particles can be entrained into the clouds and activated as cloud condensation nuclei, giving rise to the so-called aerosol indirect effects
Our recent studies in the area include:
- An invited review paper on the emerging techniques to retrieve ACA properties (Yu and Zhang 2013 Atmospheric Environment)
- A novel technique for simultaneous retrieval of the properties of ACA and underlying clouds (Meyer, Zhang and Platnick 2015 JGR)
- Using NASA's new Space-borne lidar-- Cloud-Aerosol Transport System (CATS)-- to retrieve the distance between ACA and underlying clouds (Rajapakshe et al. 2017 GRL)
Interactions of Aerosol, Cloud and Radiation
Smoke-Cloud interactions in SE Atlantic
Every year from about June to October over the southeast (SE) Atlantic, the prevailing easterly winds in the free troposphere often transport the smoke and pollution aerosols from the African continent to the west, over the ocean where extensive marine boundary layer (MBL) clouds persist for most of the year. This leads to a near-persistent seasonal biomass burning aerosol layer over MBL clouds in SE Atlantic.
Our recent studies in the area include:
- Inter-comparison of satellite and ground based cloud observations at Azores (Zhang et al. 2017)
- A novel framework to explain how cloud heterogeneity influences cloud retrievals. (Zhang et al. 2016; Highlighted by JGR editor)
- Development of retrieval algorithm for retrieving cloud properties from high resolution (~30m) ASTER observations (Werner et al. 2016)
- Investigation of how cloud vertical microphysical structure influences the liquid water path retrieval (Miller et al. 2016).
Net Radiative Effects of Dust Aerosols
Dust aerosols are the most abundant aerosol in the atmosphere in terms of dry mass, contributing a large aerosol optical depth (AOD) as showed in figure below. Therefore, they play a vital role in modulating the Earth’s radiative energy budget by interacting with solar and terrestrial radiations (so-called direct radiative effects) and serving as cloud condensation nuclei and ice nuclei. In addition, they also play an important role in ocean and terrestrial biogeochemical cycles. We focus on developing an observation-based estimate of dust direct radiative effect on both solar and terrestrial radiation and in both clear and cloudy conditions.
Cloud Simulations in Global Climate Models
