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Ice formation mechanisms in generating cells near stratiform cloud tops, where mixing and entrainment occurs in the presence of supercooled water droplets, remain poorly understood. Supercooled cloud droplet temperature and lifetime may impact heterogeneous ice nucleation through contact and immersion freezing; however, modeling studies normally assume the droplet temperature to be spatially uniform and equal to the ambient temperature. Here, we present a first-of-its-kind quantitative investigation of the temperature and lifetime of evaporating droplets, considering internal thermal gradients within the droplet, as well as thermal and vapor density gradients in the surrounding air. Our approach employs solving Navier–Stokes and continuity equations, coupled with heat and vapor transport, using an advanced numerical model. For typical ranges of cloud droplet sizes and environmental conditions, the droplet internal thermal gradients dissipate quickly (≤ 0.3 s) when droplets are introduced to new subsaturated environments. However, the magnitude of droplet cooling is much greater than estimated from past studies of droplet evaporation, especially for drier environments. For example, for an environment with 500 hPa pressure, and ambient temperature far from the droplet of - 5 °C , the droplet temperature reduction can be as high as 24, 11, and 5 °C for initial ambient relative humidities of 10 %, 40 %, and 70 %, respectively. Droplet lifetimes are found to be tens of seconds longer compared to previous estimates, due to weaker evaporation rates because of lower droplet surface temperatures. Using these new end-of-lifetime droplet temperatures, the enhancement in the activation of ice-nucleating particles predicted by current ice nucleation parameterization schemes is discussed. [ABSTRACT FROM AUTHOR]

The poor representation of the macrophysical properties of shallow oceanic cumuli in climate models contributes to the large uncertainty in cloud feedback. These properties are also difficult to measure because it requires high-resolution satellite imagery that is seldomly collected over ocean. Here, we examine cumulus cloud macrophysical properties, their size, shape, and spatial distributions, over the tropical western Pacific using 170 15 m resolution scenes from Terra's Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) collected during the 2019 Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP 2 Ex) mission. The average cloud fraction (CF) was 0.12, half of which was contributed by clouds less than 1.6 km in area-equivalent diameter. This compared well to Terra's Multi-angle Imaging SpectroRadiometer (MISR) resolution-corrected CF of 0.14 but less than the 0.19 measured by Terra's Moderate Resolution Imaging Spectroradiometer (MODIS). The cloud size distribution exhibited a power law form with an exponent of 2.93 and an area–perimeter power law with a dimension of 1.25. ASTER, MISR, and CAMP 2 Ex aircraft lidar showed excellent agreement in the cloud top height (CTH) distribution peak altitude of ∼ 750 m. We examined cumulus properties in relation to meteorological variables and found that the variation in mean CTH is controlled most by the total column water vapor, lower-tropospheric stability (LTS), and estimated inversion strength (EIS). The variation in CF is most controlled by surface wind speed and near-cloud relative humidity instead of LTS/EIS, suggesting the need to improve low-cloud parameterizations in climate models that use LTS/EIS based on stratocumulus studies. [ABSTRACT FROM AUTHOR]

The poor representation of the macrophysical properties of shallow oceanic cumuli in climate models contributes to the large uncertainty in cloud feedback. These properties are also difficult to measure because it requires high-resolution satellite imagery that are seldomly collected over ocean. Here, we examine cumulus cloud macrophysical properties, their size, shape, and spatial distributions, over the tropical western Pacific using 170 15-m resolution scenes from Terra's Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) collected during the 2019 Cloud, Aerosol and Monsoon Processes Philippines Experiment (CAMP2Ex) mission. The average cloud fraction (CF) was 0.12, half of which was contributed by clouds less than 1.6 km in area-equivalent diameter. This compared well to Terra's Multiangle Imaging Spectroradiometer (MISR) resolution-corrected CF of 0.13, but less than the 0.19 measured by Terra's Moderate Resolution Imaging Spectroradiometer (MODIS). The cloud size distribution exhibited a power law form with exponent of 2.93 and an area-perimeter power law with dimension of 1.24. ASTER, MISR, and CAMP2Ex aircraft lidar showed excellent agreement in the cloud top height (CTH) distribution peak altitude of ~750 m. We examined cumulus properties in relation to meteorological variables and found that the variation in mean CTH is most controlled by the total column water vapor, lowertropospheric stability (LTS), and estimated inversion strength (EIS). The variation in CF is most controlled by surface wind speed and near-cloud relative humidity instead of LTS/EIS, suggesting the need to improve low cloud parameterizations in climate models that use LTS/EIS based on stratocumulus studies. [ABSTRACT FROM AUTHOR]

Aerosol particles in Southeast Asia are challenging to characterize due to their complex life cycle within the diverse topography and weather of the region. An emerging aerosol climatology was established based on AErosol RObotic NETwork (AERONET) data (December 2009 to October 2018) for clear-sky days in Metro Manila, the Philippines. Aerosol optical depth (AOD) values were highest from August to October, partly from fine urban aerosol particles, including soot, coinciding with the burning season in insular Southeast Asia when smoke is often transported to Metro Manila during the southwest monsoon. Clustering of AERONET volume size distributions (VSDs) resulted in five aerosol particle sources based on the position and magnitude of their peaks in the VSD and the contributions of specific particle species to AOD per cluster based on MERRA-2. The clustering showed that the majority of aerosol particles above Metro Manila were from a clean marine source (58 %), which could be related to AOD values there being relatively low compared to other cities in the region. The following are the other particle sources over Metro Manila: fine polluted sources (20 %), mixed-dust sources (12 %), urban and industrial sources (5 %), and cloud processing sources (5 %). Furthermore, MERRA-2 AOD data over Southeast Asia were analyzed using empirical orthogonal functions. Along with AOD fractional compositional contributions and wind regimes, four dominant aerosol particle air masses emerged: two sulfate air masses from East Asia, an organic carbon source from Indonesia, and a sulfate source from the Philippines. Knowing the local and regional aerosol particle air masses that impact Metro Manila is useful in identifying the sources while gaining insight into how aerosol particles are affected by long-range transport and their impact on regional weather. [ABSTRACT FROM AUTHOR]

Our longest, stable record of cloud‐top pressure (CTP) and cloud‐top height (CTH) are derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi‐Angle Imaging Spectroradiometer (MISR) on Terra. Because of single cloud‐layer assumptions in their standard algorithms, they provide only single CTP/CTH retrievals in multi‐layered situations. In the predominant multi‐layered regime of thin cirrus over low clouds, MODIS significantly overestimates cirrus CTP and emissivity, while MISR accurately retrieves low‐cloud CTH. Utilizing these complementary capabilities, we develop a retrieval algorithm for accurately determining both‐layer CTP and cirrus emissivity for such 2‐layered clouds, by applying the MISR low‐cloud CTH as a boundary condition to a modified MODIS CO2‐slicing retrieval. We evaluate our 2‐layered retrievals against collocated Cloud‐Aerosol Transport System (CATS) lidar observations. Relative to CATS, the mean bias of the upper cloud CTP and emissivity are reduced by ∼90% and ∼75% respectively in the new technique, compared to standard MODIS products. We develop an error model for the 2‐layered retrieval accounting for systematic and random errors. We find up to 87% of all residuals lie within modeled 95% confidence intervals, indicating a near‐closure of error budget. This reduction in error leads to a reduction in modeled atmospheric longwave radiative flux biases ranging between 5 and 40 W m−2, depending on the position and optical properties of the layers. Given this large radiative impact, we recommend that the pixel‐level 2‐layered MODIS + MISR fusion algorithm be applied over the entire MISR swath for the 22‐year Terra record, leading to a first‐of‐its‐kind 2‐layered cloud climatology from Terra's morning orbit. Plain Language Summary: Our longest climate‐quality record of global cloud‐top heights (CTH) comes from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi‐Angle Imaging Spectroradiometer (MISR) on the Terra satellite. These sensors assume a single cloud‐layer in retrieving CTH, even though ∼30% of global cloud cover is multi‐layered. Multi‐layered clouds predominantly consist of thin ice clouds over low clouds. Under such conditions, MISR accurately retrieves low‐cloud CTH, while MODIS systematically underestimates upper‐cloud‐layer CTH. Here, we have developed a 2‐layered MODIS + MISR fusion CTH retrieval by using MISR's accurate low‐cloud CTH as an input to a modified MODIS algorithm. This algorithm combines the complementary capabilities of MISR and MODIS in distinguishing higher and lower clouds and estimates both‐layer cloud heights and high‐cloud emissivity. Through comparisons against coincident Cloud‐Aerosol Transport System lidar observations, we find that the new algorithm improves the accuracies in retrieved CTH and cloud emissivities by ∼75% over standard MODIS products. We further demonstrate significant improvements in estimates of simulated atmospheric longwave radiation from our implementation. Owing to its large radiative impact, we suggest that the pixel‐level fusion algorithm be applied to all 22 years of Terra record to facilitate public dissemination of the first 2‐layered cloud record from its morning orbit. Key Points: Accurate, high‐precision Multi‐Angle Imaging Spectroradiometer low cloud heights are employed in a physics‐based correction to Moderate Resolution Imaging Spectroradiometer CO2‐slicing in multi‐layered scenesCloud‐top pressure (CTP) bias drops from 65 to 5 hPa, resulting in a quartering of cloud‐height and emissivity bias for cirrus over low cloudUp to 87% of CTP retrieval errors are bound by theoretical estimates, resulting in near‐closure of CO2‐slicing error budget [ABSTRACT FROM AUTHOR]

Aerosol particles in Southeast Asia have a complex life cycle and consequently are challenging to characterize. The diverse topography and weather in the region complicate the situation. An aerosol climatology was established based on AERONET data (December 2009 to October 2018) for clear sky days in Metro Manila, Philippines. Aerosol optical depth (AOD) values were highest in August, coinciding with the summer southwest monsoon, due partly to fine particles from urban aerosol particles, including soot. Also, August corresponds to the burning season in Insular Southeast Asia when smoke is often transported to Metro Manila. Clustering of AERONET volume size distributions (VSD) resulted in five aerosol particle sources based on the position and magnitude of their peaks in the VSD and the contributions of specific particle species to AOD per cluster based on MERRA-2. The clustering showed that the majority of aerosol particles above Metro Manila were from a clean marine source (58%), which could be related to AOD values there being relatively smaller than in other cities in the region. The following are the other particle sources over Metro Manila: fine polluted (20%), mixed polluted (12%), urban/industrial (5%), and cloud processing (5%). Furthermore, MERRA-2 AOD data over Southeast Asia were analyzed using empirical orthogonal functions. Along with AOD fractional compositional contributions and wind regimes, four dominant aerosol particle air masses emerged: two sulfate air masses from East Asia, an organic carbon source from Indonesia, and a sulfate source from the Philippines. Knowing the local and regional aerosol particle air masses that impact Metro Manila is useful in identifying the sources while gaining insight on how aerosol particles are affected by long-range transport and their impact on regional weather. [ABSTRACT FROM AUTHOR]

Overshooting cloud tops (OTs) form in deep convective storms when strong updrafts overshoot the tropopause. An OT is a well-known indicator of convective updrafts and severe weather conditions. Here, we develop an OT detection algorithm using thermal infrared (IR) channels and apply this algorithm to about 20 years' worth of MODIS data from both Terra and Aqua satellites to form an extensive, near-global climatology of OT occurrences. The algorithm is based on a logistic model which is trained using A-Train observations. We demonstrate that the overall accuracy of our approach is about 0.9 when the probability of the OT candidates is larger than 0.9. The OT climatology reveals a pattern that follows the climatology of deep convection and shallow convection over the midlatitude oceans during winter cold-air outbreaks. OTs appear most frequently over the Intertropical Convergence Zone (ITCZ), central and southeastern North America, tropical and subtropical South America, southeastern and southern Asia, tropical and subtropical Africa, and northern middle–high latitudes. OT spatial distributions show strong seasonal and diurnal variabilities. Seasonal OT variations shift with large-scale climate systems such as the ITCZ and local monsoonal systems, including the South Asian monsoon, North American monsoon, and West African monsoon. OT diurnal variations agree with the known diurnal cycle of convection. Maximum OT occurrences are in the afternoon over most land areas and around midnight over ocean, and the OT diurnal cycle is stronger and more varied over land than over ocean. OTs over land are usually colder than over ocean, except at around 10:30 LT (Equator-crossing time). The top 10 coldest OTs from both Terra and Aqua mostly occur over land and at night. This study provides OT climatology for the first time, as derived from 2 decades of MODIS data, that represents the longest and stable satellite records. [ABSTRACT FROM AUTHOR]

Overshooting cloud tops (OT) form in deep convective storms when strong updrafts overshoot the tropopause. An OT is a well-known indicator for convective updrafts and severe weather conditions. Here, we develop an OT detection algorithm using thermal IR channels and apply this algorithm to about 20-year MODIS data from both Terra and Aqua satellites to form an extensive, near global climatology of OT occurrences. The algorithm is based on a logistic model which is trained using A-Train observations. We demonstrate that the overall accuracy of our approach is about 0.9 when the probability of the OT candidates is larger than 0.9. The OT climatology reveals a pattern that follows the climatology of deep convection, as well as shallow convection over the mid-latitude oceans during winter cold air outbreaks. OTs appear most frequently over the Intertropical Convergence Zone (ITCZ), central and southeast North America, tropical and subtropical South America, southeast and south Asia, tropical and subtropical Africa, and northern middle-high latitudes. OT spatial distributions show strong seasonal and diurnal variabilities. Seasonal OT variations shift with large-scale climate systems such as the ITCZ and local monsoonal systems, including the South Asian Monsoon, North American Monsoon and West African Monsoon. OT diurnal variations agree with the known diurnal cycle of convection: Maximum OT occurrences are in the afternoon over most land area and around midnight over ocean; and the OT diurnal cycle is stronger and more varied over land than over ocean. OTs over land are usually colder than over ocean except around 10:30 am. The top 10 coldest OTs from both Terra and Aqua mostly occur over land and at night. This study provides OT climatology for the first time derived from two-decade MODIS data that represents the longest and stable satellite records. [ABSTRACT FROM AUTHOR]

Our global understanding of clouds and aerosols relies on the remote sensing of their optical, microphysical, and macrophysical properties using, in part, scattered solar radiation. These retrievals assume clouds and aerosols form plane-parallel, homogeneous layers and utilize 1D radiative transfer (RT) models, limiting the detail that can be retrieved about the 3D variability of cloud and aerosol fields and inducing biases in the retrieved properties for highly heterogeneous structures such as cumulus clouds and smoke plumes. To overcome these limitations, we introduce and validate an algorithm for retrieving the 3D optical or microphysical properties of atmospheric particles using multi-angle, multi-pixel radiances and a 3D RT model. The retrieval software, which we have made publicly available, is called Atmospheric Tomography with 3D Radiative Transfer (AT3D). It uses an iterative, local optimization technique to solve a generalized least-squares problem and thereby find a best-fitting atmospheric state. The iterative retrieval uses a fast, approximate Jacobian calculation, which we have extended from Levis et al. (2020) to accommodate open as well as periodic horizontal boundary conditions (BC) and an improved treatment of non-black surfaces. We validated the accuracy of the approximate Jacobian calculation for derivatives with respect to both the 3D volume extinction coefficient and the parameters controlling the open horizontal boundary conditions across media with a range of optical depths and single scattering properties and find that it is highly accurate for a majority of cloud and aerosol fields over oceanic surfaces. Relative root-mean-square errors in the approximate Jacobian for 3D volume extinction coefficient in media with cloud-like single scattering properties increase from 2% to 12% as the Maximum Optical Depths (MOD) of the medium increases from 0.2 to 100.0 over surfaces with Lambertian albedos < 0.2. Over surfaces with albedos of 0.7, these errors increase to 20%. Errors in the approximate Jacobian for the optimization of open horizontal boundary conditions exceed 50% unless the plane-parallel media providing the boundary conditions are very optically thin (~0.1). We use the theory of linear inverse RT to provide insight into the physical processes that control the cloud tomography problem and identify its limitations, supported by numerical experiments. We show that the Jacobian matrix becomes increasing ill-posed as the optical size of the medium increases and the forward scattering peak of the phase function decreases. This suggests that tomographic retrievals of clouds will become increasingly difficult as clouds becoming optically thicker. Retrievals of asymptotically thick clouds will likely require other sources of information to be successful. In Part 2 of this study, we examine how the accuracy of the retrieved 3D volume extinction coefficient varies as the optical size of the target medium increases using synthetic data. We do this to explore how the increasing error in the approximate Jacobian and increasingly ill-posed nature of the inversion in the optically thick limit affects the retrieval. We develop a method to improve retrieval accuracy in the optically thick limit. We also assess the accuracy of retrieved optical depths and surface irradiances and compare them to retrievals using 1D radiative transfer. [ABSTRACT FROM AUTHOR]

We present the first observational study on the potential of band-differenced angular signatures for cloud detection. A band-differenced angular signature is the reflectance difference between the blue and near-infrared channels examined as a function of view angle, and it is observed in this study from the Multi-angle Imaging SpectroRadiometer (MISR). The observed band-differenced angular signatures over a variety of scene types qualitatively compare well with previously published theoretical predictions. Examples of cloud detection in polar and desert regions using band-differenced angular signatures from MISR are shown to highlight its contribution to cloud detection in areas where traditional spectral techniques tend to perform poorly. Index Terms--Angular signature, cloud detection, Multi-angle Imaging SpectroRadiometer (MISR), polar.

This paper describes a new type of global, gridded product being created by the Multi-angle Imaging Spectro-Radiometer (MISR) team. The product is a compressed version, or summary, of MISR geophysical data products on a 1[degrees] monthly global grid. Data belonging to each grid cell are summarized by a multivariate histogram. The numbers, sizes, and shapes of the histogram bins vary among cells, and they adapt to the shape of the data in high-dimensional space. Also, bin representatives are means rather than midpoints. These modifications allow data to be summarized parsimoniously and with lower error than is possible using customary, simple, descriptive statistics. The method is demonstrated by compressing test MISR aerosol data, and performance is assessed by comparing computations using compressed data with those using the original. Index Terms--Clustering algorithms, data compression, entropy-constrained vector quantization, Level 3 products, massive datasets.