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UW Atmos Sci

And the rain tossed about us
in the garden of the world
But a flame arrives to guide us,
cast in gold
between the anvils of the storms


Clouds: their structure, microphysics, and variability


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              John Constable, Cloud Study (1822)                  Fractal low clouds over the NE Atlantic

                                                                                                                                     observed from space with MODIS (2004)

                                                                                                                                                 [image courtesy of NASA]




Clouds have fascinated us for thousands of years. Behind their natural beauty lies a tremendous complexity that continues to elude complete theoretical understanding. The fundamental role that clouds play in the Earth’s climate system means that it is important that we search for better ways to understand and describe the processes than control their formation, maintenance, and dissipation. From submicron-sized cloud condensation nuclei activated in seconds, to cloud systems spanning thousands of kilometers and lasting several days, cloud processes span an enormous range of temporal and spatial scales. Furthermore, the ability to better understand the behavior of clouds requires novel combinations of knowledge drawn from the forefront of physics, chemistry and mathematics.     


Clouds over the Eastern Pacific Ocean observed using MODIS. The underlying colors show the sea surface temperature and arrows show satellite winds observed with Quikscat. Organization in these clouds occurs on scales from tens of meters to thousands of kilometers.


The large range of scales makes the observation and measurement of clouds a particularly challenging problem. No one measurement system can provide a complete observational picture of cloud processes. The major goal of my research is to use observational datasets in novel ways to examine the structural and behavioral properties of cloud systems and the underlying meteorology controlling them. Simple model frameworks are used to assist in the distillation of observational datasets and to develop a quantitative basis for understanding. Increasingly, it is through the combination of observational datasets from more than one platform that the greatest strides in our understanding are taken.






Specific projects being undertaken at present are:

1. Synthesis of observational datasets and modeling to improve our understanding and parameterizations of boundary layer clouds over the southeast Pacific Ocean (SEP). The sparsely observed SEP is covered by the world’s largest subtropical stratocumulus regime. Its northern edge extends over the equatorial eastern Pacific cold tongue, where it can have important feedbacks with ENSO variability. Many coupled climate models poorly simulate subtropical stratocumulus clouds. This can contribute to biases in regional top-of atmosphere radiation balance, eastern Pacific sea-surface temperature and its seasonal cycle, precipitation distribution (e.g.  ‘double ITCZ’) and surface winds, and ENSO. While simulations are improving, parameterization improvement is limited in part by gaps in our understanding of key physical processes such as drizzle, entrainment, and cloud heterogeneity. Our modeling strategy and observational synthesis is designed to optimally use SEP data being currently gathered to address these gaps. The VOCALS Program and VOCALS Regional Experiment (REx) will provide important new field, extended, spaceborne, and numerical model data to better understand the coupled climate system of the SEP, and the role that the marine stratocumulus cloud systems (MSCS) play in this coupling.


This work is coordinated by the VOCALS Science Working Group (C. Roberto Mechoso - chair, Chris Bretherton, Barry Huebert, Bob Weller, Robert Wood) and with VOCALS scientists in the US and abroad. We expect that many opportunities for graduate students to become involved in this work will be available at the participating institutions. VOCALS is primarily supported by grants from NOAA and NSF.


2. Exploring the links between cloud structure and meteorology over the oceans. Data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and other satellites are being used to investigate cloud structural properties (cloud optical thickness and cloud top height) of cloud ensembles over the warm regions of the tropical oceans. Many questions concerning these clouds remain unanswered: what are the relative importances of the local SST and horizontal gradients of SST in determining the cloud ensemble properties in the tropics? Is the observed cancellation between shortwave and longwave cloud forcing (SWCF and LWCF) over the tropical warm pool fortuitous or the result of physical feedbacks? Over what temporal and spatial scales does this cancellation occur? How do tropical clouds respond to climate change? What are the interactions between deep convection and the structure of the tropical tropopause? How will low clouds change under a changed climate? We are addressing these questions using multiplatform observational datasets. This work is being carried out with Dennis Hartmann, Terry Kubar, and Jian Yuan at UW, and is funded by NASA.


Mesoscale cellularity in marine stratocumulus clouds. Open and closed mesoscale cellular convection are the dominant forms of organization of low clouds over the remote eastern oceans, but an understanding of the physics of these mesoscale systems continues to be elusive. This MODIS image (approximately 800 km across) shows the sharp transitions that occur between the closed and open cells, which observations suggest may be driven by precipitation.


3. The Marine Stratocumulus Cloud System (MSCS) is an interconnected ensemble of marine boundary layer clouds in which both radiation and precipitation work together to provide the key forcings on the marine boundary layer (MBL). Sometimes, most commonly when these systems move within roughly 500 km of coastlines, the MSCS can become perturbed by ingesting cloud condensation nuclei from continental sources. This can alter their structure and dynamics in ways that are barely understood. The hypotheses that precipitation can be an important component of the mesoscale and turbulent dynamics of these systems are supported by the few studies that have well-documented the MSCS boundary layer. My aim is to assist in the pursuit of understanding the MSCS system. The importance of the MSCS over the global oceans is rarely disputed, but remoteness is at the heart of the sampling problem. Radars need to be designed to sample these clouds, and these radars need first-class sampling platforms, on ships, on aircraft, and in space. More effort needs to be spent in learning how to better coordinate and retrieve information from our satellite missions to sample these clouds, and in devising ways to incorporate this data into our numerical models. GPS provides a wonderful, as yet largely untapped, opportunity for sampling the MBL thermodynamic structure, and should be invested in. Scatterometry is our only true spaceborne dataset to sample lower atmosphere dynamics, and should not be neglected. CloudSat and CALIPSO are providing an unprecedented dataset on the properties and organization of the precipitation and cloud structure in MSCS, and the phalanx of colocated instruments on the other A-train satellites is providing the essential context for these groundbreaking measurements. The VOCALS Program and VOCALS Regional Experiment (REx) will provide important new datasets for the understanding of the MSCS.


CloudSat's sensitive radar is able to detect precipitation from shallow marine clouds below 1.5 km. The data shown above are from September 9th 2006 over the cool waters of the Southeast Pacific Ocean where stratocumulus clouds organize into the largest sheet of such clouds on the planet. Prior to the launch of CloudSat, the precipitation (typically in the form of drizzle) falling from these cloud systems was undetectable from space. It is now becoming clear that precipitation has a profound impact upon the structure, dynamics and coverage of marine stratocumulus. CloudSat will provide important insights into the role of precipitation in stratocumulus cloud systems over the remote ocean.




4. Precipitation error characterization over the global oceans. Microwave estimates of precipitation from satellites such as SSM/I, TRMM, and AQUA form the basis for our understanding of oceanic precipitation processes. These estimates are indirect and are the focus of considerable efforts to improve them. Our approach is to construct novel methods to compare the microwave estimates with those from spaceborne radar (TRMM Precipitation Radar and, in future, the Global Precipitation Mission dual-wavelength radar) and ground based radar (Kwajelein) using regime-dependence of precipitation structure as a physical basis for compositing. This work is conducted in collaboration with Sandra Yuter, Daniel Horn ( North Carolina State ), and John Stout ( George Mason University ) and is funded by NASA.