2012 CIRA Seminars
Atmospheric aerosols have important influences on climate through their interaction with clouds and radiation. The extent of aerosols' climate impact depends greatly on the aerosol concentration, size and composition. A major source of aerosol particles in the atmosphere is nucleation, where low-volatility vapors combine to form ~1 nm diameter particles. These particles must grow to diameters around 100 nm in order to significantly affect clouds and radiation, yet many newly formed particles will not survive long enough to grow to these climate-relevant sizes.
We still largely lack a process-based understanding of how mesoscale terrain-related forcings and feedbacks act to shape the response of mesoscale climate to large-scale forcings. However, such an understanding must be central in any attempt to assess the regional impacts of climate change. I will describe how I am working to build such an understanding with examples from two current research projects.
The pineapple express (PE) phenomenon is responsible for producing extreme winter precipitation events on the west coast of the US and Canada. We study regional climate models' ability to reproduce these events by defining a quantity which captures the spatial extent and intensity of PE events. We use bivariate extreme value theory to model the tail dependence of this quantity as seen in observational data and the Weather Research and Forecasting (WRF) regional climate model driven by reanalysis, and we find tail dependence between the two.
The eddy-driven jet is located in the midlatitudes, bounded on one side by the pole and often bounded on the opposite side by a strong Hadley-driven jet. This work explores how the eddy-driven jet and its variability persist within these limits. It is demonstrated that as the jet is located at higher latitudes, the leading mode of variability of the jet changes from a meridional shift to a pulse, while the persistence decreases. Looking equatorward, a similar change in eddy-driven jet variability is observed when the jet moves equatorward toward strong subtropical winds.
The response of clouds to a warming climate represents one of the largest sources of disagreement among model projections of future climate change. My research seeks to quantify, understand, and ultimately constrain cloud feedbacks using theory, observations, and global climate models. Here I will present a novel technique to compute and partition cloud feedbacks. At the heart of the technique is the cloud radiative kernel, which quantifies the impact on top-of-atmosphere radiative fluxes of changes in cloud fraction segregated by cloud top pressure and optical depth.
I will present new ideas explaining two central questions about the response of the tropical Pacific climate to increasing greenhouse gases:
For over three decades, the climate science community has sought to clarify the most important forcing and feedback mechanisms related to global climate change. Significant uncertainties remain in our projections of climate change, in particular related to the myriad of feedback mechanisms in the earth system. These can operate on a variety of time scales. The "fast" feedback mechanisms, such as those related to clouds and water vapor, operate on time scales of days to several years.
Late last year, nearly 25 students, research staff, and faculty from the department went to the Indian Ocean to participate in DYNAMO (Dynamics of the Madden-Julian Oscillation), a field campaign to study mechanisms for the initiation of the MJO. At this informal colloquium, students and staff will share their experiences in the field along with preliminary science findings with an aim to furthering research collaborations across the department and CIRA.
Paul van Delst will be visiting CIRA from the Joint Center for Satellite Data Assimilation (JCSDA). His talk will focus on development of the CRTM. The CRTM is a widely used radiative transfer model that is suited to create synthetic satellite imagery from numerical model output. In addition to generating synthetic GOES imagery, the CRTM can be used to create top of the atmosphere radiances for a wide range of geostationary and polar orbiting platforms.
2011 CIRA Seminars
Although the Madden-Julian Oscillation (MJO) has now been studied for 40 years, terms like "MJO initiation" remain mysterious. Does "MJO initiation" mean the onset of deep convection in the Indian Ocean? Or can MJO initiation occur anywhere? Does initiation necessarily imply the onset of convection? Or could it be tied to winds? Or moisture? Can initiation really be pinned down to a particular date? Or is it a more gradual process?
This talk discusses how Arctic Oscillation/Northern Annular Mode (AO/NAM) variability is related to surface forcing from the tropics and extratropics. In observations and GCMs, a lot of wintertime tropospheric AO/NAM variability is tied to the amount of wave activity that propagates into the polar stratosphere. I will show that much of this wave activity variability is simply related to how well planetary wave anomalies align with the climatological stationary wave field - a linear interference effect.
The University of Wyoming instrumented the Wyoming King Air 200 in 1976. The aircraft instrumentation documents both the atmospheric environment and the response of the aircraft to this environment. This state-of-the-art cloud physics aircraft has been operated in support of various weather modification and in-flight icing projects. These projects involved the search for super cooled liquid water. From this body of data and experience a new concept of in-flight icing was developed.
IPCC reports have extensively used the concepts of radiative forcing and climate feedbacks to help understand expected climate response. In recent years new methods have been developed to estimate forcings from standard climate model diagnostics. These forcing estimates include "rapid adjustments" to the troposphere. The talk will discuss the idea of rapid adjustment, its physical interpretation, and its implication for our understanding of climate feedbacks and future climate change.
Recent 50-year trends of surface temperature and precipitation around the globe have been strongly influenced by the warming trend of the tropical oceans. The current generation of coupled climate models with prescribed changes in radiative forcing generally fail to capture the spatial structure of these trends. On the other hand, even uncoupled atmospheric models without prescribed changes in radiative forcing, but with the observed sea surface temperature (SST) changes prescribed only in the tropics, are more successful in this regard.
Some Earth System Models include crop-specific algorithms for the simulation of interactive planting, growth, and harvesting, to better represent effects of human management on land surface properties with minimal need for new data sets. We evaluate such a model (the CESM1) with two present-day coupled atmosphereland (CAM4/CLM4CN) simulations: A CONTROL that represents crops as unmanaged grasses, and a CROP that includes special algorithms for midlatitude corn, wheat, and soybean carbon allocation and plant phenology.
An extensively instrumented NOAA WP-3D research aircraft made airborne measurements of the gaseous and aerosol composition of air over the Deepwater Horizon (DWH) oil spill that occurred in April-July of 2010 in the Gulf of Mexico. A narrow plume of hydrocarbons was observed downwind from DWH that is attributed to the prompt evaporation of fresh oil on the sea surface.
In this talk a satellite data analysis is presented to explore the thermodynamic evolution of tropical and subtropical atmospheres prior and subsequent to moist convection, motivated to offer an observational testbed for convective adjustment central to the quasi-equilibrium hypothesis. Tropical Rainfall Measuring Mission (TRMM) and Aqua satellite measurements are projected onto a composite temporal sequence over an hourly to daily timescale by exploiting the temporal gap between the local satellite overpasses that changes from one day to another.
Improved predictability of severe storm behavior is expected to emerge from a better understanding of storm dependence on microphysical characteristics, which to date remains relatively unknown. Dual-polarization Doppler radars and disdrometers are used to examine the microphysical characteristics of supercell thunderstorms in the Great Plains during the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) conducted in 2009 and 2010.
2010 CIRA Seminars
Weather extremes are reviewed in the context of behavior in present-day climate compared to climate change projections for the future. A relatively small shift in the average produces a very large change in extremes - more extreme heat and less extreme cold, and more record high maximum temperatures and fewer record low minimum temperatures. Global warming by itself doesn't cause extreme conditions, but it makes naturally occurring events more extreme.
The EUCAARI project aims to advance our understanding of climate and air quality by integrating laboratory studies, both short and long term field investigations, satellite data, and modeling at regional and global scales. Airborne observations were used in an intensive study to provide a local and regional scale overview of the general composition trends of atmospheric aerosol over northern Europe. Time dependent measurements obtained by aerosol mass spectrometry enable the chemical transformation time scales of aerosol material to be explored.
This study of hurricanes passing through the Caribbean in the period 1950-2005 reveals that seasons with more intense hurricanes occur with the onset of Pacific La Nina events and when Atlantic SSTs west of Africa are above normal. Composites of NCEP reanalysis fields w.r.t. Caribbean hurricanes reveal development of an anomalous equatorial Atlantic zonal overturning circulation (upper easterly / lower westerly) that shifts toward the Caribbean coincident with a westward spread of the cold tongue in the east Pacific.
The planet's water cycle is thought to be 'accelerating' under the scenario of global warming with projected increases in the occurrence of heavier precipitation among other expected responses. The physical basis for these claims is not firmly advanced and this lack of firm basis casts a large shadow on validity of projected changes in precipitation that appear in recent climate assessment studies.
The temporal-spatial distribution of thin moist layers in the midtroposphere over the tropical eastern Pacific is studied by data analyses of radiosonde soundings and downscaling numerical experiments with a regional model. The downscaling experiments with global objective analyses are completed for 2005-06, September and December of 1999-2004, and March of 2000-04. The vertical distribution of thin moist layers has three local maxima at 5, 10, and 16 km, where bimodality of the frequency distribution of water vapor is evident.
In spite of the unprecedented quantity and quality of meteorological data and models, there is still no consensus about either the atmospheres or the models elementary statistical properties as functions of scale in either time or in space.
It is becoming apparent that even modest amounts of precipitation falling from low clouds can have a profound effect upon their coverage, thickness and organization. It is therefore imperative that we understand the frequency and strength of precipitation produced by low clouds, together with the factors controlling it.