Interactions of the water cycle with climate and weather

Whereas in the past, meteorology and climatology were separate fields, be it only because of disparate time (and length scales as well), it appears today that the two fields are strongly coupled, not only as the climate gives the boundaries for investigating the weather, but also because localized events can influence the larger climatological scales. The specific items on which ESSL scientists focused in this year are related to the role of aerosols in climate and weather, to the coupling of eco-systems, biochemistry and climate, to climate change, climate variability and extreme weather such as hurricanes, to interactions of the water cycle with climate and weather, to the impacts of climate and weather on society and ecosystems and finally to megacities and the effects of urbanization; the latter priority is a highlight for NCAR and concerns the international multi-agency field campaign that took place in Mexico city and combined interactive modeling as well. The laboratory highlights are related to the role of aerosols, to the regional carbon cycle, to a numerical simulation of turbulence, to landfall of hurricanes, to the global and regional water cycles and to polar climate.

Global and regional water cycle [Highlight] - TIIMES
Impact of Amazonian landcover change on isoprene emissions - TIIMES
BEACHON program objectives and plans - TIIMES

Global and regional water cycle

The NCAR Water Across Scales Program has as its main goal to improve the water cycle in climate models. In order to achieve this goal, studies are being conducted to understand how processes are controlling the water cycle globally, regionally, and locally. In addition, we are diagnosing the behavior of the water cycle in current climate models in order to focus on areas for improvement. In particular, the focus has been on understanding and diagnosing the diurnal cycle of precipitation in terms of frequency, intensity, timing, and phase across scales.

Convection Episodes over North Africa: Arlene Laing analyzed the propagation characteristics and diurnal cycle of deep convective clouds over Africa using five years of digital infrared images.  Reduced dimension techniques were used to determine span, duration, and phase speeds of convective cloud episodes.  Coherent episodes span an average distance of about 1000 km and last 25 hours, which is greater than those of episodes over the contiguous U.S. and East Asia, although the phase speeds of 10 - 20 ms-1 are similar to the other continents. Approximately five episodes occur across the zonal domain per day.  Episodes propagate westward and occur with moderate vertical shear of the zonal wind, which itself varies with the migration of the African Easterly Jet.  The initiation of convection in the lee of high terrain and westward propagating features are evident in the diurnal cycle (Fig. 3).  Cold cloud frequency minimum occurs two to four hours before local noon across the longitudinal domain.  A few episodes undergo stages of decay and regeneration through as many as five diurnal cycles.  (a) Cold cloud streaks and (b) frequency of Tbb < 233K for 16 -30 June 1999 over Tropical North Africa. Black dashed lines highlight the axes of maximum frequency and thin dotted lines denote areas where the frequency is less than 5%.  Time zones are noted as hours relative to Greenwich Mean Time.  The diurnal cycle is repeated. High resolution figure

Aiguo Dai has examined global precipitation in 18 coupled climate models and compared this to Tropical Rainfall Measuring Mission (TRMM) satellite observations of precipitation. He found that climate models produce precipitation too frequently, of too-light intensity, and too early. These results point to the need for better parameterizations of convection in climate models, including the coupling between the land surface, boundary layer, and deep convection. A question related to this study is the often strong coupling of precipitation in climate models to soil moisture. Recent studies have shown that observations do not fully support this behavior. David Lawrence is leading a research effort to examine this behavior in the Community Climate System Model (CCSM) and other climate models in an attempt to understand why climate models have this bias. Results from Fei Chen and Peggy LeMone using International H₂O Project (IHOP) data have shown that latent and sensible heat fluxes are related to the upstream vegetation, and that precipitation can be influenced by soil moisture under certain conditions. Future research will focus on identifying these conditions in order to properly represent them in climate models. Cloud resolving simulations by Mitch Moncrieff and Changhai Liu have served as the basis for the development of an improved convective parameterization that takes into account mesoscale downdrafts. James Hack and Julie Caron are using these high resolution simulations to examine the convective parameterization in Community Atmosphere Model (CAM) in Single Column mode.

Another important research thread is the role of organized propagating convection in producing the observed diurnal cycle of precipitation at locations around the world. Ongoing studies include the central U.S., Mexico, Africa, Brazil, China, and Australia. All of these regions, as well as other regions located downstream of significant orography, show evidence of organized propagating convection that strongly influences the phase and amplitude of the diurnal cycle. A key result this year discovered by John Tuttle and Christopher Davis is the key role that the nocturnal low level jet plays in setting up the proper environmental conditions (shear, Convective Available Potential Energy (CAPE), lifting, convergence) that helps to determine favored “corridors” within which the organized convection propogates.

Future work in the water cycle will continue to focus on understanding precipitation processes over continental regions; diagnosing the simulated precipitation processes in global and regional global models; and improving parameterizations so that the climatology of precipitation can be more accurately simulated in climate models. These improvements are key to answering the question whether future climate warming will produce increasing floods and droughts, as well as the regional impact of climate change on the hydrological cycle.

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Impact of Amazonian landcover change on isoprene emissions

Isoprene emission map estimated using the NCAR MEGAN model (left panel) compared to measurements using NCAR aircraft measurements (right panel). The gray line on the left panel depicts the flight track. Numbers 1-6 on left panel indicate different landcover types: 1: mixed forest/plantation, 2: primary tropical forest, 3: soybean plantations, 4: mixed forest/plantation, 5: water, 6: urban. High resolution figure

Biogenic emission measurements from tropical ecosystems are scarce due to logistical challenges and are usually confined to a specific accessible location. The understanding of regional emission distributions in Amazonia has been greatly extended with the FY06 analysis of selected flights from the 2004 Chemistry and Production of Smoke (CAPOS) airborne experiment conducted in collaboration with Paulo Artaxo of the University Sao Paulo and Robert Yokelson of the University of Montana. The flight path covered various landscapes.

Areas indicated by numbers in the left panel above are based on different landcover information: 1: mixed forest/plantation, 2: primary tropical forest, 3: soybean plantations, 4: mixed forest/plantation, 5: water, 6: urban. The left panel above depicts the NCAR Model of the Exchange of Gases between the Atmosphere and Nature (MEGAN) model distribution of isoprene emission factors at standard conditions (30 C and 1000 PAR [Photosynthetically Active Radiation]). On top of the emission map, the racetrack pattern is mapped in gray. The right panel above shows the surface flux based on airborne variance measurements. The Volatile Organic Compound (VOC) variability was calculated for a 60-second running average. The qualitative agreement between model and measurements is reasonable. Both indicate that landcover change can greatly perturb isoprene emissions which will likely result in significant changes in oxidants, particles and other important atmospheric constituents. MEGAN isoprene emissions are compared with measurements in the scatterplot figure which shows a tight linear correlation (R2=0.99) between the model and measurements for isoprene which demonstrates that MEGAN is able to represent the variability associated with landcover change in this region. However, the MEGAN isoprene emission estimates are ~40% lower (slope: 0.6) than our observations. This is within the model uncertainty, but indicates that the MEGAN estimates may be a lower, rather than an upper, limit at least for this Amazonian region.

	Scatterplot comparing isoprene fluxes estimated with the NCAR MEGAN model and from NCAR aircraft measurements. High resolution figure

These observations are in stark contrast to most modeling studies, which assume the opposite (i.e., up to 50% reduced isoprene emissions compared to MEGAN). Isoprene emissions incorporated in these large scale chemical-transport models are therefore most likely too low and yet still greatly overestimate PBL isoprene concentrations.

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BEACHON program objectives and plans

Early warning of potential impacts from Earth system changes are increasingly viewed as important to human health, the global economy, and ecosystems. The Biosphere-atmosphere Exchange of Aerosols within Cloud, Carbon and Hydrologic cycles, including Organics & Nitrogen (BEACHON) program will improve the predictive skill associated with Earth system behavior over a time scale of months to a decade by explicitly coupling water, energy and biogeochemical cycles in a multi-scale modeling framework. Improved predictive skill will be obtained through coordinated modeling, observational and experimental studies focused on interfacial exchanges of water, energy and biogeochemical constituents at multiple temporal and spatial scales.

The figure depicts a schematic of biogenic aerosol regulation and linkage of the carbon and water cycles with the overall goal to characterize and understand interactions between biogeochemical and water cycles across scales and their response to climate and land-use change.  The individual processes that will be investigated by the BEACHON program are: A. Improve parameterizations (numbered on the figure) including (1) CO2 exchange, (2) VOC and bio-particle emissions, (3) CCN and IN, (4) Cloud properties, (5) Impact of clouds on environment, (6) Soil moisture, and (7) Transpiration and energy flux; B. Simulate influence of climate and landuse change on coupled feedback system; C. Assess model with field observations. High resolution figure

The BEACHON program will quantify coupled water, carbon, trace gas and aerosol cycles across scales for selected natural and managed landscapes and will devise consistent representations of the coupled system at canopy-, meso-, regional- and global scales that are applicable from weather through climate timescales. This will be accomplished by examining the dominant processes controlling water and biogeochemical cycle coupling, the importance of antecedent conditions, the response of a coupled system to change in one or more components, and how climate variability and land management decisions interact to affect water and biogeochemical cycles.

BEACHON FY07 objectives include:

• Science planning and implementation. The BEACHON science plan and detailed implementation plan will be completed in FY07.
• Site development. BEACHON core sites will include a long-term research site near Boulder, Colorado and a multi-site region that will be selected through the science planning and implementation objective. The Colorado research site will be used for education and outreach, instrument testing, and monthly to decadal investigations of water, energy and biogeochemical cycles. FY07 tasks include locating the sites and initiating infrastructure development.
• Model development and analysis. Modeling activities in FY07 will focus on land surface models and enhancements to the Large-Eddy Simulation (LES) and Weather Research and Forecast (WRF) model that will facilitate coupled cycle representations.
• Instrument development. FY07 activities will include the development of systems for characterizing regional carbon cycling, canopy scale turbulence, and trace gases and aerosols.
• CHATS field study. The Canopy Horizontal Array Turbulence Study (CHATS) will make spatial measurements of the velocity and scalar turbulence fields in a uniformly vegetated canopy using arrays of sonic anemometer/thermometers augmented with fast response water vapor and carbon dioxide sensors and scanning lidar and sodar. With this spatial information, the three-dimensional fields of velocity and scalar fluctuations will be studied to quantify turbulence transport processes and coherent structures throughout the canopy layer. BEACHON will enhance CHATS with water and biogeochemical surface exchange measurements and modeling activities that will take advantage of the CHATS framework.

Process studies. BEACHON efforts will include observational and numerical investigations of the individual processes controlling coupled systems on canopy and regional scales. FY07 studies will investigate trace gas and aerosol emission and uptake, carbon and nitrogen interactions, and cloud and precipitation processes.

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