Julio T. Bacmeister

Increased stratospheric ozone depletion due to mountain-induced atmospheric waves

Chemical reactions on polar stratospheric cloud (PSC) particles are responsible for the production of reactive chlorine species (chlorine ‘activation’) which cause ozone destruction. Gas-phase deactivation of these chlorine species can take several weeks in the Arctic winter stratosphere, so that ozone destruction can be sustained even in air parcels that encounter PSCs only intermittently,. Chlorine activation during a PSC encounter proceeds much faster at low temperatures when cloud particle surface area and heterogeneous reaction rates are higher. Although mountain-induced atmospheric gravity waves are known to cause local reductions in stratospheric temperature of as much as 10–15 K (refs 5-9), and are often associated with mesoscale PSCs, their effect on chlorine activation and ozone depletion has not been considered. Here we describe aircraft observations of mountain-wave-induced mesoscale PSCs in which temperatures were 12 K lower than expected synoptically. Model calculations show that despite their localized nature, these PSCs can cause almost complete conversion of inactive chlorine species to ozone-destroying forms in air flowing through the clouds. Using a global mountain-wave model, we identify regions where mountain waves can develop, and show that they can cause frequent chlorine activation of air in the Arctic stratosphere. Such mesoscale processes offer a possible explanation for the underprediction of reactive chlorine concentrations and ozone depletion rates calculated by three-dimensional models of the Arctic stratosphere.

Subseasonal Variability Associated with Asian Summer Monsoon Simulated by 14 IPCC AR4 Coupled GCMs

Abstract This study evaluates the subseasonal variability associated with the Asian summer monsoon in 14 coupled general circulation models (GCMs) participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). Eight years of each model’s twentieth-century climate simulation are analyzed. The authors focus on the three major components of Asian summer monsoon: the Indian summer monsoon (ISM), the western North Pacific summer monsoon (WNPSM), and the East Asian summer monsoon (EASM), together with the two dominant subseasonal modes: the eastward- and northward-propagating boreal summer intraseasonal oscillation (BSIO) and the westward-propagating 12–24-day mode. The results show that current state-of-the-art GCMs still have difficulties and display a wide range of skill in simulating the subseasonal variability associated with Asian summer monsoon. During boreal summer (May–October), most of the models produce reasonable seasonal-mean precipitation over the ISM region,...

Multiple equilibria in a single‐column model of the tropical atmosphere

We used the fact that the humidity at that level is zero.Equation (8) gives us one constraint that must be satisfied in or-der for the dry solution to occur: the ventilation of the ABL by thesubsidence has to compensate the evaporation. On the other hand,Equation (7) shows that the warming by surface sensible heat fluxand subsidence has to be compensated by the radiative cooling. Thetwo equations can be rewritten as a single condition for the exis-tence of the non-convective equilibrium:−hRi+Hs

North American Monsoon and Convectively Coupled Equatorial Waves Simulated by IPCC AR4 Coupled GCMs

Abstract This study evaluates the fidelity of North American monsoon and associated intraseasonal variability in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) coupled general circulation models (CGCMs). Twenty years of monthly precipitation data from each of the 22 models’ twentieth-century climate simulations, together with the available daily precipitation data from 12 of them, are analyzed and compared with Global Precipitation Climatology Project (GPCP) monthly and daily precipitation. The authors focus on the seasonal cycle and horizontal pattern of monsoon precipitation in conjunction with the two dominant convectively coupled equatorial wave modes: the eastward-propagating Madden–Julian oscillation (MJO) and the westward-propagating easterly waves. The results show that the IPCC AR4 CGCMs have significant problems and display a wide range of skill in simulating the North American monsoon and associated intraseasonal variability. Most of the models reproduce the...

Scale dependence of tracer microstructure: PDFs, intermittency and the dissipation scale

Thestatisticsoftracervariabilityonsmallscales (< 200 km) is investigated using high resolution aircraft measurements of ozone in the northern winter middle/high latitudes. Conditioning based on potential temperature is used to isolate the statistics of lamentation on isentropic surfacesfromspuriousvariabilityduetocross-isentropicmo- tion of the platform. The distribution of isentropic incre- ments r in the tracer eld across a horizontal scale r have non-Gaussian tails and are consistent with stretched exponential functions of the form P(r)exp(-ajrj p ), where a is a scale-dependent parameter and the exponent p increases overall with r. A scale break in the second order structure function suggests a dissipation scale rd 20 km during northern winter 91-92, but the scale break is closer to 100 km during northern winter 88-89. Possible reasons for this are discussed.

A view of Hurricane Katrina with early 21st century technology

Observing, modeling, and forecasting systems have been undergoing rapid development in the past two to three decades. For example, Atlantic hurricanes are closely monitored by the U.S. National Oceanic and Atmospheric Administrations (NOAA) National Weather Service through a significantly improved upper-air and ground-based observational network supplemented by aircraft, ship, and ocean buoy data. Given initial conditions and lateral boundary conditions provided by larger-scale model analyses, regional models have been widely utilized to predict hurricane track and intensity. Nowadays, satellite observations are playing an increasingly important role in providing global estimations of precipitation, radiative fluxes, clouds, and winds, with unprecedented temporal and spatial coverage. Global atmospheric models and global operational analyses are moving toward providing forecasts and products at resolutions ranging from 0.1° to 0.5° (10–50 kilometers).There is evidence that improved hurricane structure and track forecasts could result in part from such increases in model resolution.