Kenneth P. Bowman

The TRMM Multisatellite Precipitation Analysis (TMPA): Quasi-Global, Multiyear, Combined-Sensor Precipitation Estimates at Fine Scales

Abstract The Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA) provides a calibration-based sequential scheme for combining precipitation estimates from multiple satellites, as well as gauge analyses where feasible, at fine scales (0.25° × 0.25° and 3 hourly). TMPA is available both after and in real time, based on calibration by the TRMM Combined Instrument and TRMM Microwave Imager precipitation products, respectively. Only the after-real-time product incorporates gauge data at the present. The dataset covers the latitude band 50°N–S for the period from 1998 to the delayed present. Early validation results are as follows: the TMPA provides reasonable performance at monthly scales, although it is shown to have precipitation rate–dependent low bias due to lack of sensitivity to low precipitation rates over ocean in one of the input products [based on Advanced Microwave Sounding Unit-B (AMSU-B)]. At finer scales the TMPA is successful at approximately reproducing the s...

Large‐scale isentropic mixing properties of the Antarctic polar vortex from analyzed winds

Winds derived from analyzed geopotential height fields are used to study quasi-horizontal mixing by the large-scale flow in the lower stratosphere during austral spring. This is the period when the Antarctic ozone hole appears and disappears. Trajectories are computed for large ensembles of particles initially inside and outside the main polar vortex. Mixing and transport are diagnosed through estimates of finite time Lyapunov exponents and Lagrangian dispersion statistics of the tracer trajectories. At 450 K and above prior to the vortex breakdown: Lyapunov exponents are a factor of 2 smaller inside the vortex than outside; diffusion coefficients are an order of magnitude smaller inside than outside the vortex; and the trajectories reveal little exchange of air across the vortex boundary. At lower levels (425 and 400 K) mixing is greater, and there is substantial exchange of air across the vortex boundary. In some years there are large wave events that expel small amounts of vortex air into the mid-latitudes. At the end of the spring season during the vortex breakdown there is rapid mixing of air across the vortex boundary, which is evident in the mixing diagnostics and the tracer trajectories.

Global Patterns of the Quasi-biennial Oscillation in Total Ozone

Abstract Nine years of total ozone measurements from the Total Ozone Mapping Spectrometer (TOMS) on Nimbus 7 are used to study the global structure of the quasi-biennial oscillation (QBO) in total ozone. Interannual variability of total ozone near the equator (10°S to 10°N) is dominated by the QBO. The equatorial ozone anomalies are independent of season and are well correlated (r > 0.8) with the equatorial zonal wind. In both hemispheres midlatitude anomalies are two to three times larger in winter than in summer. Global patterns of the ozone QBO are identified by computing lagged correlations between the zonal-mean equatorial ozone and ozone elsewhere on the globe. Correlations between equatorial and extratropical ozone are weak during the summer season (r ∼ 0) and large and negative during the winter (r < − 0.8 in the Southern Hemisphere and r − 0.6 in the Northern Hemisphere). There are nodes or phase shifts in the correlation patterns at ±10° latitude, at 60°S, and at 50°N. Large negative correlation...

Comparison of TRMM Precipitation Retrievals with Rain Gauge Data from Ocean Buoys

Abstract Four years of precipitation retrievals from the Tropical Rainfall Measuring Mission (TRMM) satellite are compared with data from 25 surface rain gauges on the National Oceanic and Atmospheric Administration/Pacific Marine Environment Laboratory (NOAA/PMEL) Tropical Atmosphere–Ocean Array/Triangle Trans-Ocean Buoy Network TAO/TRITON buoy array in the tropical Pacific. The buoy gauges have a significant advantage over island-based gauges for this purpose because they represent open-ocean conditions and are not affected by island orography or surface heating. Because precipitation is correlated with itself in both space and time, comparisons between the two data sources can be improved by properly averaging in space and/or time. When comparing gauges with individual satellite overpasses, the optimal averaging time for the gauge (centered on the satellite overpass time) depends on the area over which the satellite data are averaged. For 1° × 1° areas there is a broad maximum in the correlation for ga...

Error Reduction and Convergence in Climate Prediction

Abstract Although climate models have steadily improved their ability to reproduce the observed climate, over the years there has been little change to the wide range of sensitivities exhibited by different models to a doubling of atmospheric CO2 concentrations. Stochastic optimization is used to mimic how six independent climate model development efforts might use the same atmospheric general circulation model, set of observational constraints, and model skill criteria to choose different settings for parameters thought to be important sources of uncertainty related to clouds and convection. Each optimized model improved its skill with respect to observations selected as targets of model development. Of particular note were the improvements seen in reproducing observed extreme rainfall rates over the tropical Pacific, which was not specifically targeted during the optimization process. As compared to the default model sensitivity of 2.4°C, the ensemble of optimized model configurations had a larger and n...

The Mean-Meridional Transport Circulation of the Troposphere in an Idealized GCM

Abstract Large numbers of particle trajectories are used to characterize the mean-meridional transport circulation of an idealized general circulation model (GCM). The GCM has a uniform land surface, no topography or land–sea contrasts, and no seasonal cycle. The trajectories are analyzed using both Lagrangian-mean statistics and an approach based on the Greens function of the tracer transport equation. It is shown that the distribution of particle trajectories provides an estimate of the ensemble-mean Greens function for the inviscid transport equation. Lagrangian means have a number of problems that render their interpretation difficult, including boundary effects and selection of the appropriate averaging period. The Greens function, on the other hand, provides a quantitative description of the transport circulation that is easy to visualize and interpret. The results demonstrate that within the idealized model the atmosphere can be divided into three parts: the Southern Hemisphere extratropics, the...

Global Trends in Total Ozone

Satellite ozone data from the Total Ozone Mapping Spectrometer from 1979 through 1986 show that recent decreases of total ozone have not been confined to the Antarctic spring season (the Antarctic ozone hole), but are global in extent. The losses are about twice the estimated uncertainty in the satellite data. The decreases are largest in middle and high latitudes and occur in all seasons of the year. The decreases for this 8-year period are comparable in magnitude to the increases observed during the 1960s. Southern Hemisphere values from 1986 are generally greater than those from 1985.

A Comparison of Gamma and Lognormal Distributions for Characterizing Satellite Rain Rates from the Tropical Rainfall Measuring Mission

This study investigates the spatial characteristics of nonzero rain rates to develop a probability density function (PDF) model of precipitation using rainfall data from the Tropical Rainfall Measuring Mission (TRMM) satellite. The minimum x 2 method is used to find a good estimator for the rain-rate distribution between the gamma and lognormal distributions, which are popularly used in the simulation of the rain-rate PDF. Results are sensitive to the choice of dynamic range, but both the gamma and lognormal distributions match well with the PDF of rainfall data. Comparison with sample means shows that the parametric mean from the lognormal distribution overestimates the sample mean, whereas the gamma distribution underestimates it. These differences are caused by the inflated tail in the lognormal distribution and the small shape parameter in the gamma distribution. If shape constraint is given, the difference between the sample mean and the parametric mean from the fitted gamma distribution decreases significantly, although the resulting x 2 values slightly increase. Of interest is that a consistent regional preference between two test functions is found. The gamma fits outperform the lognormal fits in wet regions, whereas the lognormal fits are better than the gamma fits for dry regions. Results can be improved with a specific model assumption depending on mean rain rates, but the results presented in this study can be easily applied to develop the rainfall retrieval algorithm and to find the proper statistics in the rainfall data.

The Stratosphere–Troposphere Analyses of Regional Transport 2008 Experiment

The Stratosphere–Troposphere Analyses of Regional Transport 2008 (START08) experiment investigated a number of important processes in the extratropical upper troposphere and lower stratosphere (UTLS) using the National Science Foundation (NSF)–NCAR Gulfstream V (GV) research aircraft. The main objective was to examine the chemical structure of the extratropical UTLS in relation to dynamical processes spanning a range of scales. The campaign was conducted during April–June 2008 from Broomfield, Colorado. A total of 18 research flights sampled an extensive geographical region of North America (25°–65°N, 80°–120°W) and a wide range of meteorological conditions. The airborne in situ instruments measured a comprehensive suite of chemical constituents and microphysical variables from the boundary layer to the lower stratosphere, with flights specifically designed to target key transport processes in the extratropical UTLS. The flights successfully investigated stratosphere–troposphere exchange (STE) processes, ...

Comparison of Freezing-Level Altitudes from the NCEP Reanalysis with TRMM Precipitation Radar Brightband Data

Abstract A global climatology of the altitude of the freezing level (0°C isotherm) is computed using 20 yr of 6-hourly output from the National Centers for Environmental Prediction (NCEP) reanalysis system. Mean statistics discussed include monthly means and climatological monthly means. Variance statistics include the standard deviation of the 6-hourly values with the month and the standard deviation of the monthly means. In the Tropics, freezing levels are highest (∼5000 m) and both intramonth and interannual variability is lowest. Freezing levels are lower and variability is higher in the subtropics and midlatitudes. In 1998 there are unusually high freezing levels in the eastern Pacific Ocean relative to the 20-yr climatology, consistent with elevated sea surface temperatures associated with the 1997–98 El Nino. Freezing levels return to near-climatological values during the last half of 1998. The individual monthly means for 1998 and the 20-yr climatology are compared with monthly means of the altitu...