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K. D. Williams and M. E. Brooks

Abstract

The Met Office unified forecast–climate model is used to compare the properties of simulated climatological cloud regimes with those produced in short-range forecasts initialized from operational analyses. The regimes are defined as principal clusters of joint cloud-top pressure–optical depth histograms. In general, the cloud regime properties are found to be similar at all forecast times, including the climatological mean. This suggests that weaknesses in the representation of fast local processes are responsible for errors in the simulation of the cloud regimes. The increased horizontal resolution of the model used for numerical weather prediction generally has little impact on the cloud regimes, although the simulation of tropical shallow cumulus is improved, while the relative frequency of tropical deep convection and cirrus compare less favorably with observations. Analysis of the initial temperature tendency profiles for each cloud regime indicates that some of the initial temperature tendency, which leads to a systematic bias in the model climatology, is associated with a particular cloud regime.

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C. L. Parsons, J. C. Gerlach, and M. E. Williams

Abstract

Five ground-based total ozone spectrophotometers were intercompared at Wallops Island, Virginia between October 1979 and January 1981. The tests were conducted to evaluate the stability and accuracy of each instrument over an extended time period. Acceptable performance regarding these two characteristics is essential if an instrument is to be useful in field measurements and network monitoring of the atmospheric total ozone content. The Dobson spectrophotometer was used as the standard of comparison for the Brewer grating spectrophotometer, the USSR M-83 ozonometer, the Canterbury filter photometer, and the SenTran filter photometer. The grating instrument was found to be potentially the equal of the Dobson but was subject to unreliable performance by its rather sophisticated electronic components. The filter photometers performed acceptably for short periods but filter aging and eventual degradation rendered both units unusable before the end of the intercomparison. Finally, the M-83 results were found to be in acceptable agreement with the Dobson throughout the period when certain qualifications are invoked. The accuracy of a single M-83 ozone measurement may be low. Averages tend to improve its agreement with the Dobson. Airmass dependencies appear to be appropriately accounted for, but zenith cloudy measurements are too high by ∼30%.

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E. Anyamba, E. Williams, J. Susskind, A. Fraser-Smith, and M. Fullekrug

Abstract

This study determines the relationship between intraseasonal oscillations observed in two independent measures of global lightning activity: a global mean convective index (a proxy for deep convection) derived from the Goddard Television Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) Pathfinder infrared cloud observations, and Schumann resonance magnetic intensity recorded at Arrival Heights, Antarctica. The study was initiated when previous results indicated a possible link between intraseasonal variations in Schumann resonances and variability of sunspot numbers on the timescale of the solar rotation period. The authors used seven years (1989–95) of daily records, though the Schumann resonance record had a number of gaps. Results of cross-spectrum and composite analysis show that intraseasonal oscillations in deep convection modulate the global variations in the Schumann resonance intensity. In the Tropics, the intraseasonal wave in deep convection has a wavenumber-1 structure with the region from 120°W to 60°E having one phase, while the other hemisphere has the opposite phase. The Schumann resonances are enhanced when a maximum in deep convection lies in the former hemisphere that comprises the main lightning-producing regions of South America and Africa. Conversely, Schumann resonances are suppressed when the convection propagates eastward to the Indian Ocean and the western Pacific Ocean. This relationship between the deep convection and Schumann resonances was best defined during the Northern Hemisphere springs of 1990 and 1992 but was less evident in 1993 and 1994.

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G. S. Kent, E. R. Williams, P-H. Wang, M. P. McCormick, and K. M. Skeens

Abstract

Data from the Stratospheric Aerosol and Gas Experiment II (SAGE II) solar occultation satellite instrument have been used to study the properties of tropical cloud over the altitude range 10.5–18.5 km. By virtue of its limb viewing measurement geometry, SAGE II has good vertical resolution and sensitivity to subvisual cloud not detectable by most other satellite instruments. The geographical distribution and temporal variation of the cloud occurrence have been examined over all longitudes on timescales from less than 1 day to that of the El Niño-Southern Oscillation (ENSO) cycle. Significant variations in cloud occurrence are found on each of these scales and have been compared with the underlying surface temperature changes. Maximum cloud occurs over the warm pool region of the Pacific Ocean, with secondary maxima over the South American and Central African landmasses, where the percentage of cloud occurrence in the upper troposphere can exceed 75%. Cloud occurrence at all altitudes within the Tropics, over both land and ocean, increases with the underlying surface temperature at a rate of approximately 13%°C−1. Extrapolated threshold temperatures for the formation of cloud are about 5°C lower than those found from nadir viewing observations. This difference is believed to be a consequence of the averaging process and the inclusion of outliers in the dataset. ENSO cycle changes in cloud occurrence are observed, not only over the Tropics but also over the subtropics, indicating a difference in the meridional Hadley circulation between ENSO warm and cold years. Sunrise–sunset cloud differences indicate that large-scale variations, whose form resembles that of the Hadley and Walker circulations, are present, with a timescale of 1 day or less. The global distribution of upper-tropospheric ice and its positive correlation with surface temperature on all timescales are generally consistent with the behavior of lightning and the global electrical circuit.

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Anthony C. Riddle, Leslie M. Hartten, David A. Carter, Paul E. Johnston, and Christopher R. Williams

Abstract

One limiting factor in atmospheric radar observations is the inability to distinguish the often weak atmospheric signals from fluctuations of the noise. This study presents a minimum threshold of usability, SNRmin, for signal-to-noise ratios obtained from wind profiling radars. The basic form arises from theoretical considerations of radar noise; the final form includes empirical modifications based on radar observations. While SNRmin was originally developed using data from the 50-MHz profiler at Poker Flat, Alaska, it works well with data collected from a wide range of locations, frequencies, and parameter settings. It provides an objective criterion to accept or reject individual spectra, can be quickly applied to a large quantity of data, and has a false-alarm rate of approximately 0.1%. While this threshold’s form depends on the methods used to calculate SNR and spectral moments, variations of the threshold could be developed for use with data processed by other methods.

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Skylar S. Williams, Kiel L. Ortega, Travis M. Smith, and Anthony E. Reinhart

Abstract

The Multi-Year Reanalysis of Remotely Sensed Storms (MYRORSS) data set blends radar data from the WSR-88D network and Near-Storm Environmental (NSE) model analyses using the Multi-Radar Multi-Sensor (MRMS) framework. The MYRORSS data set uses the WSR-88D archive starting in 1998 through 2011, processing all valid single-radar volumes to produce a seamless three-dimensional reflectivity volume over the entire contiguous United States with an approximate 5-min update frequency. The three-dimensional grid has an approximate 1-km by 1-km horizontal dimension and is on a stretched vertical grid that extends to 20 km MSL with a maximal vertical spacing of 1 km. Several reflectivity-derived, severe storm related products are also produced, which leverage the ability to merge the MRMS and NSE data. Two Doppler velocity-derived azimuthal shear layer maximum products are produced at a higher horizontal resolution of approximately 0.5-km by 0.5-km. The initial period of record for the data set is 1998-2011.

The data set underwent intensive manual quality control to ensure that all available and valid data were included while excluding highly problematic radar volumes that were a negligible percentage of the overall data set, but which caused large data errors in some cases. This data set has applications towards radar-based climatologies, post-event analysis, machine learning applications, model verification, and warning improvements. Details of the manual quality control process are included and examples of some of these applications are presented.

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Arianna M. Varuolo-Clarke, Jason E. Smerdon, A. Park Williams, and Richard Seager

Abstract

Southeastern South America (SESA; encompassing Paraguay, southern Brazil, Uruguay, and northern Argentina) experienced a 27% increase in austral summer precipitation from 1902 to 2019, one of the largest observed trends in seasonal precipitation globally. Previous research identifies Atlantic multidecadal variability and anthropogenic forcing from stratospheric ozone depletion and greenhouse gas emissions as key factors contributing to the positive precipitation trends in SESA. We analyze multimodel ensemble simulations from phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP) and find that not only do Earth system models simulate positive SESA precipitation trends that are much weaker over the historical interval, but some models persistently simulate negative SESA precipitation trends under historical forcings. Similarly, 16-member ensembles from two atmospheric models forced with observed historical sea surface temperatures never simulate precipitation trends that even reach the lower bound of the observed trend’s range of uncertainty. Moreover, while future twenty-first-century projections from CMIP6 yield positive ensemble mean precipitation trends over SESA that grow with increasing greenhouse gas emissions, the mean forced response never exceeds the observed historical trend. Preindustrial control runs from CMIP6 indicate that some models do occasionally simulate centennial-scale trends in SESA that fall within the observational range, but most models do not. Results point to significant uncertainties in the attribution of anthropogenically forced influences on the observed increases in precipitation over SESA while also suggesting that internal decadal-to-centennial variability of unknown origin and not present in state-of-the-art models may have also played a large role in generating the twentieth-to-twenty-first-century SESA precipitation trend.

Open access
Xiaoyan Wei, Henk M. Schuttelaars, Megan E. Williams, Jennifer M. Brown, Peter D. Thorne, and Laurent O. Amoudry

Abstract

Asymmetric tidal turbulence (ATT) strongly influences estuarine health and functioning. However, its impact on the three-dimensional estuarine dynamics and the feedback of water motion and salinity distribution on ATT remain poorly understood, especially for short estuaries (estuarine length ≪ tidal wavelength). This study systematically investigates the abovementioned interactions in a short estuary for the first time, considering periodically weakly stratified conditions. This is done by developing a three-dimensional semi-analytical model (combining perturbation method with finite element method) that allows a dissection of the contributions of different processes to ATT, estuarine circulation, and salt transport. The generation of ATT is dominated by (i) strain-induced periodic stratification and (ii) asymmetric bottom-shear-generated turbulence, and their contributions to ATT are different both in amplitude and phase. The magnitude of the residual circulation related to ATT and the eddy viscosity–shear covariance (ESCO) is about half of that of the gravitational circulation (GC) and shows a “reversed” pattern as compared to GC. ATT generated by strain-induced periodic stratification contributes to an ESCO circulation with a spatial structure similar to GC. This circulation reduces the longitudinal salinity gradients and thus weakens GC. Contrastingly, the ESCO circulation due to asymmetric bottom-shear-generated turbulence shows patterns opposite to GC and acts to enhance GC. Concerning the salinity dynamics at steady state, GC and tidal pumping are equally important to salt import, whereas ESCO circulation yields a significant seaward salt transport. These findings highlight the importance of identifying the sources of ATT to understand its impact on estuarine circulation and salt distribution.

Open access
E. Drigeard, E. Fontaine, W. Wobrock, A. Schwarzenböck, C. Duroure, E. R. Williams, B. Russell, A. Protat, J. Delanoë, F. Cazenave, and M. Gosset

Abstract

This study addresses clouds with significant ice water content (IWC) in the stratiform regions downwind of the convective cores of African squall lines in the framework of the French–Indian satellite Megha-Tropiques project, observed in August 2010 next to Niamey (13.5°N, 2°E) in the southwestern part of Niger. The objectives included comparing the IWC–Z reflectivity relationship for precipitation radars in deep stratiform anvils, collocating reflectivity observed from ground radar with the calculated reflectivity from in situ microphysics for all aircraft locations inside the radar range, and interpreting the role of large ice crystals in the reflectivity of centimeter radars through analysis of their microphysical characteristics as ice crystals larger than 5 mm frequently occurred. It was found that, in the range of 20–30 dBZ, IWC and C-band reflectivity are not really correlated. Cloud regions with high IWC caused by important crystal number concentrations can lead to the same reflectivity factor as cloud regions with low IWC formed by a few millimeter-sized ice crystals.

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Richard J. Keane, Keith D. Williams, Alison J. Stirling, Gill M. Martin, Cathryn E. Birch, and Douglas J. Parker

Abstract

The Met Office Unified Model (MetUM) is known to produce too little total rainfall on average over India during the summer monsoon period, when assessed for multiyear climate simulations. We investigate how quickly this dry bias appears by assessing the 5-day operational forecasts produced by the MetUM for six different years. It is found that the MetUM shows a drying tendency across the five days of the forecasts, for all of the six years (which correspond to two different model versions). We then calculate each term in the moisture budget, for a region covering southern and central India, where the dry bias is worst in both climate simulations and weather forecasts. By looking at how the terms vary with forecast lead time, we are able to identify biases in the weather forecasts that have been previously identified in climate simulations using the same model, and we attempt to quantify how these biases lead to a reduction in total rainfall. In particular, an anticyclonic bias develops to the east of India throughout the forecast, and it has a complex effect on the moisture available over the peninsula, and a reduction in the wind speed into the west of the region appears after about 3 days, indicative of upstream effects. In addition, we find a new bias that the air advected from the west is too dry from very early in the forecast, and this has an important effect on the rainfall.

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