The Quarterly Journal of the Royal Meteorological Society

Newman, Stuart M.; Knuteson, Robert. O.; Zhou, Daniel K.; Larar, Allen. M.; Smith, William L.; Taylor, Jonathan P.

doi: 10.1002/qj.382pmid: N/A

The European AQUA Thermodynamic Experiment (EAQUATE) brought together collocated hyperspectral measurements from the Atmospheric Infrared Sounder (AIRS) on NASA's Aqua satellite, as well as three airborne interferometers operating in the troposphere and stratosphere. Simultaneous observations of temperature, water vapour and trace gases have been used to constrain the atmospheric state, for validation of radiometric accuracy and radiative transfer modelling. The close agreement of observations with model spectra, in the 800–1200 cm−1 region, indicates that the accuracy of modelling AIRS spectra is sufficient for beneficial use of the data in operational weather forecasting. The variability of atmospheric water vapour was found to be a fundamental constraint in the modelling of the 1400–1600 cm−1 region, emphasising the requirement for a high density of observations in future validation campaigns. Copyright © Royal Meteorological Society and Crown Copyright, 2009.

Um, Junshik; McFarquhar, Greg M.

doi: 10.1002/qj.378pmid: N/A

During the 2006 Tropical Warm Pool International Cloud Experiment (TWP‐ICE) sponsored by the Atmospheric Radiation Measurement programme, the Scaled Composites Proteus aircraft executed spiral profiles and flew horizontal legs through aging anvils, fresh anvils, and cirrus of unknown origin in the vicinity of Darwin, Australia. Pristine ice crystals including both plates and bullet rosettes, their aggregates, and unclassifiable ice crystals were observed by a Cloud Particle Imager. The widths of observed plates ranged between 80 µm and 200 µm. When a fresh dissipating anvil was observed on 2 February, aggregates of plates contributed 46.2% of the total area of measured ice crystals with maximum dimensions greater than 200 µm, while it was only 7.2% and 1.0% for 27 and 29 January, respectively, when aged cirrus was sampled. Because aggregates of plates have been observed to make large contributions to projected ice crystal area near convection during TWP‐ICE and other projects, their single‐scattering properties that have not been previously examined are investigated here.

Cetrone, Jasmine; Houze, Robert A.

doi: 10.1002/qj.389pmid: N/A

The anvil clouds of tropical mesoscale convective systems (MCSs) in West Africa, the Maritime Continent and the Bay of Bengal have been examined with TRMM and CloudSat satellite data and ARM ground‐based radar observations. The anvils spreading out from the precipitating cores of MCSs are subdivided into thick, medium and thin portions. The thick portions of anvils show distinct differences from one climatological regime to another. In their upper portions, the thick anvils of West Africa MCSs have a broad, flat histogram of reflectivity, and a maximum of reflectivity in their lower portions. The reflectivity histogram of the Bay of Bengal thick anvils has a sharply peaked distribution of reflectivity at all altitudes with modal values that increase monotonically downward. The reflectivity histogram of the Maritime Continent thick anvils is intermediate between that of the West Africa and Bay of Bengal anvils, consistent with the fact this region comprises a mix of land and ocean influences. It is suggested that the difference between the statistics of the continental and oceanic anvils is related to some combination of two factors: (1) the West African anvils tend to be closely tied to the convective regions of MCSs while the oceanic anvils are more likely to be extending outward from large stratiform precipitation areas of MCSs, and (2) the West African MCSs result from greater buoyancy, so that the convective cells are more likely to produce graupel particles and detrain them into anvils. Copyright © 2009 Royal Meteorological Society

Boos, William R.; Emanuel, Kerry A.

doi: 10.1002/qj.388pmid: N/A

The annual intensification of the Somali jet, which accompanies the onset of the Indian summer monsoon, is rapid compared to the evolution of the seasonal insolation forcing. Using observationally‐based data sets, the dynamic and thermodynamic changes accompanying the onset of this jet are presented. The abrupt component of jet onset is shown to occur over ocean about 1000km east of the East African highlands, and is accompanied by increases in both deep convection and baroclinic flow over the off‐equatorial Arabian Sea. These abrupt changes are well separated from the core of the cross‐equatorial jet, which is located over land adjacent to the East African highlands.

Zhao, Mei; Hendon, Harry H.

doi: 10.1002/qj.370pmid: N/A

Predictive skill of the Indian Ocean Dipole (IOD) using the Bureau of Meteorology's POAMA seasonal forecast model is assessed in light of the model's ability to represent the coupled nature of the IOD. The analysis is based on a ten‐member ensemble of nine‐month forecasts that are initialized from observed ocean and atmosphere states once per month for the period 1982–2006. Some aspects of the IOD are well represented by the POAMA forecast model, including the seasonal phase locking of the development of the IOD but the IOD's horizontal structure is distorted and its amplitude weakens with increasing forecast lead time, while at the same time the IOD becomes spuriously overdependent on ENSO. These deficiencies in the representation of the IOD are attributed to model error (including horizontal resolution and development of biases in the mean state), which affects depiction of important coupled dynamics for the IOD as well as the role of remote forcing of the IOD by ENSO (which itself exhibits distortion of its spatial structure attributed to biases in the simulated mean state).

Jian, Jun; Webster, Peter J.; Hoyos, Carlos D.

doi: 10.1002/qj.384pmid: N/A

Reliable water supply from the Ganges and Brahmaputra is of critical importance to the sustainability of the agricultural societies of India and Bangladesh. But, the flow in both basins is highly variable on time‐scales ranging from days to years, creating challenges for the optimization of agricultural practices, water resource management and disaster mitigation. The following questions are addressed. Is intraseasonal monsoon variability related to the subseasonal variability of river flow? Do variations in the large‐scale tropical sea‐surface temperature (SST) located both regionally and remotely promote seasonal and interannual variations of river discharge? And, if these relationships do exist, are they determinable with sufficient lead‐times to allow useful predictions for user communities in South Asia? We examine these questions using 50 years of daily river discharge data for both rivers calculated at the points where they enter Bangladesh, and with SST data in the Indo‐Pacific region. We also examine the question of determining the impact of man‐made dams, diversions and barrages on the data record, especially that of the Ganges. A comparison of discharge prior to 1974 (the time of the construction of the largest barrage) shows no statistical difference that cannot be explained by basin‐wide rainfall distributions. Changes that do occur are restricted to the dry‐season months.

Coughlin, K.; Bellone, E.; Laepple, T.; Jewson, S.; Nzerem, K.

doi: 10.1002/qj.367pmid: N/A

The interannual prediction of the number of hurricanes to strike the US coastline is of vital interest to those who live on the coast, those who insure property along the coast and policy makers who make decisions about infrastructure and mitigation measures. Here it is shown that these predictions can be made more accurate when information about the total number of Atlantic Basin hurricanes is considered. The proportion of Atlantic hurricanes that make landfall in the USA is a measure of the relationship between landfalling and basin hurricane numbers. In order to use basin numbers to help make predictions of landfalling numbers, we must first investigate the nature of this relationship. Here we run a number of statistical tests and find that there is a significant change in the relative proportion of Atlantic hurricanes that hit the US coastline between the first and second halves of the 20th century. This difference is ostensibly due to the lack of basin observations in the earlier period. However, after 1948, the proportion of Atlantic hurricanes that hit the US coast shows no evidence of change. By assuming that the proportion is constant, we are able to then use information about the total number of hurricanes in the Atlantic Basin and make more accurate estimates of landfalling hurricane activity. Copyright © 2009 Royal Meteorological Society

Smart, D. J.; Browning, K. A.

doi: 10.1002/qj.399pmid: N/A

A numerical simulation of an intense, narrow cold‐frontal rain band (NCFR) is presented. This front was associated with localized wind damage as it moved eastwards across the United Kingdom on 24 September 2007. The model used was the operational and research Weather Research and Forecasting‐‐Advanced Research (WRF‐‐ARW) mesoscale model, initialized with Global Forecast System (GFS) operational model output data. The simulation produced a front strongly resembling that seen in radar imagery and conforming to previous studies and conceptual models of the ana‐cold front. In the simulation, a strong surface horizontal wind‐speed maximum is located along the southern flank of a misocyclone; this is shown to be typical of a number that form along the front. Using trajectory analysis, descending cold air parcels, which originate in the rear inflow jet and accelerate within the circulation of the misocyclone, are identified as the origin of the strongest winds. In addition to the localized strong straight‐line winds, circumstantial evidence is presented for the possible occurrence of weak tornadogenesis. The results are discussed in relation to recent studies of NCFRs, squall lines and misocyclones. Copyright © 2009 Royal Meteorological Society

Thomsen, Gerald L.; Reeder, Michael J.; Smith, Roger K.

doi: 10.1002/qj.387pmid: N/A

The effect of the diurnal cycle on the evolution of cold fronts in the Australian subtropics is investigated in two high‐resolution numerical simulations. The simulations are made using the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5) with the initial and boundary conditions taken from the operational analyses of the European Centre for Medium‐range Weather Forecasts. These simulations are compared with the observations of two cold fronts taken during the Central Australian Fronts Experiment in 1991. The simulations show a number of features that have been suspected, but never confirmed or quantified: (i) although the wind field in the boundary layer is frontogenetic, daytime turbulent mixing is strongly frontolytic, which accounts for the weakening and deceleration of the fronts during the late morning and afternoon when convective mixing in the boundary layer is most vigorous; (ii) when the mixing subsides in the early evening, the low‐level winds increase along with the deformation and convergence, leading to a strengthening and acceleration of the fronts; (iii) bore‐like disturbances are generated during the early hours of the morning as the nocturnal inversion strengthens. These bores propagate ahead of the front, developing a series of large‐amplitude waves at their leading edge. Copyright © 2009 Royal Meteorological Society

Flamant, C.; Lavaysse, C.; Todd, M. C.; Chaboureau, J.‐P.; Pelon, J.

doi: 10.1002/qj.376pmid: N/A

The structure of the Saharan air layer over Niger and Benin during a major springtime dust event from the Bodélé region and Sudan is investigated using airborne lidar and dropsonde measurements. Aircraft operations were conducted on 13 and 14 June 2006, within the framework of the African Monsoon Multidisciplinary Analysis Special Observing Period. Complementary ground‐based and satellite observations, as well as European Centre for Medium‐range Weather Forecasts analyses are used to investigate the regional aspects of emission, transport and deposition in the period from 9 to 14 June 2006, to provide a framework for the interpretation of the airborne measurements. The study details the transport patterns of dust from eastern Saharan sources towards the southwest in the springtime, and highlights the role of the intertropical discontinuity and the Darfur mountains in injecting the aerosols from Bodélé and Sudan, respectively, over the monsoon flow, and in the African easterly jet (AEJ) region. It also illustrates the impact of the daily variability of the emissions in the source regions on the dust load advected westward by the AEJ and observed over Benin. The impact of a mesoscale convective system (MCS) on the dust load and vertical distribution observed over Benin and southern Niger was also investigated, nearly 12 hours after its passage over Benin. The only discernible impact on the dust distribution is observed to be associated with widespread subsidence in the wake of the MCS, over northern Benin and Niger. Wet scavenging related to convective or stratiform rain could not be observed, as processed air masses were replaced by fresh dust transported in the upper Saharan air layer by the AEJ. Over southern Benin, the dust distribution appeared to be mostly controlled by processes affecting the planetary boundary layer upstream, i.e. over Nigeria or Chad. Because springtime dust from remote eastern sources (such as observed in this case) is mainly transported into the AEJ, it could impact on the radiation budget in the AEJ region, thereby possibly modifying the West African weather at the synoptic scale. Copyright © 2009 Royal Meteorological Society