Showing papers in "Quarterly Journal of the Royal Meteorological Society in 2018"

Validation of the CHIRPS satellite rainfall estimates over eastern Africa

Abstract: Long and temporally consistent rainfall time series are essential in climate analyses and applications. Rainfall data from station observations are inadequate over many parts of the world due to sparse or non-existent observation networks, or limited reporting of gauge observations. As a result, satellite rainfall estimates have been used as an alternative or as a supplement to station observations. However, many satellite-based rainfall products with long time series suffer from coarse spatial and temporal resolutions and inhomogeneities caused by variations in satellite inputs. There are some satellite rainfall products with reasonably consistent time series, but they are often limited to specific geographic areas. The Climate Hazards Group Infrared Precipitation (CHIRP) and CHIRP combined with station observations (CHIRPS) are recently produced satellite-based rainfall products with relatively high spatial and temporal resolutions and quasi-global coverage. In this study, CHIRP and CHIRPS were evaluated over East Africa at daily, dekadal (10-day) and monthly time scales. The evaluation was done by comparing the satellite products with rain gauge data from about 1200 stations. The CHIRP and CHIRPS products were also compared with two similar operation satellite rainfall products: the African Rainfall Climatology version 2 (ARC2) and the Tropical Applications of Meteorology using Satellite data (TAMSAT). The results show that both CHIRP and CHIRPS products are significantly better than ARC2 with higher skill and low or no bias. These products were also found to be slightly better than the latest version of the TAMSAT product at dekadal and monthly time scales, while TAMSAT performed better at daily time scale. The performance of the different satellite products exhibits high spatial variability with weak performances over coastal and mountainous regions.

On the representation error in data assimilation

Abstract: Representation, representativity, representativeness error, forward interpolation error, forward model error, observation operator error, aggregation error and sampling error are all terms used to refer to components of observation error in the context of data assimilation. This paper is an attempt to consolidate the terminology that has been used in the earth sciences literature and was suggested at a European Space Agency workshop held in Reading in April 2014. We review the state-of-the-art, and through examples, motivate the terminology. In addition to a theoretical framework, examples from application areas of satellite data assimilation, ocean reanalysis and atmospheric chemistry data assimilation are provided. Diagnosing representation error statistics as well as their use in state-of-the-art data assimilation systems is discussed within a consistent framework.

Survey of data assimilation methods for convective-scale numerical weather prediction at operational centres

Abstract: Data assimilation methods for convective-scale numerical weather prediction at operational centres are surveyed in this paper. The operational methods include variational methods (3D-Var and 4D-Var), ensemble methods (LETKF) and hybrids between variational and ensemble methods (3DEnVar and 4DEnVar). At several of the operational centres, other assimilation algorithms, like latent heat nudging, are additionally applied to improve the model initial state, with emphasis on convective scales. It is demonstrated that the quality of forecasts based on initial data from convective-scale data assimilation is significantly better than the quality of forecasts from simple downscaling of larger-scale initial data. The duration of positive impact depends however on the weather situation, the size of the computational domain and the data that are assimilated. It is furthermore shown that more-advanced methods applied at convective scales provide improvements compared to simpler methods. This motivates continued research and development in convective-scale data assimilation. Challenges in research and development for improvements of convective-scale data assimilation are also reviewed and discussed in this paper. The difficulty of handling the wide range of spatial and temporal scales makes development of multi-scale assimilation methods and space-time covariance localization techniques important. Improved utilization of observations is also important. In order to extract more information from existing observing systems of convective-scale phenomena, for example weather radar data and satellite image data, it is necessary to provide improved statistical descriptions of the observation errors associated with these observations.

All-sky satellite data assimilation at operational weather forecasting centres

Abstract: This article reviews developments towards assimilating cloud and precipitation-affected satellite radiances at operational forecasting centres. Satellite data assimilation is moving beyond the ‘clear-sky’ approach that discards any observations affected by cloud. Some centres already assimilate cloud and precipitation-affected radiances operationally and the most popular approach is known as ‘all-sky’, which assimilates all observations directly as radiances, whether they are clear, cloudy or precipitating, using models (both for radiative transfer and forecasting) that are capable of simulating cloud and precipitation with sufficient accuracy. Other frameworks are being tried including the assimilation of humidity retrieved from cloudy observations using Bayesian techniques. Although the all-sky technique is now proven for assimilation of microwave radiances, it has yet to be demonstrated operationally for infrared radiances, though several centres are getting close. Assimilating frequently-available all-sky infrared observations from geostationary satellites could give particular benefit for short-range forecasting. More generally, assimilating cloud and precipitation-affected satellite observations improves forecasts into the medium-range globally, and it can also improve the analysis and shorter-range forecast of otherwise poorly-observed weather phenomena as diverse as tropical cyclones and wintertime low cloud.

Predicting weather forecast uncertainty with machine learning

Abstract: Weather forecasts are inherently uncertain. Therefore, for many applications forecasts are only considered valuable if an uncertainty estimate can be assigned to them. Currently, the best method to ...

The evolution, seasonality and impacts of western disturbances

Abstract: Western disturbances (WDs) are upper-level synoptic-scale systems embedded in the subtropical westerly jet stream (STWJ), often associated with extreme rainfall events in north India and Pakistan during boreal winter. Here, a tracking algorithm is applied to the upper-tropospheric vorticity field in 37 years of ERA-Interim reanalysis data, giving a catalogue of over 3000 events. These events are analysed in a composite framework: the vertical structure is explored across a large number of dynamic and thermodynamic fields, revealing a significant northwestward tilt with height, strong ascent ahead of the centre which sits above the maximum surface precipitation and a warm-over-cold, dry-over-moist structure among other signatures of strong baroclinicity. Evolution of the structures of cloud cover and vertical wind speed are investigated as the composite WD passes across northern India. Cloud cover in particular is found to be particularly sensitive to the presence of the Himalayan foothills, with a significant maximum at 300 hPa approximately one day after the WD reaches peak intensity. k-means clustering is used to classify WDs both according to dynamical structure and precipitation footprint, and the relationship between the two sets is explored. Finally, the statistical relationship between the STWJ position and WDs on interannual time scales is explored, showing that WD frequency in north India is highly sensitive to the jet location over Eurasia. Years with a greater number of WDs feature a STWJ shifted to the south, a pattern that is substantially more coherent and reaches as far west as North America during boreal winter. This suggests that it may be possible to predict the statistics of western disturbance events on seasonal time scales if suitable indicators of jet position can also be predicted.

An Overview of the TROPICS NASA Earth Venture Mission

Abstract: The Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (TROPICS) mission was selected by NASA as part of the Earth Venture-Instrument (EVI-3) program. The overarching goal for TROPICS is to provide nearly all-weather observations of 3D temperature and humidity, as well as cloud ice and precipitation horizontal structure, at high temporal resolution to conduct high-value science investigations of tropical cyclones. TROPICS will provide rapid-refresh microwave measurements (median refresh rate better than 60 min for the baseline mission) which can be used to observe the thermodynamics of the troposphere and precipitation structure for storm systems at the mesoscale and synoptic scale over the entire storm life cycle. TROPICS comprises six Cube-Sats in three low-Earth orbital planes. Each CubeSat will host a high-performance radiometer to provide temperature profiles using seven channels near the 118.75 GHz oxygen absorption line, water vapour profiles using three channels near the 183 GHz water vapour absorption line, imagery in a single channel near 90 GHz for precipitation measurements (when combined with higher-resolution water vapour channels), and a single channel near 205 GHz which is more sensitive to precipitation-sized ice particles. This observing system offers an unprecedented combination of horizontal and temporal resolution to measure environmental and inner-core conditions for tropical cyclones on a nearly global scale and is a major leap forward in the temporal resolution of several key parameters needed for assimilation into advanced data assimilation systems capable of utilizing rapid-update radiance or retrieval data.Launch readiness is currently projected for late 2019.

The Global Precipitation Measurement (GPM) mission's scientific achievements and societal contributions: reviewing four years of advanced rain and snow observations.

Abstract: Precipitation represents a life-critical energy and hydrologic exchange between the Earth’s atmosphere and its surface. As such, knowledge of where, when, and how much rain and snow falls is essential for scientific research and societal applications. Building on the 17-year success of the Tropical Rainfall Measurement Mission (TRMM), the Global Precipitation Measurement (GPM) Core Observatory (GPM-CO) is the first U.S. National Aeronautical and Space Administration (NASA) satellite mission specifically designed with sensors to observe the structure and intensities of both rain and falling snow. The GPM-CO has proved to be a worthy successor to TRMM, extending and improving high-quality active and passive microwave observations across all times of day. The GPM-CO launched in early 2014, is a joint mission between NASA and the Japanese Aerospace Exploration Agency (JAXA), with sensors that include the NASA-provided GPM Microwave Imager and the JAXA-provided Dual-frequency Precipitation Radar. These sensors were devised with high accuracy standards enabling them to be used as a reference for inter-calibrating a constellation of partner satellite data. These intercalibrated partner satellite retrievals are used with infrared data to produce merged precipitation estimates at temporal scales of 30 minutes and spatial scales of 0.1° x 0.1°. Precipitation estimates from the GPM-CO and partner constellation satellites, provided in near real time and later reprocessed with all ancillary data, are an indispensable source of precipitation data for operational and scientific users. Advances have been made using GPM data, primarily in improving sensor calibration, retrieval algorithms, and ground validation measurements, and used to further our understanding of the characteristics of liquid and frozen precipitation and the science of water and hydrological cycles for climate/weather forecasting. These advances have extended to societal benefits related to water resources, operational numerical weather prediction, hurricane monitoring, prediction, and disaster response, extremes, and disease.

The influence of the stratospheric state on North Atlantic weather regimes

Abstract: Stratosphere-troposphere coupling is often viewed from the perspective of the annular modes and their dynamics. Despite the obvious benefits of this approach, recent work has emphasised the greater tropospheric sensitivity to stratospheric variability in the Atlantic basin than in the Pacific basin. In this study, a new approach to understanding stratosphere-troposphere coupling is proposed, with a focus on the influence of the stratospheric state on North Atlantic weather regimes (during extended winter, November to March). The influence of the strength of the lower stratospheric vortex on four commonly used tropospheric weather regimes is quantified. The negative phase of the North Atlantic Oscillation is most sensitive to the stratospheric state, occurring on 33% of days following weak vortex conditions but on only 5% of days following strong vortex conditions. An opposite and slightly weaker sensitivity is found for the positive phase of the North Atlantic Oscillation and the Atlantic Ridge regime. For the North Atlantic Oscillation regimes, stratospheric conditions change both the probability of remaining in each regime and the probability of transitioning to that regime from others. A logistic regression model is developed to further quantify the sensitivity of tropospheric weather regimes to the lower stratospheric state. The logistic regression model predicts an increase of 40-60% in the probability of transition to the negative phase of the North Atlantic Oscillation for a one standard deviation reduction in the strength of the stratospheric vortex. Similarly it predicts a 10-30% increase in the probability of transition to the positive phase of the North Atlantic Oscillation for a one standard deviation increase in the strength of the stratospheric vortex. The stratosphere-troposphere coupling in the European Centre for Medium Range Weather Forecasts, Integrated Forecasting System model is found to be consistent with the re- analysis data by fitting the same logistic regression model.

Atmospheric boundary‐layer characteristics from ceilometer measurements. Part 1: A new method to track mixed layer height and classify clouds

Abstract: The use of Automatic Lidars and Ceilometers (ALC) is increasingly extended beyond monitoring cloud base height to the study of atmospheric boundary layer (ABL) dynamics. Therefore, long-term sensor networks observations require robust algorithms to automatically detect the mixed layer height (ZML). Here, a novel automatic algorithm CABAM (Characterise the Atmospheric Boundary layer based on ALC Measurements) is presented . CABAM is the first, non-proprietary mixed layer height algorithm specifically designed for the commonly deployed Vaisala CL31 ceilometer. The method: tracks ZML, takes into account precipitation, classifies the ABL based on cloud cover and cloud type, and determines the relation between ZML and cloud base height. CABAM relies solely on ALC measurements. Results perform well against independent reference (AMDAR: Aircraft Meteorological Data Relay) measurements and supervised ZML detection. AMDAR derived temperature inversion heights allow ZML evaluation throughout the day. Very good agreement is found in the afternoon when the mixed layer height extends over the full ABL. However, during night or the morning transition the temperature inversion is more likely associated with the top of the residual layer. From comparison with SYNOP reports, the ABL classification scheme generally correctly distinguishes between convective and stratiform boundary layer clouds, with slightly better performance during daytime. Applied to six years of ALC observations in central London, Kotthaus and Grimmond (2018) demonstrate CABAM results are valuable to characterise the urban boundary layer over London, UK, where clouds of various types are frequent.

Examining the role of unusually warm Indo‐Pacific sea‐surface temperatures in recent African droughts

Abstract: Southern Africa (SA) and Eastern Africa (EA) experienced a sequence of severe droughts in December-January-February (SA DJF) 2015-16, October-November-December (EA OND) 2016 and March-April-May, 2017 (EA MAM). This sequence contributed to severe food insecurity. While climate variability in these regions is very complex, the goal of this study is to analyze the potential role played by unusually warm Indo-Pacific SS, where unusual is defined as a 1-in-6 year event. We use observed sea surface temperatures (SST) and satellite-gauge rainfall observations, a 20-member ensemble of Community Atmospheric Model version 5.1 simulations (CAM5), and a 40-member ensemble of climate change simulations from the Community Earth Systems Model version 1 (CESM1) Large Ensemble Community Project (LENS) to explore climate conditions associated with warm events identified based on eastern and western Pacific sea surface temperatures (SST). Our analysis suggests that strong El Ninos may be followed by warm western Pacific SST conditions, which can lead to creating conditions conducive for successive and potentially predictable droughts in SA DJF, EA OND and EA MAM. We show that different regions of warm SST appear related to recent droughts – SA DJF: Nino 3.4, EA OND: Western Equatorial Pacific (WEP), and EA MAM: Western North Pacific (WNP). For DJF and MAM, respectively, the CAM5 model driven with observed SST and the same model driven within a climate change experiment indicate that warmer El Ninos and WNP events produce more intense atmospheric responses, potentially associated with more severe droughts. OND climate seems to be strongly influenced by the Indian Ocean Dipole, which corresponds to some WEP events. We suggest the extreme SST events responsible for 2015-17 droughts are likely to reoccur, thus humanitarian agencies should consider multiyear drought and substantial food insecurity in SA and EA.

How far in advance can we predict changes in large-scale flow leading to severe cold conditions over Europe?

Abstract: Correspondence Laura Ferranti, European Centre for Medium-Range Weather Forecasts (ECMWF), Shinfield Park, RG2 9AX, Reading, UK. Email: laura.ferranti@ecmwf.int The potential of early warning for severe cold conditions is explored using the Subseasonal to Seasonal (S2S) Prediction research project data archive. We explore the use of a two-dimensional phase space based on the leading empirical orthogonal functions (EOFs) of mid-tropospheric flow computed over the Euro-Atlantic region in order to study the time evolution of flow patterns associated with high-impact temperature anomalies. We find that the phase space is an effective tool for monitoring predictions of regime transitions at medium and extended ranges. We show that a number of S2S systems have some skill in the prediction of cold spells over Europe, even beyond the medium range. In particular, the ECMWF (European Centre for Medium-Range Weather Forecasts) model represents well the observed preferential transition paths. We reveal that the impact of the Madden–Julian Oscillation (MJO) on the predictive skill of large-scale flow over Europe is asymmetric. The impact of the MJO on the Brier skill scores and reliability is significantly positive for predictions of the negative phase of the North Atlantic Oscillation (NAO): beyond week one, forecasts with the MJO in their initial state are significantly more reliable than forecasts with no MJO in their initial conditions. In contrast, the predictive skill for positive NAO shows little sensitivity to the MJO.

Dependence of near-surface similarity scaling on the anisotropy of atmospheric turbulence.

Abstract: Turbulence data from the CASES-99 field experiment, over comparatively horizontally homogeneous and flat terrain, are separated based on the anisotropy of the Reynolds stress tensor (into isotropic, two-component axisymmetric and one-component turbulence) and flux-variance similarity scaling relations are tested. Results illustrate that different states of anisotropy correspond to different similarity relations, especially under unstable stratification. Experimental data with close to isotropic turbulence match similarity relationships well. On the other hand, very anisotropic turbulence deviates significantly from the traditional scaling relations. We examine in detail the characteristics of these states of anisotropy, identify conditions in which they occur and connect them with different governing parameters. The governing parameters of turbulence anisotropy are shown to be different for stable and unstable stratification, but are able to delineate clearly the conditions in which each of the anisotropy states occurs.

Linking Northern Hemisphere temperature extremes to Rossby wave packets

Abstract: This work investigates the statistical linkage between upper-tropospheric transient Rossby wave packets (RWPs) and lower-tropospheric temperature extremes in the Northern Hemisphere during the period 1979–2015. Data from ERA-Interim reanalyses are used for the diagnosis of RWP amplitude and temperature anomalies as well as the systematic examination of their connection. Areas of large RWP amplitude are found to be associated with an increased probability of lower-tropospheric temperature extremes in many regions of the midlatitudes. Although a seasonal and inter-regional variability is apparent, this link is always stronger than in an analysis using a circumglobal waviness metric based on Fourier wavenumber amplitudes. Further insight is gained by complementing the climatological results with an investigation of the two most severe recent heat waves in Europe, viz. during the 2003 and 2010 summers. Both events are found to be associated with conspicuous non-circumglobal RWPs, but differences between the two events suggest that the mechanisms linking RWPs and temperature extremes are case-dependent. The aforementioned results underscore the important role of upper-troposphere dynamics and open up avenues for future research on heat waves and cold spells at both weather and climate time-scales.

Rainfall types over southern West Africa: Objective identification, climatology and synoptic environment

Abstract: Southern West Africa (SWA) is characterised by a wide range of rainfall types, the relative importance of which have never been quantified on a regional level. Here, we use 16 years of three‐dimensional reflectivity data from the Tropical Rainfall Measuring Mission–Precipitation Radar (TRMM‐PR) to objectively distinguish between seven different rainfall types in three subregions of SWA. Highly organized Mesoscale Convective System (MCS) events are the dominating rain‐bearing systems in SWA. They tend to occur in highly sheared environments as a result of mid‐level northeasterlies ahead of a cyclonic vortex. Their contribution to annual rainfall decreases from 71% in the Soudanian to 56% in the coastal zone. MCSs in SWA also propagate slower than their Sahelian counterparts and occur predominantly at the start of the first coastal rainy season. However, in terms of numbers, about 90% of rainfall systems are weakly organized classes, particularly small‐sized, highly reflective and moderately deep (40 dBZ at altitude 9 h, whereas less intense rainfall types tend to be short‐lived, diurnal phenomena. This novel approach stresses the relevance of mid‐level (wave) disturbances on the type and lifetime of convective systems and thereby their regionally, seasonally and diurnally varying contribution to rainfall amount. The present study suggests further investigations into the character of the disturbances as well as possible implications for operational forecasting and the understanding of rainfall variability in SWA.

Estimates of flow-dependent predictability of wintertime Euro-Atlantic weather regimes in medium-range forecasts

Abstract: This study assesses the medium‐range flow‐dependent forecast skill of Euro‐Atlantic weather regimes: the positive and negative phases of the North Atlantic Oscillation (NAO+ and NAO−), Atlantic ridge (ATLR), and Euro‐Atlantic blocking (EABL), for extended winters (November–March) in the periods 2006/2007–2013/2014 and 1985/1986–2013/2014 using The Interactive Grand Global Ensemble (TIGGE) and the National Oceanic and Atmospheric Administration (NOAA)'s Global Ensemble Forecasting System (GEFS) reforecast datasets, respectively. The models show greater‐than‐observed (smaller‐than‐observed) frequencies of NAO− and ATLR (NAO+) with forecast lead time. The increased frequency of NAO− is not due to its excess persistence but due to more frequent transitions mainly from ATLR, but also from NAO+. In turn, NAO+ is under‐persistent. The models show the highest probabilistic skill for forecasts initialised on NAO− and the NAO− forecasts during the TIGGE period. However, the GEFS reforecast during the period 1985/1986–2013/2014 revealed that these recent high skills reflect the occurrence of four long‐lasting (>30 days) NAO− events in 2009/2010–2013/2014 and that the skill for forecasts initialised on NAO− before 2009/2010 (the longest duration was 22 days and the second‐longest 16 days) was the lowest. The longer the NAO− events persist, the higher the skill of forecasts initialised on NAO−. The skill dependency on regime duration is less clearly observed for the other regimes. In addition, the GEFS reforecast also revealed that the highest skill of the NAO− forecasts during the period 1985/1986–2013/2014 is attributed to the higher skill of the NAO− forecasts during the active NAO− periods. The EABL forecasts initialised on ATLR show the lowest skill, followed by the NAO− (EABL) forecasts initialised on NAO+ or ATLR (NAO+). These results suggest that the recent models still have difficulties in predicting the onset of blocking.

Validation of the Version 05 Level 2 precipitation products from the GPM Core Observatory and constellation satellite sensors

Abstract: The accurate measurement of precipitation, an essential source of freshwater, is key for assessing water resources and precipitation-related hazards across the globe. The measurement of global precipitation is hampered by the inadequate spatial distribution of ground-based observations such as gauges and weather radars. However, satellite instruments can provide global observations from which precipitation estimates may be generated. The satellites of the Global Precipitation Measurement (GPM) constellation carry passive microwave sensors that provide an essential set of observations from which the Goddard PROFiling (GPROF 2017v1) scheme is used to retrieve precipitation. The regional consistency and differences in the satellite retrievals need to be documented in order to refine and improve global precipitation estimates which form the basis of widely-used gridded precipitation products. Here the Level 2 instantaneous swath-based precipitation products generated by the GPROF scheme are evaluated using standard descriptive and statistical scores against national/international surface radar and dense gauge datasets, over the United States and Western Europe. Results show that over Europe the current GPROF retrieval technique tends to overestimate the occurrence of light precipitation, leading to an overestimation of the volumetric contribution by light precipitation intensities, while it underestimates moderate to heavy precipitation. Over the US the overestimation of light precipitation is reduced, with a more pronounced overestimation of moderate precipitation intensities, and an underestimation of heavier precipitation intensities.

Influence of tropical Atlantic sea surface temperature anomalies on the East Asian summer monsoon

Abstract: Funding information Natural Science Foundation of Jiangsu Province of China, BK20160956. Natural Science Foundation of China, 41330425. Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Startup Foundation for Introducing Talent of NUIST, 2015r035, 2014r006. General Program of the Natural Science Foundation of Jiangsu Higher Education Institutions, 17KJB170013. This study investigates the relationship between tropical Atlantic (TA) sea surface temperature (SST) anomalies and the East Asian summer monsoon (EASM), as well as the possible mechanism by which TA SST anomalies affect the EASM. We demonstrate a robust positive simultaneous correlation between the boreal summer TA SST anomalies and the EASM. Observational and model-based studies identify an atmospheric teleconnection in which the summer TA warming can enhance convection and induce low-level convergence in the Atlantic basin, which alters the Walker circulation and produces a sinking motion and divergence in the central Pacific. As a Rossby response to the anomalous divergence, an anomalous anticyclonic pair is generated over the western Pacific. As a result, the EASM is strengthened. Additionally, a significant lag correlation of the EASM with TA SST anomalies from the previous boreal spring is demonstrated, which implies potential applicability of the TA SST factor to the EASM prediction at seasonal time-scales.

The severe hailstorm in southwest Germany on 28 July 2013: characteristics, impacts and meteorological conditions

Abstract: At the end of July 2013, a series of severe thunderstorms associated with heavy rainfall, severe wind gusts, and large hail affected parts of Germany. On 28 July 2013, two supercells formed almost simultaneously in southern Germany, from which only the more southerly cell produced hailstones up to 10 cm in diameter on a hailswath approximately 120 km long and 15–20 km wide. For the insurance industry, this event with losses of more than EUR 1 billion was one of the most expensive natural disasters that has ever occurred in Germany. This paper investigates the creation, temporal evolution, and effects of the most severe supercell that day by considering and merging radar and satellite data, eyewitness reports, insurance loss data, and numerical model studies. Observations of hail at ground level fit very well with a cold–ring-shaped structure in the cloud top brightness temperature observed by a geostationary satellite imager. Various simulations conducted with the convection-permitting COnsortium for Small-scale MOdeling (COSMO) revealed that the track of the hailstorm could be reproduced only when convection was triggered artificially by two warm bubbles that produced single cells that were precursors of the supercell. The model results suggested that the supercell developed near a pre-existing single cell through low-level flow convergence in an environment with moderate CAPE, but substantial wind shear and storm-relative helicity, both of which persisted for several hours in the area in which the supercell moved.

On distinguishing snowfall from rainfall using near-surface atmospheric information: Comparative analysis, uncertainties and hydrologic importance

Abstract: National Aeronautics and Space Administration (NASA) GRACE [NNH15ZDA001N-GRACE]; NASA Energy and Water Cycle Study (NEWS) [NNH13ZDA001N-NEWS]; US Department of Agriculture/National Institute of Food Agriculture; National Science Foundation; Water Sustainability & Climate Program [1360506/1360507]; NASA Weather [NNH13ZDA001N-WEATHER]; NASA MIRO [NNX15AQ06A]

The activities of the international precipitation working group.

Abstract: The International Precipitation Working Group (IPWG) is a permanent International Science Working Group (ISWG) of the Coordination Group for Meteorological Satellites (CGMS), co-sponsored by CGMS and the World Meteorological Organization (WMO). The IPWG provides a focal point and forum for the international scientific community to address the issues and challenges of satellite-based quantitative precipitation retrievals, and for the operational agencies to access and make use of precipitation products. Through partnerships and biennial meetings, the group supports the exchange of information on techniques for retrieving and measuring precipitation and for enhancing the impact of space-borne precipitation retrievals in numerical weather and hydrometeorological prediction and climate studies. The group furthers the refinement of current estimation techniques and the development of new methodologies for improved global precipitation measurements, together with the validation of the derived precipitation products with ground-based precipitation measurements. The IPWG identifies critical issues, provides recommendations to the CGMS and supports upcoming precipitation-oriented missions. Training activities on precipitation retrieval from space are also part of the IPWG mandate in cooperation with WMO and other bodies.

Medicanes as subtropical cyclones: the December 2005 case from the perspective of surface pressure tendency diagnostics and atmospheric water budget

Abstract: Fil: Fita Borrell, Lluis. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmosfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmosfera; Argentina

Quasi-stationary waves and their impact on European weather and extreme events

Abstract: Large-scale, quasi-stationary atmospheric waves (QSWs) have long been known to be associated with weather extremes such as the European heatwave in 2003. There is much debate in the scientific literature as to whether QSW activity may increase under a changing climate, providing a strong motivation for developing a better understanding of the behaviour and drivers of QSWs. This paper presents the first steps in this regard: the development of a robust objective method for a simple identification and characterisation of these waves. A clear connection between QSWs and European weather and extreme events is confirmed for all seasons, indicating that blocking anticyclones are often part of a broader scale wave pattern. Investigation of the QSW climatology in the Northern Hemisphere reveals that wave activity is typically strongest in midlatitudes, particularly at the exit of the Atlantic and Pacific storm track with weaker intensities in summer. In general, the structure of individual QSW events tends to follow the climatological pattern, except in winter where the strongest and most persistent QSWs are typically shifted polewards, indicating a distinct evolution of the ’strongest’ QSW events. Modes of inter-annual variability are calculated to better understand their importance and connection to European temperatures and to identify relevant QSW patterns. This analysis highlights that European winter temperatures are strongly associated with the meridional location of QSW activity whereas warm European summer temperatures are associated with increases in the overall intensity of midlatitude QSW activity. QSWs are shown to be strongly connected to commonly used indices to describe the large scale atmospheric circulation (NAO, AO, Ni˜no 3.4, PNA) but offer a more direct link to understanding their impact on regional weather events. It is therefore hoped that objective identification of QSWs will provide a useful new viewpoint for interpreting large-scale weather alongside more traditional measures and metrics.

A proof of concept for scale-adaptive parametrizations: the case of the Lorenz '96 model

Abstract: Constructing efficient and accurate parameterizations of sub-grid scale processes is a central area of interest in the numerical modelling of geophysical fluids. Using a modified version of the two-level Lorenz ’96 model, we present here a proof of concept of a scale-adaptive parameterization constructed using statistical mechanical arguments. By a suitable use of the Ruelle response theory and of the Mori-Zwanzig projection method, it is possible to derive explicitly a parameterization for the fast variables that translates into deterministic, stochastic and non-markovian contributions to the equations of motion of the variables of interest. We show that our approach is computationally parsimonious, has great flexibility, as it is explicitly scale-adaptive, and we prove that it is competitive compared to empirical ad-hoc approaches. While the parameterization proposed here is universal and can be easily analytically adapted to changes in the parameters’ values by a simple rescaling procedure, the parameterization constructed with the ad-hoc approach needs to be recomputed each time the parameters of the systems are changed. The price of the higher flexibility of the method proposed here is having a lower accuracy in each individual case.

Observations of submeso motions and intermittent turbulent mixing across a low level jet with a 132-m tower

Abstract: The coexistence of wavelike submeso motions and anisotropic intermittent turbulence in a night time stable boundary layer is investigated. Submeso motions of different characteristics and amplitudes interact with each other. These interactions may lead to intermittent turbulence production that alters the turbulent structure of the stable boundary layer. On the other hand, the production and transfer of turbulence affect the delicate balance of submeso motions. In this work sonic anemometer data collected at eleven levels in south-eastern Brazil have been used to study a case of a nocturnal boundary layer in a coastal site. The absence of forcing at the synoptic scale allows the development of a breeze circulation on which a low level jet of moderate intensity (4 m s-1) and low height (about 50 m) takes place. The jet evolution is coupled with dirty waves, while its full development is associated with gravity waves driven by a strong vertical temperature gradient. The layer centred at the low-level jet nose is characterized by horizontal meandering and very weak turbulence intensity. The air far below and above the low-level jet maximum experiences bursts of significant increase of the turbulence intensity, showing a three-layer structure. The oscillations of the wind velocity horizontal components exhibit the same frequency of the temperature oscillations, suggesting that the presence of an adequate temperature horizontal gradient is one of the fundamental driver of the meandering phenomenon. The considered night has been studied by means of the Eulerian auto-correlation functions for the detection of the meandering hours and their oscillation time-scales and by means of the continuous Morlet wavelet function for the detection of the gravity waves and the characterization of their spatial time scales and temporal evolution.