Richard Forbes | ECMWF - Academia.edu (original) (raw)

Papers by Richard Forbes

Freezing rain events, though relatively rare, can be extremely debilitating and dangerous for soc... more Freezing rain events, though relatively rare, can be extremely debilitating and dangerous for society, with recovery times of order months or even years. Analysis of forecasts of past events by the operational ECMWF Integrated Forecast System (IFS) showed a strong tendency to incorrectly represent freezing rain as snow. Investigations highlighted that this was primarily because the re-freezing process in IFS, following hydrometeors as they descend, was parametrised with the same time-scale as the melting process. In reality the time-scale for re-freezing should, in general, be much longer. The model physics were changed accordingly, and the results in terms of forecast quality were positive and very striking. Coupled with these physics changes new IFS output was developed for users which shows precipitation type at the surface (rain, snow, wet snow, sleet, freezing rain, ice pellets). The changes to the physics will be described in detail, and their impact will be illustrated by com...

This note has been prepared by the Physical Aspects Section of the Research Department at ECMWF. ... more This note has been prepared by the Physical Aspects Section of the Research Department at ECMWF. ECMWF is an operational centre for medium range weather forecasting, monthly forecasting, seasonal forecasting and ocean wave forecasting. The operational system is also used for re-analysis projects (ERA-15, ERA-40 and ERA-Interim). ECMWF operates a state of the art global atmospheric model with an integrated data assimilation system. For monthly and seasonal forecasting the atmospheric model is coupled to an ocean model. Resolution varies from T159 (110 km) for the seasonal system to T799 (25 km) for the 10-day deterministic forecasts. A resolution upgrade to T1279 (15 km) is currently under test and will become operational late 2009 or early 2010. Intermediate resolutions are used for monthly forecasting and for the ensemble prediction system. Given these application, ECMWF is interested in atmospheric processes like radiation, convection, clouds, precipitation, and turbulence, but it...

The parametrization of clouds and precipitation in numerical weather prediction (NWP) models has ... more The parametrization of clouds and precipitation in numerical weather prediction (NWP) models has evolved significantly over the years, yet it remains a challenge to represent the complex system of microphysical interactions whose influence spans the range of scales from microns to hundreds of kilometres. Significant approximations must be made and there is always a balance between the complexity of the parametrization, the limitations of our knowledge of the real world and the computational constraints of an operational NWP system. The model is an integral part of modern data assimilation systems and for the assimilation of cloud and precipitation data, it is imperative that there is a good understanding of the representation of cloud and precipitation in the model. The purpose of this paper is to discuss some future directions for the parametrization of cloud and precipitation, which are relevant for the future development of data assimilation schemes for assimilating satellite obs...

Quarterly Journal of the Royal Meteorological Society, 2021

A cold bias in the extratropical lowermost stratosphere in forecasts is one of the most prominent... more A cold bias in the extratropical lowermost stratosphere in forecasts is one of the most prominent systematic temperature errors in numerical weather prediction models. Hypothesized causes of this bias include radiative effects from a collocated moist bias in model analyses. Such biases would be expected to affect extratropical dynamics and result in the misrepresentation of wave propagation at tropopause level. Here the extent to which these humidity and temperature biases are connected is quantified. Observations from radiosondes are compared to operational analyses and forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS) and Met Office Unified Model (MetUM) to determine the magnitude and vertical structure of these biases. Both operational models overestimate lowermost stratospheric specific humidity, with a maximum moist bias around 1 km above the tropopause where humidities are around 170% of the observed values on average. This moist bias is already present in the initial conditions and changes little in forecasts over the first five days. Though temperatures are represented well in the analyses, the IFS forecasts anomalously cool in the lower stratosphere, relative to verifying radiosonde observations, by 0.2 K day −1. The IFS single column model is used to show this temperature change can be attributed to increased long-wave radiative cooling due to the lowermost stratospheric moist bias in the initial conditions. However, the MetUM temperature biases cannot be entirely attributed to the moist bias, and another significant factor must be present. These results highlight the importance of improving the humidity analysis to reduce the extratropical lowermost stratospheric cold bias in forecast models and the need to understand and mitigate the causes of the moist bias in these models.

Quarterly Journal of the Royal Meteorological Society, 2020

Within the Copernicus Climate Change Service (C3S), ECMWF is producing the ERA5 reanalysis which,... more Within the Copernicus Climate Change Service (C3S), ECMWF is producing the ERA5 reanalysis which, once completed, will embody a detailed record of the global atmosphere, land surface and ocean waves from 1950 onwards. This new reanalysis replaces the ERA‐Interim reanalysis (spanning 1979 onwards) which was started in 2006. ERA5 is based on the Integrated Forecasting System (IFS) Cy41r2 which was operational in 2016. ERA5 thus benefits from a decade of developments in model physics, core dynamics and data assimilation. In addition to a significantly enhanced horizontal resolution of 31 km, compared to 80 km for ERA‐Interim, ERA5 has hourly output throughout, and an uncertainty estimate from an ensemble (3‐hourly at half the horizontal resolution). This paper describes the general set‐up of ERA5, as well as a basic evaluation of characteristics and performance, with a focus on the dataset from 1979 onwards which is currently publicly available. Re‐forecasts from ERA5 analyses show a gain of up to one day in skill with respect to ERA‐Interim. Comparison with radiosonde and PILOT data prior to assimilation shows an improved fit for temperature, wind and humidity in the troposphere, but not the stratosphere. A comparison with independent buoy data shows a much improved fit for ocean wave height. The uncertainty estimate reflects the evolution of the observing systems used in ERA5. The enhanced temporal and spatial resolution allows for a detailed evolution of weather systems. For precipitation, global‐mean correlation with monthly‐mean GPCP data is increased from 67% to 77%. In general, low‐frequency variability is found to be well represented and from 10 hPa downwards general patterns of anomalies in temperature match those from the ERA‐Interim, MERRA‐2 and JRA‐55 reanalyses.

Journal of the Atmospheric Sciences, 2019

The upper-level potential vorticity (PV) structure plays a key role in the evolution of extratrop... more The upper-level potential vorticity (PV) structure plays a key role in the evolution of extratropical weather systems. PV is modified by nonconservative processes, such as cloud latent heating, radiative transfer, and turbulence. Using a Lagrangian method, material PV modification near the tropopause is attributed to specific parameterized processes in the global model of the European Centre for Medium-Range Weather Forecasts (ECMWF). In a case study, several flow features identified in a vertical section across an extratropical cyclone experienced strong PV modification. In particular clear-air turbulence at the jet stream is found to be a relevant process (i) for the PV structure of an upper-level front–jet system, corroborating previous observation-based findings of turbulent PV generation; (ii) for the purely turbulent decay of a tropopause fold, identified as an effective process of stratosphere–troposphere exchange; and (iii) in the ridge, where the Lagrangian accumulated turb...

Journal of Geophysical Research: Atmospheres, 2018

We introduce the Clouds Above the United States and Errors at the Surface (CAUSES) project with i... more We introduce the Clouds Above the United States and Errors at the Surface (CAUSES) project with its aim of better understanding the physical processes leading to warm screen temperature biases over the American Midwest in many numerical models. In this first of four companion papers, 11 different models, from nine institutes, perform a series of 5 day hindcasts, each initialized from reanalyses. After describing the common experimental protocol and detailing each model configuration, a gridded temperature data set is derived from observations and used to show that all the models have a warm bias over parts of the Midwest. Additionally, a strong diurnal cycle in the screen temperature bias is found in most models. In some models the bias is largest around midday, while in others it is largest during the night. At the Department of Energy Atmospheric Radiation Measurement Southern Great Plains (SGP) site, the model biases are shown to extend several kilometers into the atmosphere. Finally, to provide context for the companion papers, in which observations from the SGP site are used to evaluate the different processes contributing to errors there, it is shown that there are numerous locations across the Midwest where the diurnal cycle of the error is highly correlated with the diurnal cycle of the error at SGP. This suggests that conclusions drawn from detailed evaluation of models using instruments located at SGP will be representative of errors that are prevalent over a larger spatial scale.

Proceedings of the National Academy of Sciences of the United States of America, Oct 10, 2017

Snowfall in Antarctica is a key term of the ice sheet mass budget that influences the sea level a... more Snowfall in Antarctica is a key term of the ice sheet mass budget that influences the sea level at global scale. Over the continental margins, persistent katabatic winds blow all year long and supply the lower troposphere with unsaturated air. We show that this dry air leads to significant low-level sublimation of snowfall. We found using unprecedented data collected over 1 year on the coast of Adélie Land and simulations from different atmospheric models that low-level sublimation accounts for a 17% reduction of total snowfall over the continent and up to 35% on the margins of East Antarctica, significantly affecting satellite-based estimations close to the ground. Our findings suggest that, as climate warming progresses, this process will be enhanced and will limit expected precipitation increases at the ground level.

Monthly Weather Review, 2014

Supercooled liquid water (SLW) layers in boundary layer clouds are abundantly observed in the atm... more Supercooled liquid water (SLW) layers in boundary layer clouds are abundantly observed in the atmosphere at high latitudes, but remain a challenge to represent in numerical weather prediction (NWP) and climate models. Unresolved processes such as small-scale turbulence and mixed-phase microphysics act to maintain the liquid layer at cloud top, directly affecting cloud radiative properties and prolonging cloud lifetimes. This paper describes the representation of supercooled liquid water in boundary layer clouds in the European Centre for Medium-Range Weather Forecasts (ECMWF) global NWP model and in particular the change from a diagnostic temperature-dependent mixed phase to a prognostic representation with separate cloud liquid and ice variables. Data from the Atmospheric Radiation Measurement site in Alaska and from the CloudSat/Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite missions are used to evaluate the model parameterizations. The prog...

cabernet.atmosfcu.unam.mx

Quarterly Journal of the Royal Meteorological Society, 2006

The evaporation (sublimation) of ice particles beneath frontal ice cloud can provide a significan... more The evaporation (sublimation) of ice particles beneath frontal ice cloud can provide a significant source of diabatic cooling which can lead to enhanced slantwise descent below the frontal surface. The strength and vertical extent of the cooling play a role in determining the dynamic response of the atmosphere, and an adequate representation is required in numerical weather-prediction (NWP) models for accurate forecasts of frontal dynamics. In this paper, data from a vertically pointing 94 GHz radar are used to determine the characteristic depth-scale of ice particle sublimation beneath frontal ice cloud. A statistical comparison is made with equivalent data extracted from the NWP mesoscale model operational at the Met Office, defining the evaporation depth-scale as the distance for the ice water content to fall to 10% of its peak value in the cloud. The results show that the depth of the ice evaporation zone derived from observations is less than 1 km for 90% of the time. The model significantly overestimates the sublimation depth-scales by a factor of between two and three, and underestimates the local ice water content by a factor of between two and four. Consequently the results suggest the model significantly underestimates the strength of the evaporative cooling, with implications for the prediction of frontal dynamics. A number of reasons for the model discrepancy are suggested. A comparison with radiosonde relative humidity data suggests part of the overestimation in evaporation depth may be due to a high RH bias in the dry slot beneath the frontal cloud, but other possible reasons include poor vertical resolution and deficiencies in the evaporation rate or ice particle fall-speed parametrizations.

Quarterly Journal of the Royal Meteorological Society, 2013

Observations are described of a Mesoscale Convective System (MCS) with a cold pool which propagat... more Observations are described of a Mesoscale Convective System (MCS) with a cold pool which propagated across southern England on 25 August 2005. The observations were made as part of the Convective Storm Initiation Project (CSIP). The observed MCS structure broadly followed well-established theories, including the presence of a weak rear-inflow jet. In detail, however, unsteady transitions occurred involving the formation of two distinct lines of showers ahead of the initial linear system. In each case the cold pool merged with cold downdraughts from the new showers leading to a discontinuous propagation of the system. One of these lines formed independently of the MCS, very probably on a sea-breeze convergence line. The mechanism for formation of the other is unknown, but it is possible that it was triggered by ascent associated with gravity waves generated by the MCS. The merged cold pool was deeper and colder and propagated faster than the original system, eventually forming a bow echo and arc cloud as it propagated across the English Channel. Until completion of the merger, the propagation velocity of the overall system had been controlled by a combination of the above mechanisms rather than simply by cold pool dynamics.

Quarterly Journal of the Royal Meteorological Society, 2013

Simulations using the Met Office Unified Model at 1 km horizontal grid spacing of a Mesoscale Con... more Simulations using the Met Office Unified Model at 1 km horizontal grid spacing of a Mesoscale Convective System (MCS) with a cold pool which propagated across southern England on 25 August 2005 are validated using detailed observations from the Convective Storm Initiation Project (CSIP). Early organisation of the system is not especially well treated, but the model goes on to form a system which developed qualitative and quantitative features remarkably similar to the observations. A sensitivity study suggests that the initial linear system is driven by the position of a low-level 'lid' and upper-level instability, the linear organisation being promoted by a weak rear-inflow jet forced by the upper-level warm anomaly in the cloud anvil. A weak cold pool develops in the absence of ice-phase processes, but this does not promote system propagation. Strengthening and descent of the rear-inflow jet, and acceleration of the system, is promoted by the additional heating through glaciation and cooling through snow evaporation. The surface cold pool and gust front are further strengthened by snow melting and rainfall evaporation. With ice-phase processes present, the cold pool strengthens as a result of the system development and its strength is broadly correlated with system propagation speed during the middle phase of the system's lifetime. Propagation enables the convective band to 'sweep up' any convective cells which trigger ahead of the system. The differing scales of microphysical processes means that it is difficult to form a steady-state system. The observed transition phase corresponds to an increase in the slantwise nature of the flow in the storm and in the low-level cooling by rainfall evaporation near the gust front, combined with a change in ambient conditions (advection over the sea) which eventually enable the cold pool to propagate ahead of the system.

Quarterly Journal of the Royal Meteorological Society, 2012

This article explores the uncertainties associated with evaluating a global atmospheric model wit... more This article explores the uncertainties associated with evaluating a global atmospheric model with radar reflectivity observations. A forward operator for radar reflectivity (ZmVar) is described and used for the comparison of the ECMWF global numerical weather prediction model short-range forecasts with radar data from CloudSat. A sensitivity study is performed to determine which differences can be attributed to either specific radar forward operator assumptions or to deficiencies in the global model. The results show that model-derived reflectivities are particularly sensitive to the definition of subgrid precipitation fraction, as precipitation dominates the radar reflectivity signal, but also to the choice of particle size distribution and scattering properties of the different hydrometeor categories. However, there are a number of consistent differences in the reflectivity comparison that are significantly larger than can be explained by the sensitivity tests. This suggests that these discrepancies are due to deficiencies in the model cloud and precipitation frequency of occurrence and hydrometeor water contents. These include too frequent occurrence at high altitudes, too low occurrence in the Southern Hemisphere storm track and an overestimate of rain in warm-phase low cloud. The study shows the value of CloudSat for evaluating the model in terms of radar reflectivity and highlights the importance of taking into account forward operator uncertainties for both model evaluation and data assimilation applications.

Monthly Weather Review, 2014

This study introduces a concept of global versus local forecast skill of the Madden–Julian oscill... more This study introduces a concept of global versus local forecast skill of the Madden–Julian oscillation (MJO). The global skill, measured by a commonly used MJO index [the Real-time Multivariate MJO (RMM)], evaluates the model’s capability of forecasting global patterns of the MJO, with an emphasis on the zonal wind fields. The local skill is measured by a method of tracking the eastward propagation of MJO precipitation. It provides quantitative information of the strength, propagation speed, and timing of MJO precipitation in a given region, such as the Indian Ocean. Both global and local MJO forecast skills are assessed for ECMWF forecasts of three MJO events during the 2011–12 Dynamics of the MJO (DYNAMO) field campaign. Characteristics of error growth differ substantially between global and local MJO forecast skills, and between the three MJO quantities (strength, speed, and timing) of the local skill measure. They all vary considerably among the three MJO events. Deterioration i...

Meteorological Applications, 2009

On the 8 th January 2005 the city of Carlisle in northwest England was severely flooded following... more On the 8 th January 2005 the city of Carlisle in northwest England was severely flooded following two days of almost continuous rain over the nearby hills. Orographic enhancement of the rain through the seeder-feeder mechanism led to the very high rainfall totals. This paper shows the impact of running the Met Office Unified Model (UM) with a grid spacing of 4 and 1 km compared to the 12 km available at the time of the event. These forecasts, and forecasts from the Nimrod nowcasting system, were fed into the Probability Distributed Model (PDM) to predict river flow at the outlets of two catchments important for flood warning. The results show the benefit of increased resolution in the UM, the benefit of coupling the highresolution rainfall forecasts to the PDM and the improvement in timeliness of flood warning that might have been possible. High-resolution NWP hydrological modelling flood prediction orographic rainfall

Climate Dynamics, 2012

Tropical upper tropospheric humidity, clouds, and ice water content, as well as outgoing longwave... more Tropical upper tropospheric humidity, clouds, and ice water content, as well as outgoing longwave radiation (OLR), are evaluated in the climate model EC Earth with the aid of satellite retrievals. The Atmospheric Infrared Sounder and Microwave Limb Sounder together provide good coverage of relative humidity. EC Earth's relative humidity is in fair agreement with these observations. CloudSat and CALIPSO data are combined to provide cloud fractions estimates throughout the altitude region considered (500 to 100 hPa). EC Earth is found to overestimate the degree of cloud cover above 200 hPa and underestimate it below. Precipitating and non-precipitating EC Earth ice definitions are combined to form a complete ice water content. EC Earth's ice water content is below the uncertainty range of CloudSat above 250 hPa, but can be twice as high as CloudSat's estimate in the melting layer. CERES data show that the model underestimates the impact of clouds on OLR, on average with about 9 W m −2. Regionally, EC Earth's outgoing longwave radiation can be ∼20 W m −2 higher than the observation. A comparison to ERA-Interim provides further perspectives on the model's performance. Limitations of the satellite observations are emphasised and their uncertainties are, throughout, consid

Bulletin of the American Meteorological Society, 2007

The Convective Storm Initiation Project (CSIP) is an international project to understand precisel... more The Convective Storm Initiation Project (CSIP) is an international project to understand precisely where, when, and how convective clouds form and develop into showers in the mainly maritime environment of southern England. A major aim of CSIP is to compare the results of the very high resolution Met Office weather forecasting model with detailed observations of the early stages of convective clouds and to use the newly gained understanding to improve the predictions of the model. A large array of ground-based instruments plus two instrumented aircraft, from the U.K. National Centre for Atmospheric Science (NCAS) and the German Institute for Meteorology and Climate Research (IMK), Karlsruhe, were deployed in southern England, over an area centered on the meteorological radars at Chilbolton, during the summers of 2004 and 2005. In addition to a variety of ground-based remote-sensing instruments, numerous rawinsondes were released at one- to two-hourly intervals from six closely space...

Freezing rain events, though relatively rare, can be extremely debilitating and dangerous for soc... more Freezing rain events, though relatively rare, can be extremely debilitating and dangerous for society, with recovery times of order months or even years. Analysis of forecasts of past events by the operational ECMWF Integrated Forecast System (IFS) showed a strong tendency to incorrectly represent freezing rain as snow. Investigations highlighted that this was primarily because the re-freezing process in IFS, following hydrometeors as they descend, was parametrised with the same time-scale as the melting process. In reality the time-scale for re-freezing should, in general, be much longer. The model physics were changed accordingly, and the results in terms of forecast quality were positive and very striking. Coupled with these physics changes new IFS output was developed for users which shows precipitation type at the surface (rain, snow, wet snow, sleet, freezing rain, ice pellets). The changes to the physics will be described in detail, and their impact will be illustrated by com...

This note has been prepared by the Physical Aspects Section of the Research Department at ECMWF. ... more This note has been prepared by the Physical Aspects Section of the Research Department at ECMWF. ECMWF is an operational centre for medium range weather forecasting, monthly forecasting, seasonal forecasting and ocean wave forecasting. The operational system is also used for re-analysis projects (ERA-15, ERA-40 and ERA-Interim). ECMWF operates a state of the art global atmospheric model with an integrated data assimilation system. For monthly and seasonal forecasting the atmospheric model is coupled to an ocean model. Resolution varies from T159 (110 km) for the seasonal system to T799 (25 km) for the 10-day deterministic forecasts. A resolution upgrade to T1279 (15 km) is currently under test and will become operational late 2009 or early 2010. Intermediate resolutions are used for monthly forecasting and for the ensemble prediction system. Given these application, ECMWF is interested in atmospheric processes like radiation, convection, clouds, precipitation, and turbulence, but it...

The parametrization of clouds and precipitation in numerical weather prediction (NWP) models has ... more The parametrization of clouds and precipitation in numerical weather prediction (NWP) models has evolved significantly over the years, yet it remains a challenge to represent the complex system of microphysical interactions whose influence spans the range of scales from microns to hundreds of kilometres. Significant approximations must be made and there is always a balance between the complexity of the parametrization, the limitations of our knowledge of the real world and the computational constraints of an operational NWP system. The model is an integral part of modern data assimilation systems and for the assimilation of cloud and precipitation data, it is imperative that there is a good understanding of the representation of cloud and precipitation in the model. The purpose of this paper is to discuss some future directions for the parametrization of cloud and precipitation, which are relevant for the future development of data assimilation schemes for assimilating satellite obs...

Quarterly Journal of the Royal Meteorological Society, 2021

A cold bias in the extratropical lowermost stratosphere in forecasts is one of the most prominent... more A cold bias in the extratropical lowermost stratosphere in forecasts is one of the most prominent systematic temperature errors in numerical weather prediction models. Hypothesized causes of this bias include radiative effects from a collocated moist bias in model analyses. Such biases would be expected to affect extratropical dynamics and result in the misrepresentation of wave propagation at tropopause level. Here the extent to which these humidity and temperature biases are connected is quantified. Observations from radiosondes are compared to operational analyses and forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS) and Met Office Unified Model (MetUM) to determine the magnitude and vertical structure of these biases. Both operational models overestimate lowermost stratospheric specific humidity, with a maximum moist bias around 1 km above the tropopause where humidities are around 170% of the observed values on average. This moist bias is already present in the initial conditions and changes little in forecasts over the first five days. Though temperatures are represented well in the analyses, the IFS forecasts anomalously cool in the lower stratosphere, relative to verifying radiosonde observations, by 0.2 K day −1. The IFS single column model is used to show this temperature change can be attributed to increased long-wave radiative cooling due to the lowermost stratospheric moist bias in the initial conditions. However, the MetUM temperature biases cannot be entirely attributed to the moist bias, and another significant factor must be present. These results highlight the importance of improving the humidity analysis to reduce the extratropical lowermost stratospheric cold bias in forecast models and the need to understand and mitigate the causes of the moist bias in these models.

Quarterly Journal of the Royal Meteorological Society, 2020

Within the Copernicus Climate Change Service (C3S), ECMWF is producing the ERA5 reanalysis which,... more Within the Copernicus Climate Change Service (C3S), ECMWF is producing the ERA5 reanalysis which, once completed, will embody a detailed record of the global atmosphere, land surface and ocean waves from 1950 onwards. This new reanalysis replaces the ERA‐Interim reanalysis (spanning 1979 onwards) which was started in 2006. ERA5 is based on the Integrated Forecasting System (IFS) Cy41r2 which was operational in 2016. ERA5 thus benefits from a decade of developments in model physics, core dynamics and data assimilation. In addition to a significantly enhanced horizontal resolution of 31 km, compared to 80 km for ERA‐Interim, ERA5 has hourly output throughout, and an uncertainty estimate from an ensemble (3‐hourly at half the horizontal resolution). This paper describes the general set‐up of ERA5, as well as a basic evaluation of characteristics and performance, with a focus on the dataset from 1979 onwards which is currently publicly available. Re‐forecasts from ERA5 analyses show a gain of up to one day in skill with respect to ERA‐Interim. Comparison with radiosonde and PILOT data prior to assimilation shows an improved fit for temperature, wind and humidity in the troposphere, but not the stratosphere. A comparison with independent buoy data shows a much improved fit for ocean wave height. The uncertainty estimate reflects the evolution of the observing systems used in ERA5. The enhanced temporal and spatial resolution allows for a detailed evolution of weather systems. For precipitation, global‐mean correlation with monthly‐mean GPCP data is increased from 67% to 77%. In general, low‐frequency variability is found to be well represented and from 10 hPa downwards general patterns of anomalies in temperature match those from the ERA‐Interim, MERRA‐2 and JRA‐55 reanalyses.

Journal of the Atmospheric Sciences, 2019

The upper-level potential vorticity (PV) structure plays a key role in the evolution of extratrop... more The upper-level potential vorticity (PV) structure plays a key role in the evolution of extratropical weather systems. PV is modified by nonconservative processes, such as cloud latent heating, radiative transfer, and turbulence. Using a Lagrangian method, material PV modification near the tropopause is attributed to specific parameterized processes in the global model of the European Centre for Medium-Range Weather Forecasts (ECMWF). In a case study, several flow features identified in a vertical section across an extratropical cyclone experienced strong PV modification. In particular clear-air turbulence at the jet stream is found to be a relevant process (i) for the PV structure of an upper-level front–jet system, corroborating previous observation-based findings of turbulent PV generation; (ii) for the purely turbulent decay of a tropopause fold, identified as an effective process of stratosphere–troposphere exchange; and (iii) in the ridge, where the Lagrangian accumulated turb...

Journal of Geophysical Research: Atmospheres, 2018

We introduce the Clouds Above the United States and Errors at the Surface (CAUSES) project with i... more We introduce the Clouds Above the United States and Errors at the Surface (CAUSES) project with its aim of better understanding the physical processes leading to warm screen temperature biases over the American Midwest in many numerical models. In this first of four companion papers, 11 different models, from nine institutes, perform a series of 5 day hindcasts, each initialized from reanalyses. After describing the common experimental protocol and detailing each model configuration, a gridded temperature data set is derived from observations and used to show that all the models have a warm bias over parts of the Midwest. Additionally, a strong diurnal cycle in the screen temperature bias is found in most models. In some models the bias is largest around midday, while in others it is largest during the night. At the Department of Energy Atmospheric Radiation Measurement Southern Great Plains (SGP) site, the model biases are shown to extend several kilometers into the atmosphere. Finally, to provide context for the companion papers, in which observations from the SGP site are used to evaluate the different processes contributing to errors there, it is shown that there are numerous locations across the Midwest where the diurnal cycle of the error is highly correlated with the diurnal cycle of the error at SGP. This suggests that conclusions drawn from detailed evaluation of models using instruments located at SGP will be representative of errors that are prevalent over a larger spatial scale.

Proceedings of the National Academy of Sciences of the United States of America, Oct 10, 2017

Snowfall in Antarctica is a key term of the ice sheet mass budget that influences the sea level a... more Snowfall in Antarctica is a key term of the ice sheet mass budget that influences the sea level at global scale. Over the continental margins, persistent katabatic winds blow all year long and supply the lower troposphere with unsaturated air. We show that this dry air leads to significant low-level sublimation of snowfall. We found using unprecedented data collected over 1 year on the coast of Adélie Land and simulations from different atmospheric models that low-level sublimation accounts for a 17% reduction of total snowfall over the continent and up to 35% on the margins of East Antarctica, significantly affecting satellite-based estimations close to the ground. Our findings suggest that, as climate warming progresses, this process will be enhanced and will limit expected precipitation increases at the ground level.

Monthly Weather Review, 2014

Supercooled liquid water (SLW) layers in boundary layer clouds are abundantly observed in the atm... more Supercooled liquid water (SLW) layers in boundary layer clouds are abundantly observed in the atmosphere at high latitudes, but remain a challenge to represent in numerical weather prediction (NWP) and climate models. Unresolved processes such as small-scale turbulence and mixed-phase microphysics act to maintain the liquid layer at cloud top, directly affecting cloud radiative properties and prolonging cloud lifetimes. This paper describes the representation of supercooled liquid water in boundary layer clouds in the European Centre for Medium-Range Weather Forecasts (ECMWF) global NWP model and in particular the change from a diagnostic temperature-dependent mixed phase to a prognostic representation with separate cloud liquid and ice variables. Data from the Atmospheric Radiation Measurement site in Alaska and from the CloudSat/Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite missions are used to evaluate the model parameterizations. The prog...

cabernet.atmosfcu.unam.mx

Quarterly Journal of the Royal Meteorological Society, 2006

The evaporation (sublimation) of ice particles beneath frontal ice cloud can provide a significan... more The evaporation (sublimation) of ice particles beneath frontal ice cloud can provide a significant source of diabatic cooling which can lead to enhanced slantwise descent below the frontal surface. The strength and vertical extent of the cooling play a role in determining the dynamic response of the atmosphere, and an adequate representation is required in numerical weather-prediction (NWP) models for accurate forecasts of frontal dynamics. In this paper, data from a vertically pointing 94 GHz radar are used to determine the characteristic depth-scale of ice particle sublimation beneath frontal ice cloud. A statistical comparison is made with equivalent data extracted from the NWP mesoscale model operational at the Met Office, defining the evaporation depth-scale as the distance for the ice water content to fall to 10% of its peak value in the cloud. The results show that the depth of the ice evaporation zone derived from observations is less than 1 km for 90% of the time. The model significantly overestimates the sublimation depth-scales by a factor of between two and three, and underestimates the local ice water content by a factor of between two and four. Consequently the results suggest the model significantly underestimates the strength of the evaporative cooling, with implications for the prediction of frontal dynamics. A number of reasons for the model discrepancy are suggested. A comparison with radiosonde relative humidity data suggests part of the overestimation in evaporation depth may be due to a high RH bias in the dry slot beneath the frontal cloud, but other possible reasons include poor vertical resolution and deficiencies in the evaporation rate or ice particle fall-speed parametrizations.

Quarterly Journal of the Royal Meteorological Society, 2013

Observations are described of a Mesoscale Convective System (MCS) with a cold pool which propagat... more Observations are described of a Mesoscale Convective System (MCS) with a cold pool which propagated across southern England on 25 August 2005. The observations were made as part of the Convective Storm Initiation Project (CSIP). The observed MCS structure broadly followed well-established theories, including the presence of a weak rear-inflow jet. In detail, however, unsteady transitions occurred involving the formation of two distinct lines of showers ahead of the initial linear system. In each case the cold pool merged with cold downdraughts from the new showers leading to a discontinuous propagation of the system. One of these lines formed independently of the MCS, very probably on a sea-breeze convergence line. The mechanism for formation of the other is unknown, but it is possible that it was triggered by ascent associated with gravity waves generated by the MCS. The merged cold pool was deeper and colder and propagated faster than the original system, eventually forming a bow echo and arc cloud as it propagated across the English Channel. Until completion of the merger, the propagation velocity of the overall system had been controlled by a combination of the above mechanisms rather than simply by cold pool dynamics.

Quarterly Journal of the Royal Meteorological Society, 2013

Simulations using the Met Office Unified Model at 1 km horizontal grid spacing of a Mesoscale Con... more Simulations using the Met Office Unified Model at 1 km horizontal grid spacing of a Mesoscale Convective System (MCS) with a cold pool which propagated across southern England on 25 August 2005 are validated using detailed observations from the Convective Storm Initiation Project (CSIP). Early organisation of the system is not especially well treated, but the model goes on to form a system which developed qualitative and quantitative features remarkably similar to the observations. A sensitivity study suggests that the initial linear system is driven by the position of a low-level 'lid' and upper-level instability, the linear organisation being promoted by a weak rear-inflow jet forced by the upper-level warm anomaly in the cloud anvil. A weak cold pool develops in the absence of ice-phase processes, but this does not promote system propagation. Strengthening and descent of the rear-inflow jet, and acceleration of the system, is promoted by the additional heating through glaciation and cooling through snow evaporation. The surface cold pool and gust front are further strengthened by snow melting and rainfall evaporation. With ice-phase processes present, the cold pool strengthens as a result of the system development and its strength is broadly correlated with system propagation speed during the middle phase of the system's lifetime. Propagation enables the convective band to 'sweep up' any convective cells which trigger ahead of the system. The differing scales of microphysical processes means that it is difficult to form a steady-state system. The observed transition phase corresponds to an increase in the slantwise nature of the flow in the storm and in the low-level cooling by rainfall evaporation near the gust front, combined with a change in ambient conditions (advection over the sea) which eventually enable the cold pool to propagate ahead of the system.

Quarterly Journal of the Royal Meteorological Society, 2012

This article explores the uncertainties associated with evaluating a global atmospheric model wit... more This article explores the uncertainties associated with evaluating a global atmospheric model with radar reflectivity observations. A forward operator for radar reflectivity (ZmVar) is described and used for the comparison of the ECMWF global numerical weather prediction model short-range forecasts with radar data from CloudSat. A sensitivity study is performed to determine which differences can be attributed to either specific radar forward operator assumptions or to deficiencies in the global model. The results show that model-derived reflectivities are particularly sensitive to the definition of subgrid precipitation fraction, as precipitation dominates the radar reflectivity signal, but also to the choice of particle size distribution and scattering properties of the different hydrometeor categories. However, there are a number of consistent differences in the reflectivity comparison that are significantly larger than can be explained by the sensitivity tests. This suggests that these discrepancies are due to deficiencies in the model cloud and precipitation frequency of occurrence and hydrometeor water contents. These include too frequent occurrence at high altitudes, too low occurrence in the Southern Hemisphere storm track and an overestimate of rain in warm-phase low cloud. The study shows the value of CloudSat for evaluating the model in terms of radar reflectivity and highlights the importance of taking into account forward operator uncertainties for both model evaluation and data assimilation applications.

Monthly Weather Review, 2014

This study introduces a concept of global versus local forecast skill of the Madden–Julian oscill... more This study introduces a concept of global versus local forecast skill of the Madden–Julian oscillation (MJO). The global skill, measured by a commonly used MJO index [the Real-time Multivariate MJO (RMM)], evaluates the model’s capability of forecasting global patterns of the MJO, with an emphasis on the zonal wind fields. The local skill is measured by a method of tracking the eastward propagation of MJO precipitation. It provides quantitative information of the strength, propagation speed, and timing of MJO precipitation in a given region, such as the Indian Ocean. Both global and local MJO forecast skills are assessed for ECMWF forecasts of three MJO events during the 2011–12 Dynamics of the MJO (DYNAMO) field campaign. Characteristics of error growth differ substantially between global and local MJO forecast skills, and between the three MJO quantities (strength, speed, and timing) of the local skill measure. They all vary considerably among the three MJO events. Deterioration i...

Meteorological Applications, 2009

On the 8 th January 2005 the city of Carlisle in northwest England was severely flooded following... more On the 8 th January 2005 the city of Carlisle in northwest England was severely flooded following two days of almost continuous rain over the nearby hills. Orographic enhancement of the rain through the seeder-feeder mechanism led to the very high rainfall totals. This paper shows the impact of running the Met Office Unified Model (UM) with a grid spacing of 4 and 1 km compared to the 12 km available at the time of the event. These forecasts, and forecasts from the Nimrod nowcasting system, were fed into the Probability Distributed Model (PDM) to predict river flow at the outlets of two catchments important for flood warning. The results show the benefit of increased resolution in the UM, the benefit of coupling the highresolution rainfall forecasts to the PDM and the improvement in timeliness of flood warning that might have been possible. High-resolution NWP hydrological modelling flood prediction orographic rainfall

Climate Dynamics, 2012

Tropical upper tropospheric humidity, clouds, and ice water content, as well as outgoing longwave... more Tropical upper tropospheric humidity, clouds, and ice water content, as well as outgoing longwave radiation (OLR), are evaluated in the climate model EC Earth with the aid of satellite retrievals. The Atmospheric Infrared Sounder and Microwave Limb Sounder together provide good coverage of relative humidity. EC Earth's relative humidity is in fair agreement with these observations. CloudSat and CALIPSO data are combined to provide cloud fractions estimates throughout the altitude region considered (500 to 100 hPa). EC Earth is found to overestimate the degree of cloud cover above 200 hPa and underestimate it below. Precipitating and non-precipitating EC Earth ice definitions are combined to form a complete ice water content. EC Earth's ice water content is below the uncertainty range of CloudSat above 250 hPa, but can be twice as high as CloudSat's estimate in the melting layer. CERES data show that the model underestimates the impact of clouds on OLR, on average with about 9 W m −2. Regionally, EC Earth's outgoing longwave radiation can be ∼20 W m −2 higher than the observation. A comparison to ERA-Interim provides further perspectives on the model's performance. Limitations of the satellite observations are emphasised and their uncertainties are, throughout, consid

Bulletin of the American Meteorological Society, 2007

The Convective Storm Initiation Project (CSIP) is an international project to understand precisel... more The Convective Storm Initiation Project (CSIP) is an international project to understand precisely where, when, and how convective clouds form and develop into showers in the mainly maritime environment of southern England. A major aim of CSIP is to compare the results of the very high resolution Met Office weather forecasting model with detailed observations of the early stages of convective clouds and to use the newly gained understanding to improve the predictions of the model. A large array of ground-based instruments plus two instrumented aircraft, from the U.K. National Centre for Atmospheric Science (NCAS) and the German Institute for Meteorology and Climate Research (IMK), Karlsruhe, were deployed in southern England, over an area centered on the meteorological radars at Chilbolton, during the summers of 2004 and 2005. In addition to a variety of ground-based remote-sensing instruments, numerous rawinsondes were released at one- to two-hourly intervals from six closely space...