scholarly journals Effects of Saharan Dust on African Easterly Waves: The Impact of Aerosol-Affected Satellite Radiances on Data Assimilation

2021 ◽  
Author(s):  
Dustin Francis Phillip Grogan ◽  
Cheng-Hsuan Lu ◽  
Shih-Wei Wei ◽  
Sheng-Po Chen
Keyword(s):  
Data Assimilation ◽  
Large Scale ◽  
West African Monsoon ◽  
Forecast Model ◽  
Saharan Dust ◽  
African Easterly Waves ◽  
Radiative Effects ◽  
Easterly Waves ◽  
Brightness Temperatures ◽  
The Impact

Abstract. This study incorporates time-varying aerosols into satellite radiance calculations within the Global Data Assimilation System (GDAS) to investigate its impact on African easterly waves (AEWs) and their environment. Comparison of analysis fields from the aerosol-aware experiment and an aerosol-blind control during August 2017 showed that the aerosol-affected radiances accelerated the African easterly jet and West African monsoon flow; warmed the Saharan boundary layer; and modified the AEW vorticity structure, with increases in the northern circulation and decreases in the southern circulation. Analysis fields from each experiment were used in the Global Forecast System (GFS) to examine differences in forecasting two AEW cases that developed hurricanes over the Atlantic, but were structurally different over North Africa. The aerosol-aware experiment reduced errors in forecasting the AEW case whose northern circulation interacted with a large-scale Saharan dust plume; neutral improvement was found for the other AEW, which did not contain a northern circulation nor interacted with a dust plume. The changes to the analysis fields by the aerosol-aware assimilation are reminiscent of dust radiative effects that operate on AEWs and their environment. That is, the aerosol-affected radiances produce corrections to the brightness temperatures that modify the analysis fields like dust aerosols that are radiatively coupled to the atmospheric variables in the forecast model. We show qualitatively that dust radiative effects are captured by the aerosol-affected radiances for the AEW case that interacted with a dust plume, which served to improve forecasts of the wave downstream.

Favorable monsoon environment over eastern Africa for subsequent tropical cyclogenesis of African easterly waves

2021 ◽  
Author(s):  
Kelly M. Núñez Ocasio ◽  
Alan Brammer ◽  
Jenni L. Evans ◽  
George S. Young ◽  
Zachary L. Moon
Keyword(s):  
Large Scale ◽  
West African Monsoon ◽  
Eastern Africa ◽  
Tropical Cyclogenesis ◽  
African Easterly Waves ◽  
Easterly Waves ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Vertically Aligned ◽  
Somali Jet

AbstractEastern Africa is a common region of African easterly wave (AEW) onset and AEW early-life. How the large-scale environment over east Africa relates to the likelihood of an AEW subsequently undergoing tropical cyclogenesis in a climatology has not been documented. This study addresses the following hypothesis: AEWs that undergo tropical cyclogenesis (i.e., developing AEWs) initiate and propagate under a more favorable monsoon large-scale environment over eastern Africa when compared to non-developing AEWs. Using a 21-year August-to-September (1990-2010) climatology of AEWs, differences in the large-scale environment between developers and non-developers are identified and are propose to be used as key predictors of subsequent tropical cyclone formation and could informtropical cyclogenesis prediction. TC precursors when compared to non-developing AEWs experience: an anomalously active West African Monsoon, stronger northerly flow, more intense zonal Somali jet, anomalous convergence over the Marrah Mountains (region of AEW forcing), and a more intense and elongated African easterly jet (AEJ). These large-scale conditions are linked to near-trough attributes of developing AEWs which favor more moisture ingestion, vertically aligned circulation, a stronger initial 850-hPa vortex, deeper wave pouch, and arguably more AEW and Mesoscale convective systems interactions. AEWs that initiate over eastern Africa and cross the west coast of Africa are more likely to undergo tropical cyclogenesis than those initiating over central or west Africa. Developing AEWs are more likely to be southern-track AEWs than non-developing AEWs.

A New Approach for Estimating the Observation Impact in Ensemble–Variational Data Assimilation

2018 ◽  
Vol 146 (2) ◽  
pp. 447-465 ◽  
Author(s):  
Mark Buehner ◽  
Ping Du ◽  
Joël Bédard
Keyword(s):  
Data Assimilation ◽  
Forecast Error ◽  
Forecast Model ◽  
Variational Data Assimilation ◽  
Observation Error ◽  
New Approach ◽  
Regional Prediction ◽  
Observation Impact ◽  
Adjoint Approach ◽  
The Impact

Abstract Two types of approaches are commonly used for estimating the impact of arbitrary subsets of observations on short-range forecast error. The first was developed for variational data assimilation systems and requires the adjoint of the forecast model. Comparable approaches were developed for use with the ensemble Kalman filter and rely on ensembles of forecasts. In this study, a new approach for computing observation impact is proposed for ensemble–variational data assimilation (EnVar). Like standard adjoint approaches, the adjoint of the data assimilation procedure is implemented through the iterative minimization of a modified cost function. However, like ensemble approaches, the adjoint of the forecast step is obtained by using an ensemble of forecasts. Numerical experiments were performed to compare the new approach with the standard adjoint approach in the context of operational deterministic NWP. Generally similar results are obtained with both approaches, especially when the new approach uses covariance localization that is horizontally advected between analysis and forecast times. However, large differences in estimated impacts are obtained for some surface observations. Vertical propagation of the observation impact is noticeably restricted with the new approach because of vertical covariance localization. The new approach is used to evaluate changes in observation impact as a result of the use of interchannel observation error correlations for radiance observations. The estimated observation impact in similarly configured global and regional prediction systems is also compared. Overall, the new approach should provide useful estimates of observation impact for data assimilation systems based on EnVar when an adjoint model is not available.

Probabilistic discrimination between large-scale environments of intensifying and decaying African Easterly Waves

2010 ◽  
Vol 36 (7-8) ◽  
pp. 1379-1401 ◽  
Author(s):  
Paula A. Agudelo ◽  
Carlos D. Hoyos ◽  
Judith A. Curry ◽  
Peter J. Webster
Keyword(s):  
Large Scale ◽  
African Easterly Waves ◽  
Easterly Waves

African Easterly Waves and convective activity in wet and dry sequences of the West African Monsoon

2006 ◽  
Vol 27 (2-3) ◽  
pp. 319-332 ◽  
Author(s):  
Christophe Lavaysse ◽  
Arona Diedhiou ◽  
Henri Laurent ◽  
Thierry Lebel
Keyword(s):  
West African Monsoon ◽  
West African ◽  
Convective Activity ◽  
African Easterly Waves ◽  
The West ◽  
African Monsoon ◽  
Easterly Waves

Simulations of the West African Monsoon with a Superparameterized Climate Model. Part II: African Easterly Waves

2014 ◽  
Vol 27 (22) ◽  
pp. 8323-8341 ◽  
Author(s):  
Rachel R. McCrary ◽  
David A. Randall ◽  
Cristiana Stan
Keyword(s):  
General Circulation ◽  
Climate Model ◽  
Deep Convection ◽  
West African Monsoon ◽  
General Circulation Models ◽  
African Easterly Waves ◽  
Climate System Model ◽  
Easterly Waves ◽  
Cloud Resolving Model ◽  
The Waves

Abstract The relationship between African easterly waves and convection is examined in two coupled general circulation models: the Community Climate System Model (CCSM) and the “superparameterized” CCSM (SP-CCSM). In the CCSM, the easterly waves are much weaker than observed. In the SP-CCSM, a two-dimensional cloud-resolving model replaces the conventional cloud parameterizations of CCSM. Results show that this allows for the simulation of easterly waves with realistic horizontal and vertical structures, although the model exaggerates the intensity of easterly wave activity over West Africa. The simulated waves of SP-CCSM are generated in East Africa and propagate westward at similar (although slightly slower) phase speeds to observations. The vertical structure of the waves resembles the first baroclinic mode. The coupling of the waves with convection is realistic. Evidence is provided herein that the diabatic heating associated with deep convection provides energy to the waves simulated in SP-CCSM. In contrast, horizontal and vertical structures of the weak waves in CCSM are unrealistic, and the simulated convection is decoupled from the circulation.

scholarly journals Impact of the 4 April 2014 Saharan dust outbreak on the photovoltaic power generation in Germany

2017 ◽  
Vol 17 (21) ◽  
pp. 13391-13415 ◽  
Author(s):  
Daniel Rieger ◽  
Andrea Steiner ◽  
Vanessa Bachmann ◽  
Philipp Gasch ◽  
Jochen Förstner ◽  
...  
Keyword(s):  
Power Generation ◽  
Solar Radiation ◽  
Numerical Weather Prediction ◽  
Mineral Dust ◽  
Weather Prediction ◽  
Aerosol Particles ◽  
Saharan Dust ◽  
Radiative Effects ◽  
Numerical Weather ◽  
The Impact

Abstract. The importance for reliable forecasts of incoming solar radiation is growing rapidly, especially for those countries with an increasing share in photovoltaic (PV) power production. The reliability of solar radiation forecasts depends mainly on the representation of clouds and aerosol particles absorbing and scattering radiation. Especially under extreme aerosol conditions, numerical weather prediction has a systematic bias in the solar radiation forecast. This is caused by the design of numerical weather prediction models, which typically account for the direct impact of aerosol particles on radiation using climatological mean values and the impact on cloud formation assuming spatially and temporally homogeneous aerosol concentrations. These model deficiencies in turn can lead to significant economic losses under extreme aerosol conditions. For Germany, Saharan dust outbreaks occurring 5 to 15 times per year for several days each are prominent examples for conditions, under which numerical weather prediction struggles to forecast solar radiation adequately. We investigate the impact of mineral dust on the PV-power generation during a Saharan dust outbreak over Germany on 4 April 2014 using ICON-ART, which is the current German numerical weather prediction model extended by modules accounting for trace substances and related feedback processes. We find an overall improvement of the PV-power forecast for 65 % of the pyranometer stations in Germany. Of the nine stations with very high differences between forecast and measurement, eight stations show an improvement. Furthermore, we quantify the direct radiative effects and indirect radiative effects of mineral dust. For our study, direct effects account for 64 %, indirect effects for 20 % and synergistic interaction effects for 16 % of the differences between the forecast including mineral dust radiative effects and the forecast neglecting mineral dust.

scholarly journals Dust radiative effects on atmospheric thermodynamics and tropical cyclogenesis over the Atlantic Ocean using WRF-Chem coupled with an AOD data assimilation system

2017 ◽  
Vol 17 (12) ◽  
pp. 7917-7939 ◽  
Author(s):  
Dan Chen ◽  
Zhiquan Liu ◽  
Chris Davis ◽  
Yu Gu
Keyword(s):  
Boundary Layer ◽  
Data Assimilation ◽  
Atlantic Ocean ◽  
Saharan Dust ◽  
Tropical Cyclogenesis ◽  
Moist Convection ◽  
Radiative Effects ◽  
Modis Aod ◽  
Atmospheric Thermodynamics ◽  
Convective Inhibition

Abstract. This study investigated the dust radiative effects on atmospheric thermodynamics and tropical cyclogenesis over the Atlantic Ocean using the Weather Research and Forecasting Model with Chemistry (WRF-Chem) coupled with an aerosol data assimilation (DA) system. MODIS AOD (aerosol optical depth) data were assimilated with the Gridpoint Statistical Interpolation (GSI) three-dimensional variational (3DVAR) DA scheme to depict the Saharan dust outbreak events in the 2006 summer. Comparisons with Ozone Monitoring Instrument (OMI), AErosol RObotic NETwork (AERONET), and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) observations showed that the system was capable of reproducing the dust distribution. Two sets of 180 h forecasts were conducted with the dust radiative effects activated (RE_ON) and inactivated (RE_OFF) respectively. Differences between the RE_ON and RE_OFF forecasts showed that low-altitude (high-altitude) dust inhibits (favors) convection owing to changes in convective inhibition (CIN). Heating in dust layers immediately above the boundary layer increases inhibition, whereas sufficiently elevated heating allows cooling above the boundary layer that reduces convective inhibition. Semi-direct effects in which clouds are altered by thermodynamic changes are also noted, which then alter cloud-radiative temperature (T) changes. The analysis of a tropical cyclone (TC) suppression case on 5 September shows evidence of enhanced convective inhibition by direct heating in dust, but it also suggests that the low-predictability dynamics of moist convection reduces the determinism of the effects of dust on timescales of TC development (days).

An Impact Assessment of GPS Radio Occultation Data on Prediction of a Rapidly Developing Cyclone over the Southern Ocean*

2014 ◽  
Vol 142 (11) ◽  
pp. 4187-4206 ◽  
Author(s):  
Shu-Ya Chen ◽  
Tae-Kwon Wee ◽  
Ying-Hwa Kuo ◽  
David H. Bromwich
Keyword(s):  
Data Assimilation ◽  
Southern Ocean ◽  
Large Scale ◽  
Radio Occultation ◽  
Three Dimensional ◽  
Wrf Model ◽  
The Other ◽  
Gps Radio Occultation ◽  
The Antarctic ◽  
The Impact

Abstract The impact of global positioning system (GPS) radio occultation (RO) data on an intense synoptic-scale storm that occurred over the Southern Ocean in December 2007 is evaluated, and a synoptic explanation of the assessed impact is offered. The impact is assessed by using the three-dimensional variational data assimilation scheme (3DVAR) of the Weather Research and Forecasting (WRF) Model Data Assimilation system (WRFDA), and by comparing two experiments: one with and the other without assimilating the refractivity data from four different RO missions. Verifications indicate significant positive impacts of the RO data in various measures and parameters as well as in the track and intensity of the Antarctic cyclone. The analysis of the atmospheric processes underlying the impact shows that the assimilation of the RO data yields substantial improvements in the large-scale circulations that in turn control the development of the Antarctic storm. For instance, the RO data enhanced the strength of a 500-hPa trough over the Southern Ocean and prevented the katabatic flow near the coast of East Antarctica from an overintensification. This greatly influenced two low pressure systems of a comparable intensity, which later merged together and evolved into the major storm. The dominance of one low over the other in the merger dramatically changed the track, intensity, and structure of the merged storm. The assimilation of GPS RO data swapped the dominant low, leading to a remarkable improvement in the subsequent storm’s prediction.

scholarly journals Generalizing Cloud Overlap Treatment to Include Solar Zenith Angle Effects on Cloud Geometry

2007 ◽  
Vol 64 (6) ◽  
pp. 2116-2125 ◽  
Author(s):  
Adrian M. Tompkins ◽  
Francesca Di Giuseppe
Keyword(s):  
Zenith Angle ◽  
Large Scale ◽  
Solar Zenith Angle ◽  
Forecast Model ◽  
Simple Extension ◽  
Scale Models ◽  
Field Geometry ◽  
The Mean ◽  
The Impact ◽  
First Time

Shortwave radiative transfer depends on the cloud field geometry as viewed from the direction of the sun. To date, the radiation schemes of large-scale models only consider a zenith view of the cloud field, and the apparent change in the cloud geometry with decreasing solar zenith angle is neglected. A simple extension to an existing cloud overlap scheme is suggested to account for this for the first time. It is based on the assumption that at low sun angles, the overlap between cloud elements is random for an unscattered photon. Using cloud scenes derived from radar retrievals at two European sites, it is shown that the increase of the apparent cloud cover with a descending sun is reproduced very well with the new scheme. Associated with this, there is a marked reduction in the mean radiative biases averaged across all solar zenith angles with respect to benchmark calculations. The scheme is implemented into the ECMWF global forecast model using imposed sea surface temperatures, and while the impact on the radiative statistics is significant, the feedback on the large-scale dynamics is minimal.

scholarly journals Assimilation of Synthetic GOES-R ABI Infrared Brightness Temperatures and WSR-88D Radar Observations in a High-Resolution OSSE

2016 ◽  
Vol 144 (9) ◽  
pp. 3159-3180 ◽  
Author(s):  
Rebecca M. Cintineo ◽  
Jason A. Otkin ◽  
Thomas A. Jones ◽  
Steven Koch ◽  
David J. Stensrud
Keyword(s):  
High Resolution ◽  
Data Assimilation ◽  
Radial Velocity ◽  
Doppler Radar ◽  
Forecast Accuracy ◽  
Central Plains ◽  
Radar Observations ◽  
Brightness Temperatures ◽  
Observing System Simulation Experiment ◽  
The Impact

This study uses an observing system simulation experiment to explore the impact of assimilating GOES-R Advanced Baseline Imager (ABI) 6.95-μm brightness temperatures and Weather Surveillance Radar-1988 Doppler (WSR-88D) reflectivity and radial velocity observations in an ensemble data assimilation system. A high-resolution truth simulation was used to create synthetic radar and satellite observations of a severe weather event that occurred across the U.S. central plains on 4–5 June 2005. The experiment employs the Weather Research and Forecasting Model at 4-km horizontal grid spacing and the ensemble adjustment Kalman filter algorithm in the Data Assimilation Research Testbed system. The ability of GOES-R ABI brightness temperatures to improve the analysis and forecast accuracy when assimilated separately or simultaneously with Doppler radar reflectivity and radial velocity observations was assessed, along with the use of bias correction and different covariance localization radii for the brightness temperatures. Results show that the radar observations accurately capture the structure of a portion of the storm complex by the end of the assimilation period, but that more of the storms and atmospheric features are reproduced and the accuracy of the ensuing forecast improved when the brightness temperatures are also assimilated.

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