Genesis of Hurricane Julia (2010) within an African Easterly Wave: Sensitivity Analyses of WRF-LETKF Ensemble Forecasts

2014 ◽  
Vol 71 (9) ◽  
pp. 3180-3201 ◽  
Author(s):  
Stefan F. Cecelski ◽  
Da-Lin Zhang
Keyword(s):  
Deep Convection ◽  
Empirical Orthogonal Functions ◽  
Sensitivity Analyses ◽  
Tropical Cyclogenesis ◽  
Tropospheric Temperature ◽  
Orthogonal Functions ◽  
Ensemble Forecasts ◽  
Low Level ◽  
Modes Of Variability ◽  
Two Parameters

Abstract In this study, the predictability of tropical cyclogenesis (TCG) is explored by conducting ensemble sensitivity analyses on the TCG of Hurricane Julia (2010). Using empirical orthogonal functions (EOFs), the dominant patterns of ensemble disagreements are revealed for various meteorological parameters such as mean sea level pressure (MSLP) and upper-tropospheric temperature. Using the principal components of the EOF patterns, ensemble sensitivities are generated to elucidate which mechanisms drive the parametric ensemble differences. The dominant pattern of MSLP ensemble spread is associated with the intensity of the pre–tropical depression (pre-TD), explaining nearly half of the total variance at each respective time. Similar modes of variance are found for the low-level absolute vorticity, though the patterns explain substantially less variance. Additionally, the largest modes of variability associated with upper-level temperature anomalies closely resemble the patterns of MSLP variance, suggesting interconnectedness between the two parameters. Sensitivity analyses at both the pre-TD and TCG stages reveal that the MSLP disturbance is strongly correlated to upper-tropospheric temperature and, to a lesser degree, surface latent heat flux anomalies. Further sensitivity analyses uncover a statistically significant correlation between upper-tropospheric temperature and convective anomalies, consistent with the notion that deep convection is important for augmenting the upper-tropospheric warmth during TCG. Overall, the ensemble forecast differences for the TCG of Julia are strongly related to the processes responsible for MSLP falls and low-level cyclonic vorticity growth, including the growth of upper-tropospheric warming and persistent deep convection.

scholarly journals A model intercomparison analysing the link between ozone and geopotential height anomalies in January

2007 ◽  
Vol 7 (6) ◽  
pp. 15409-15451 ◽  
Author(s):  
P. Braesicke ◽  
C. Brühl ◽  
M. Dameris ◽  
R. Deckert ◽  
V. Eyring ◽  
...  
Keyword(s):  
Geopotential Height ◽  
Climate Models ◽  
Horizontal Resolution ◽  
Empirical Orthogonal Functions ◽  
Orthogonal Functions ◽  
Modes Of Variability ◽  
Statistical Framework ◽  
Column Ozone ◽  
Linear Correlations ◽  
Upper Boundary

Abstract. A statistical framework to evaluate the performance of chemistry-climate models with respect to the interaction between meteorology and ozone during northern hemisphere mid-winter, in particularly January, is used. Different statistical diagnostics from four chemistry-climate models (E39C, ME4C, UMUCAM, ULAQ) are compared with the ERA-40 re-analysis. First, we analyse vertical coherence in geopotential height anomalies as described by linear correlations between two different pressure levels (30 and 200 hPa) of the atmosphere. In addition, linear correlations between (partial) column ozone and geopotential height anomalies at 200 hPa are discussed to motivate a simple picture of the meteorological impacts on ozone on interannual timescales. Secondly, we discuss characteristic spatial structures in geopotential height and (partial) column ozone anomalies as given by their first two empirical orthogonal functions. Finally, we describe the covariance patterns between reconstructed anomalies of geopotential height and (partial) column ozone. In general we find good agreement between the models with higher horizontal resolution (E39C, ME4C, UMUCAM) and ERA-40. Some diagnostics seem to be capable of picking up model similarities (either that the models use the same dynamical core (E39C, ME4C), or that they have a high upper boundary (ME4C, UMUCAM)). The methodology allows to identify the leading modes of variability contributing to the overall ozone-geopotential height correlations and points to interesting differences between the chemistry-climate models and ERA-40. Those discrepancies have to be taken into account when providing confidence intervals for climate change integrations.

scholarly journals Mesoscale Processes during the Genesis of Hurricane Karl (2010)

2019 ◽  
Vol 76 (8) ◽  
pp. 2235-2255 ◽  
Author(s):  
Michael M. Bell ◽  
Michael T. Montgomery
Keyword(s):  
Structural Changes ◽  
Doppler Radar ◽  
Deep Convection ◽  
Tropical Cyclogenesis ◽  
Primary Role ◽  
Low Level ◽  
Stratiform Precipitation ◽  
Environmental Air ◽  
Mesoscale Processes

Abstract Observations from the Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT), Genesis and Rapid Intensification Processes (GRIP), and Intensity Forecast Experiment (IFEX) field campaigns are analyzed to investigate the mesoscale processes leading to the tropical cyclogenesis of Hurricane Karl (2010). Research aircraft missions provided Doppler radar, in situ flight level, and dropsonde data documenting the structural changes of the predepression disturbance. Following the pre-Karl wave pouch, variational analyses at the meso-β and meso-α scales suggest that the convective cycle in Karl alternately built the low- and midlevel circulations leading to genesis episodically rather than through a sustained lowering of the convective mass flux from increased stabilization. Convective bursts that erupt in the vorticity-rich environment of the recirculating pouch region enhance the low-level meso-β- and meso-α-scale circulation through vortex stretching. As the convection wanes, the resulting stratiform precipitation strengthens the midlevel circulation through convergence associated with ice microphysical processes, protecting the disturbance from the intrusion of dry environmental air. Once the column saturation fraction returns to a critical value, a subsequent convective burst below the midlevel circulation further enhances the low-level circulation, and the convective cycle repeats. The analyses suggest that the onset of deep convection and associated low-level spinup were closely related to the coupling of the vorticity and moisture fields at low and midlevels. Our interpretation of the observational analysis presented in this study reaffirms a primary role of deep convection in the genesis process and provides a hypothesis for the supporting role of stratiform precipitation and the midlevel vortex.

Understanding the role of water vapor and temperature in easterly wave-related convection

2020 ◽  
Author(s):  
Rosa Vargas Martes ◽  
Angel Adames Corraliza
Keyword(s):  
Water Vapor ◽  
Rainfall Variability ◽  
Large Fraction ◽  
Deep Convection ◽  
Linear Regression Analysis ◽  
Empirical Orthogonal Functions ◽  
Specific Humidity ◽  
Coupling Mechanism ◽  
Orthogonal Functions ◽  
The Relationship

<p>Easterly Waves (EW) in the Pacific Ocean (PEW) and over Africa (AEW) account for a large fraction of rainfall variability in their respective regions. Although multiple studies have been conducted to better understand EWs, many questions remain regarding their structure, development, and coupling to deep convection. Recent studies have highlighted the relationship between water vapor and precipitation in tropical motion systems. However, EW have not been studied within this context. On the basis of Empirical Orthogonal Functions (EOFs) and a novel plume-buoyancy framework, the thermodynamic processes associated with EW-related convection are elucidated. A linear regression analysis reveals the relationship between temperature, moisture, and precipitation in EW. Temperature anomalies are found to be highly correlated in space and time with anomalies in specific humidity. However, this coupling between temperature and moisture is more robust in AEWs than PEWs. In PEWs moisture accounts for a larger fraction of precipitation variability. Results suggest that the convective coupling mechanism in AEW may differ from the coupling mechanism of PEWs.</p>

scholarly journals Geospatial Trends and Decadal Anomalies in Extreme Rainfall over Uganda, East Africa

2016 ◽  
Vol 2016 ◽  
pp. 1-15 ◽  
Author(s):  
Charles Onyutha
Keyword(s):  
North Atlantic Ocean ◽  
Rainfall Variability ◽  
Daily Rainfall ◽  
Extreme Rainfall ◽  
Empirical Orthogonal Functions ◽  
Orthogonal Functions ◽  
Indicator Method ◽  
Significance Level ◽  
Modes Of Variability ◽  
In Series

Trends and variability in series comprising the mean of fifteen highest daily rainfall intensities in each year were analyzed considering entire Uganda. The data were extracted from high-resolution (0.5° × 0.5°) gridded daily series of the Princeton Global Forcings covering the period 1948–2008. Variability was analyzed using nonparametric anomaly indicator method and empirical orthogonal functions. Possible drivers of the rainfall variability were investigated. Trends were analyzed using the cumulative rank difference approach. Generally, rainfall was above the long-term mean from the mid-1950s to the late 1960s and again in the 1990s. From around 1970 to the late 1980s, rainfall was characterized by a decrease. The first and second dominant modes of variability correspond with the variation in Indian Ocean Dipole and North Atlantic Ocean index, respectively. The influence of Niño 3 on the rainfall variability of some parts of the country was also evident. The southern and northern parts had positive and negative trends, respectively. The null hypothesisH0(no trend) was collectively rejected at the significance level of 5% in the series from 7 out of 168 grid points. The insights from the findings of this study are vital for planning and management of risk-based water resources applications.

Observations and Predictability of a Nondeveloping Tropical Disturbance over the Eastern Atlantic

2018 ◽  
Vol 146 (9) ◽  
pp. 3079-3096 ◽  
Author(s):  
Alan Brammer ◽  
Chris D. Thorncroft ◽  
Jason P. Dunion
Keyword(s):  
Deep Convection ◽  
Recirculation Region ◽  
Tropical Cyclogenesis ◽  
Short Term ◽  
Ensemble Forecasts ◽  
Dry Air ◽  
Easterly Wave ◽  
Closed Circulation ◽  
West African Coast ◽  
Favorable Conditions

Abstract A strong African easterly wave (AEW) left the West African coast in early September 2014 and operational global numerical forecasts suggested a potential for rapid tropical cyclogenesis of this disturbance in the eastern Atlantic, despite the presence of a large region of dry air northwest of the disturbance. Analysis and in situ observations show that after leaving the coast, the closed circulation associated with the AEW trough was not well aligned vertically, and therefore, low-level or midlevel dry air was advected below or above, respectively, areas of closed circulation. GPS dropwindsonde observations highlight the dry air undercutting the midlevel recirculation region in the southwestern quadrant. This advection of dry air constrains the spatial extent of deep convection within the AEW trough, leading to the vortex decaying. As the column continues to be displaced horizontally, losing vertical alignment, this enables increased horizontal advection of dry air into the system further limiting convective activity. Ensemble forecasts indicate that short-term errors in precipitation rate and vorticity generation can lead to an over intensified and well-aligned vortex, which then interacts less with the unfavorable environment, allowing for further convection and intensification. The stronger vortex provides more favorable conditions for precipitation through a more vertically coherent closed circulation and thus a positive feedback loop is initiated. The short-term forecasts of precipitation were shown to be sensitive to lower-tropospheric moisture anomalies around the AEW trough through ensemble sensitivity analysis from Global Ensemble Forecast System real-time forecasts.

scholarly journals Genesis of Hurricane Julia (2010) within an African Easterly Wave: Low-Level Vortices and Upper-Level Warming

2013 ◽  
Vol 70 (12) ◽  
pp. 3799-3817 ◽  
Author(s):  
Stefan F. Cecelski ◽  
Da-Lin Zhang
Keyword(s):  
Model Simulation ◽  
Deep Convection ◽  
Mesoscale Convective System ◽  
Tropical Cyclogenesis ◽  
Convective System ◽  
Easterly Waves ◽  
Low Level ◽  
Mesoscale Convective ◽  
Upper Level ◽  
Scale Surface

Abstract While a robust theoretical framework for tropical cyclogenesis (TCG) within African easterly waves (AEWs) has recently been developed, little work explores the development of low-level meso-β-scale vortices (LLVs) and a meso-α-scale surface low in relation to deep convection and upper-tropospheric warming. In this study, the development of an LLV into Hurricane Julia (2010) is shown through a high-resolution model simulation with the finest grid size of 1 km. The results presented expand upon the connections between LLVs and the AEW presented in previous studies while demonstrating the importance of upper-tropospheric warming for TCG. It is found that the significant intensification phase of Hurricane Julia is triggered by the pronounced upper-tropospheric warming associated with organized deep convection. The warming is able to intensify and expand during TCG owing to formation of a storm-scale outflow beyond the Rossby radius of deformation. Results confirm previous ideas by demonstrating that the intersection of the AEW's trough axis and critical latitude is a preferred location for TCG, while supplementing such work by illustrating the importance of upper-tropospheric warming and meso-α-scale surface pressure falls during TCG. It is shown that the meso-β-scale surface low enhances boundary layer convergence and aids in the bottom-up vorticity development of the meso-β-scale LLV. The upper-level warming is attributed to heating within convective bursts at earlier TCG stages while compensating subsidence warming becomes more prevalent once a mesoscale convective system develops.

scholarly journals Initial state perturbations in ensemble forecasting

2008 ◽  
Vol 15 (5) ◽  
pp. 751-759 ◽  
Author(s):  
L. Magnusson ◽  
E. Källén ◽  
J. Nycander
Keyword(s):  
Initial Conditions ◽  
Weather Prediction ◽  
Empirical Orthogonal Functions ◽  
Initial State ◽  
Orthogonal Functions ◽  
Ensemble Forecasts ◽  
Long Time ◽  
Chaotic Nature ◽  
Prediction Systems ◽  
Open Question

Abstract. Due to the chaotic nature of atmospheric dynamics, numerical weather prediction systems are sensitive to errors in the initial conditions. To estimate the forecast uncertainty, forecast centres produce ensemble forecasts based on perturbed initial conditions. How to optimally perturb the initial conditions remains an open question and different methods are in use. One is the singular vector (SV) method, adapted by ECMWF, and another is the breeding vector (BV) method (previously used by NCEP). In this study we compare the two methods with a modified version of breeding vectors in a low-order dynamical system (Lorenz-63). We calculate the Empirical Orthogonal Functions (EOF) of the subspace spanned by the breeding vectors to obtain an orthogonal set of initial perturbations for the model. We will also use Normal Mode perturbations. Evaluating the results, we focus on the fastest growth of a perturbation. The results show a large improvement for the BV-EOF perturbations compared to the non-orthogonalised BV. The BV-EOF technique also shows a larger perturbation growth than the SVs of this system, except for short time-scales. The highest growth rate is found for the second BV-EOF for the long-time scale. The differences between orthogonal and non-orthogonal breeding vectors are also investigated using the ECMWF IFS-model. These results confirm the results from the Loernz-63 model regarding the dependency on orthogonalisation.

scholarly journals Sensitivity to the Representation of Microphysical Processes in Numerical Simulations during Tropical Storm Formation

2016 ◽  
Vol 144 (10) ◽  
pp. 3611-3630 ◽  
Author(s):  
Andrew B. Penny ◽  
Patrick A. Harr ◽  
James D. Doyle
Keyword(s):  
Heating Rate ◽  
Diabatic Heating ◽  
Deep Convection ◽  
Initial Period ◽  
Tropical Cyclogenesis ◽  
Heating Rates ◽  
Low Level ◽  
Convective Scale ◽  
Microphysical Processes ◽  
Heating Profiles

An analysis of in situ observations from the nondeveloping tropical disturbance named TCS025 revealed that a combination of unfavorable system-scale and environmental factors limited further development. In this study, a multiphysics ensemble of high-resolution simulations of TCS025 are analyzed and compared. A simulation that overdeveloped the TCS025 disturbance is compared with one that correctly simulated nondevelopment and reveals that convection was stronger and diabatic heating rates were larger in the developing simulation. This led to continued spinup of the low-level circulation primarily through vorticity stretching. In contrast, convection was much weaker in the nondeveloping simulation, and after an initial period of deep convection, average vorticity tendencies from stretching became weakly negative, which allowed for the frictional spindown of the low-level circulation. Convective-scale differences identified early in the simulations appear to have resulted from the explicit representation of graupel in the developing simulation. The net impacts resulting from these differences in convection are manifest in the average diabatic heating profiles that are important for determining the developmental outcome. Additional simulations are conducted whereby the diabatic heating rates are artificially adjusted. Relatively small changes in the diabatic heating rate led to significantly different outcomes with respect to storm development, and the degree of overdevelopment is largely dictated by the diabatic heating rate. These findings suggest the correct representation of convective processes and associated diabatic heating are necessary to adequately forecast tropical cyclogenesis, especially for systems near a threshold of development like TCS025.

scholarly journals Comparison of catchment grouping methods for flow duration curve estimation at ungauged sites in France

2011 ◽  
Vol 15 (8) ◽  
pp. 2421-2435 ◽  
Author(s):  
E. Sauquet ◽  
C. Catalogne
Keyword(s):  
Regression Models ◽  
Empirical Orthogonal Functions ◽  
Parametric Models ◽  
The Other ◽  
Orthogonal Functions ◽  
Flow Duration ◽  
Ungauged Sites ◽  
Two Parameters ◽  
Homogeneous Regions ◽  
The Impact

Abstract. The study aims at estimating flow duration curves (FDC) at ungauged sites in France and quantifying the associated uncertainties using a large dataset of 1080 FDCs. The interpolation procedure focuses here on 15 percentiles standardised by the mean annual flow, which is assumed to be known at each site. In particular, this paper discusses the impact of different catchment grouping procedures on the estimation of percentiles by regional regression models. In a first step, five parsimonious FDC parametric models are tested to approximate FDCs at gauged sites. The results show that the model based on the expansion of Empirical Orthogonal Functions (EOF) outperforms the other tested models. In the EOF model, each FDC is interpreted as a linear combination of regional amplitude functions with spatially variable weighting factors corresponding to the parameters of the model. In this approach, only one amplitude function is required to obtain a satisfactory fit with most of the observed curves. Thus, the considered model requires only two parameters to be applicable at ungauged locations. Secondly, homogeneous regions are derived according to hydrological response, on the one hand, and geological, climatic and topographic characteristics on the other hand. Hydrological similarity is assessed through two simple indicators: the concavity index (IC) representing the shape of the dimensionless FDC and the seasonality ratio (SR), which is the ratio of summer and winter median flows. These variables are used as homogeneity criteria in three different methods for grouping catchments: (i) according to an a priori classification of French Hydro-EcoRegions (HERs), (ii) by applying regression tree clustering and (iii) by using neighbourhoods obtained by canonical correlation analysis. Finally, considering all the data, and subsequently for each group obtained through the tested grouping techniques, we derive regression models between physiographic and/or climatic variables and the two parameters of the EOF model. Results on percentile estimation in cross validation show that a significant benefit is obtained by defining homogeneous regions before developing regressions, particularly when grouping methods make use of hydrogeological information.

scholarly journals Comparison of catchment grouping methods for flow duration curve estimation at ungauged sites in France

2011 ◽  
Vol 8 (2) ◽  
pp. 3233-3269 ◽  
Author(s):  
E. Sauquet ◽  
C. Catalogne
Keyword(s):  
Regression Models ◽  
Regression Tree ◽  
Empirical Orthogonal Functions ◽  
Parametric Models ◽  
The Other ◽  
Orthogonal Functions ◽  
Flow Duration ◽  
Ungauged Sites ◽  
Two Parameters ◽  
Homogeneous Regions

Abstract. The study aims at estimating flow duration curves (FDC) at ungauged sites in France and quantifying the associated uncertainties using a large dataset of 1080 FDCs. The interpolation procedure focuses here on 15 percentiles standardised by the mean annual flow, which is supposed to be known at each site. In particular, this paper discusses the relevance of different catchments grouping procedures on percentiles estimation by regional regression models. First, five parsimonious FDC parametric models were tested to approximate FDCs at gauged sites. The results show that the model based on Empirical Orthogonal Functions (EOF) expansion outperforms the other ones. In this model each FDC is interpreted as a linear combination of regional amplitude functions with weights – the parameters of the model – varying in space. Here, only one amplitude function was found sufficient to fit well most of the observed curves. Thus the considered model requires only two parameters to be estimated at ungauged locations. Second, homogeneous regions were derived according to hydrological response on one hand, and geological, climatic and topographic characteristics on the other hand. Hydrological similarity was assessed through two simple indicators: the concavity index (IC) that represents the shape of the standardized FDC and the seasonality ratio (SR) which is the ratio of summer and winter median flows. These variables were used as homogeneity criteria in three different methods for grouping catchments: (i) according to their membership in one of an a priori French classification into Hydro-Eco-Regions (HERs), (ii) by applying a regression tree clustering and (iii) by using hydrological neighbourhood obtained by canonical correlation analysis. Finally, regression models between physiographic and/or climatic variables and the two parameters of the EOF model were derived considering all the data and thereafter for each group obtained through the tested grouping techniques. Results on percentiles estimation in cross validation show a significant benefit to form homogeneous regions before developing regressions, particularly when grouping methods use hydrogeological information.

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