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William A. Komaromi, Sharanya J. Majumdar, and Eric D. Rappin

Abstract

The response of Weather Research and Forecasting (WRF) model predictions of two tropical cyclones to perturbations in the initial conditions is investigated. Local perturbations to the vorticity field in the synoptic environment are created in features considered subjectively to be of importance to the track forecast. The rebalanced analysis is then integrated forward and compared with an unperturbed “control” simulation possessing similar errors to those in the corresponding operational model forecasts. In the first case, Typhoon Sinlaku (2008), the premature recurvature in the control simulation is found to be corrected by a variety of initial perturbations; in particular, the weakening of an upper-level low directly to its north, and the weakening of a remote short-wave trough in the midlatitude storm track. It is suggested that one or both of the short waves may have been initialized too strongly. In the second case, the forecasts for Hurricane Ike (2008) initialized 4 days prior to its landfall in Texas were not sensitive to most remote perturbations. The primary corrections to the track of Ike arose from a weakening of a midlevel ridge directly to its north, and the strengthening of a short-wave trough in the midlatitudes. For both storms, the targets selected by the ensemble transform Kalman filter (ETKF) were often, but not always, consistent with the most sensitive regions found in this study. Overall, the results can be used to retrospectively diagnose features in which the initial conditions require improvement, in order to improve forecasts of tropical cyclone track.

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Eric D. Rappin, David S. Nolan, and Sharanya J. Majumdar

Abstract

A highly configurable vortex initialization methodology has been constructed in order to permit manipulation of the initial vortex structure in numerical models of tropical cyclones. By using distinct specifications of the flow in the boundary layer and free atmosphere, an array of parameters is available to modify the structure. A nonlinear similarity model that solves the steady-state, height-dependent equations for a neutrally stratified, axisymmetric vortex is solved for the boundary layer flow. Above the boundary layer, a steady-state, moist-neutral, hydrostatic and gradient wind balanced model is used to generate the angular momentum distribution in the free atmosphere. In addition, an unbalanced mass-conserving secondary circulation is generated through the assumption of conservation of mass and angular momentum above the boundary layer. Numerical simulations are conducted using a full-physics mesoscale model to explore the sensitivity of the vortex evolution to different prescriptions of the initial vortex. Dynamical adjustment is found to be dominant in the early evolution of the simulations, thereby masking any sensitivity to initial changes in the secondary circulation and boundary layer structure. The adjustment time can be significantly reduced by arbitrarily enhancing the moisture in the eyewall region.

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Eric D. Rappin, Michael C. Morgan, and Gregory J. Tripoli

Abstract

In this study, the impacts of regions of weak inertial stability on tropical cyclone intensification and peak strength are examined. It is demonstrated that weak inertial stability in the outflow layer minimizes an energy sink of the tropical cyclone secondary circulation and leads to more rapid intensification to the maximum potential intensity. Using a full-physics, three-dimensional numerical weather prediction model, a symmetric distribution of environmental inertial stability is generated using a variable Coriolis parameter. It is found that the lower the value of the Coriolis parameter, the more rapid the strengthening. The lower-latitude simulation is shown to have a significantly stronger secondary circulation with intense divergent outflow against a comparatively weak environmental resistance. However, the impacts of differences in the gradient wind balance between the different latitudes on the core structure cannot be neglected. A second study is then conducted using an asymmetric inertial stability distribution generated by the presence of a jet stream to the north of the tropical cyclone. The initial intensification is similar, or even perhaps slower, in the presence of the jet as a result of increased vertical wind shear. As the system evolves, convective outflow from the tropical cyclone modifies the jet resulting in weaker shear and more rapid intensification of the tropical cyclone–jet couplet. It is argued that the generation of an outflow channel as the tropical cyclone outflow expands into the region of weak inertial stability on the anticyclonic shear side of the jet stream minimizes the energy expenditure of forced subsidence by ventilating all outflow in one long narrow path, allowing radiational cooling to lessen the work of subsidence. Furthermore, it is hypothesized that evolving conditions in the outflow layer modulate the tropical cyclone core structure in such a way that tropical cyclone outflow can access weak inertial stability in the environment.

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Jesse Winchester, Rezaul Mahmood, William Rodgers, Faisal Hossain, Eric Rappin, Joshua Durkee, and Themis Chronis

Abstract

Land-use land-cover change (LULCC) plays an important role in weather and climate systems. Human modifications of land cover include building reservoirs and thus creating artificial lakes for multipurpose use. In this research, the authors have completed a Weather Research and Forecasting (WRF) Model–based assessment of impacts of two large parallel lakes on precipitation. This area is located in the western part of the states of Kentucky and Tennessee and known as the Land between the Lakes (LBL). To determine the impacts, this study has replaced the lakes with grass, deciduous forests, and bare soil and conducted model simulations for three precipitation events of different magnitudes.

The analysis suggests that precipitation increased in some cases and reduced in others. One of the key impacts of LULCC in the LBL area is the relocation of precipitation cells and also the timing of precipitation. Local precipitation amounts increased or decreased with these relocations. In summary, establishment of lakes or replacement of lakes with alternate land cover may modify local precipitation in the LBL area.

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Rezaul Mahmood, Megan Schargorodski, Eric Rappin, Melissa Griffin, Patrick Collins, Kevin Knupp, Andrew Quilligan, Ryan Wade, Kevin Cary, and Stuart Foster
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Rezaul Mahmood, Megan Schargorodski, Eric Rappin, Melissa Griffin, Patrick Collins, Kevin Knupp, Andrew Quilligan, Ryan Wade, and Kevin Cary

Abstract

A total solar eclipse traversed the continental United States on 21 August 2017. It was the first such event in 99 years and provided a rare opportunity to observe the atmospheric response from a variety of instrumented observational platforms. This paper discusses the high-quality observations collected by the Kentucky Mesonet (www.kymesonet.org), a research-grade meteorological and climatological observation network consisting of 72 stations and measuring air temperature, precipitation, relative humidity, solar radiation, wind speed, and wind direction. The network samples the atmosphere, for most variables, every 3 s and then calculates and records observations every 5 min. During the total solar eclipse, these observations were complemented by observations collected from three atmospheric profiling systems positioned in the path of the eclipse and operated by the University of Alabama in Huntsville (UAH). Observational data demonstrate that solar radiation at the surface dropped from >800 to 0 W m‒2, the air temperature decreased by about 4.5°C, and, most interestingly, a land-breeze–sea-breeze-type wind developed. In addition, due to the high density of observations, the network recorded a detailed representation of the spatial variation of surface meteorology. The UAH profiling system captured collapse and reformation of the planetary boundary layer and related changes during the total solar eclipse.

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Eric Rappin, Rezaul Mahmood, Udaysankar Nair, Roger A. Pielke Sr., William Brown, Steve Oncley, Joshua Wurman, Karen Kosiba, Aaron Kaulfus, Chris Phillips, Emilee Lachenmeier, Joseph Santanello Jr., Edward Kim, and Patricia Lawston-Parker

Abstract

Extensive expansion in irrigated agriculture has taken place over the last half century. Due to increased irrigation and resultant land use land cover change, the central United States has seen a decrease in temperature and changes in precipitation during the second half of 20th century. To investigate the impacts of widespread commencement of irrigation at the beginning of the growing season and continued irrigation throughout the summer on local and regional weather, the Great Plains Irrigation Experiment (GRAINEX) was conducted in the spring and summer of 2018 in southeastern Nebraska. GRAINEX consisted of two, 15-day intensive observation periods. Observational platforms from multiple agencies and universities were deployed to investigate the role of irrigation in surface moisture content, heat fluxes, diurnal boundary layer evolution, and local precipitation.

This article provides an overview of the data collected and an analysis of the role of irrigation in land-atmosphere interactions on time scales from the seasonal to the diurnal. The analysis shows that a clear irrigation signal was apparent during the peak growing season in mid-July. This paper shows the strong impact of irrigation on surface fluxes, near-surface temperature and humidity, as well as boundary layer growth and decay.

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