Browsing by Author "Allen J. Riordan, Committee Member"

Now showing 1 - 5 of 5

Analysis of Model QPF Errors During the 2-4 December 2000 Snowstorm in North Carolina
(2006-01-31) Caldwell, Raymond Jason; Gary M. Lackmann, Committee Chair; Jerry M. Davis, Committee Member; Allen J. Riordan, Committee Member
Model forecasts of an early season snowstorm for 2 – 4 December 2000 followed the historical blizzard of 24 – 25 January 2000 that dumped 20.3 inches of snowfall at the Raleigh-Durham International Airport. Much like the January 2000 storm, operational models exhibited a significant lack of skill, particularly in the realm of quantitative precipitation forecasting. As early as 1800 UTC 1 December, operational models from the National Centers for Environmental Prediction (NCEP), including the 32-kilometer Eta, generated liquid equivalent precipitation totals approaching two inches for the Raleigh–Durham metropolitan area. Later forecasts indicated as much as 2.77 inches of precipitation would fall. In reality, only a trace of precipitation was observed at the Raleigh–Durham airport. A local, real–time version of the fifth-generation, mesoscale modeling system (MM5) was operational at the time of the event and provided a much-improved forecast scenario compared to the NCEP Eta model. Remarkably, the initial conditions and lateral boundary conditions in the MM5 were identical to those used to initialize the Eta model at 1200 UTC 2 December. In this study, an examination of the potential sources of error in the quantitative precipitation forecast is performed to challenge prior studies that suggest that data quality issues with sea surface temperature analyses led to spurious precipitation generation. The study includes a case study of the 2 – 4 December snowstorm, model sensitivity experiments, and quasi-geostrophic analysis to identify and diagnose the quantitative precipitation errors in the Eta model and the superior forecast guidance available from the local MM5 model. The case study showed that several potential sources of model error existed including missing upper air soundings, sea surface temperatures, model design, and misdiagnosed topographic flow. This study will test the hypothesis that errors at the 500–hPa level led to limited precipitation early in the period and, hence, produced errors in the cold air damming, coastal front, and cyclogenesis in later periods responsible for the heaviest precipitation in model forecasts. Results from sensitivity experiments with the MM5 model failed to exhibit significant differences in the representation of topographically induced phenomena or the westward extent of the precipitation shield into central North Carolina. The Eta model produced an anomalously strong 850 hPa jet at the North Carolina coast which transported warm air and moisture inland over the region. Better representation of the initial 500–hPa shortwave trough and associated vorticity maximum in the MM5 model is shown in the results to strengthen the low-level damming episode and shift the coastal front farther offshore. The results of this study provide basis for further investigation into both models and concludes that the effect of the upper–level forcing on the evolution of the low-level topographically induced flow and the surface–based forcing of upper–level dynamics can be of equal magnitude and importance in winter season precipitation forecasting. Results of this study will be coupled with local efforts to improve forecasting through conceptual model development by providing operational forecasting with the knowledge that individual models can have independent and opposing response to initial condition errors based on the physical and dynamical make–up of the mesoscale modeling system.
The Effect of Upstream Convection on Downstream Precipitation
(2005-07-22) Mahoney, Kelly Marie; Gary M. Lackmann, Committee Chair; Allen J. Riordan, Committee Member; Lian Xie, Committee Member
Numerical weather prediction (NWP) models have demonstrated a weakness in the ability to accurately forecast precipitation amounts downstream of strong, organized convection in the Southeast US. This weakness has also been communicated by operational forecasters, as past events have exhibited reduced downstream precipitation amounts relative to the model quantitative precipitation forecast (QPF), particularly when the upstream convection (UC) feature moves rapidly eastward. Conventional forecaster wisdom evolving from such events thus advocates reducing downstream QPF in the presence of UC. The purposes of this study are (i) to identify the physical processes by which downstream precipitation may be reduced or enhanced in the presence of UC; (ii) distinguish UC cases in which downstream QPF should be reduced from those in which it should be enhanced; (iii) understand why operational models are challenged to produce an accurate downstream forecast in these situations; (iv) identify synoptic settings associated with different types of UC events; (v) find ways in which human forecasters may anticipate and improve upon erroneous model forecasts; and (vi) investigate optimal model configurations for the representation of UC and downstream QPF. A brief climatology of National Centers for Environmental Prediction (NCEP) Eta model QPF error revealed that the UC problem is observed to occur in three relatively distinct synoptic settings, in which different QPF biases tended to occur. The three scenarios are based on the orientation and movement of the UC. Scenario 1 (S1) is characterized by UC that is oriented parallel to mean flow, and the organized convective system propagates quickly eastward relative to the primary synoptic system in a direction perpendicular to flow. These cases were generally characterized by a positive model QPF bias. Scenario 2 (S2) features UC that is oriented parallel to mean flow as in S1, but that propagates slowly (or not at all) with respect to the primary synoptic system. It was hypothesized that in these cases a diabatically-enhanced low-level jet (LLJ) would act to enhance moisture transport ahead of the synoptic system, and these cases were therefore hypothesized to yield increased downstream precipitation amounts, albeit with smaller QPF errors. Scenario 3 (S3) is characterized by UC oriented perpendicular to flow, usually aligning with a coastal and/or warm frontal features. S3 cases were found to often transition into an S1 or S2 event. They were consequently treated as precursors to S1 or S2 cases and are not investigated in great detail here. Several potential physical mechanisms of downstream precipitation alteration were examined for each scenario. These physical mechanisms include (i) moisture consumption; (ii) stabilization of the downstream environment; (iii) alteration of lower-tropospheric moisture transport through interruption (S1) or enhancement (S2) of the LLJ; and (iv) alteration of synoptic dynamics. Two main case studies were undertaken to investigate S1 and S2. For the S1 case it was found that a high-resolution (4-km grid-spacing) convection-resolving forecast using the Weather Research and Forecast (WRF) model vastly improved the downstream QPF relative to the large overprediction of precipitation produced by the operational Eta forecast. Comparisons were made between the successful WRF run, analyses, and the erroneous operational Eta forecast. Errors in the low-level wind and horizontal moisture flux fields from the operational Eta forecast revealed that the delayed speed of the model-forecasted UC did not permit an accurate representation of low-level moisture transport (physical mechanism (iii)), causing the model to allow too much moisture to infiltrate the downstream area, and likely contributing to the model overprediction of precipitation. The S1 case showed that despite excellent operational Eta forecasts of the placement and intensity of the synoptic system, errors in model-forecasted convective motion resulted in large operational QPF errors downstream. The S2 case study revealed that UC does not always lead to a decrease in downstream precipitation. Potential vorticity (PV) diagnostics were used to demonstrate that the diabatic influence of the UC acted to enhance the low-level jet (LLJ) and increase moisture transport (physical mechanism (iii)). A quasi-geostrophic PV inversion showed that the diabatic cyclonic QGPV anomaly associated with the UC feature contributed significantly to the southerly LLJ preceding the convective line. S2 proved to be an important counter-example to the notion that UC generally reduces downstream precipitation, as this case study demonstrates a physical process associated with the UC that instead acts to enhance downstream precipitation. The findings of this study may be helpful to forecasters in anticipating future UC events, as well as to numerical modelers facing decisions regarding what type of model run will yield the best results for future UC events. The results are further analyzed in the context of the ability of cumulus parameterization (CP) schemes to accurately represent convective movement, and the implications of such a shortcoming for UC forecasts. Future work in this area is suggested.
The Formation and Impact of an Incipient Cold-Air Precipitation Feature on the 24-25 January 2000 East Coast Cyclone
(2005-07-22) Brennan, Michael Joseph; Gary M. Lackmann, Committee Chair; Allen J. Riordan, Committee Member; Lian Xie, Committee Member; Sethu Raman, Committee Member
The 24–25 January 2000 East Coast cyclone was characterized by a major operational forecast failure. In an effort to understand why short-range operational numerical weather prediction (NWP) model forecasts were so poor, the impact of a cold-air incipient precipitation (IP) feature that developed prior to the rapid cyclogenesis on 24 January is investigated using potential vorticity (PV) analysis. The IP was poorly forecasted by the operational NWP models, and these models failed to produce heavy precipitation far enough inland over the Carolinas and Virginia later in the cyclone event. Here the formation of the IP is examined from an observational perspective, the impact of the IP is quantified using PV methodology, and the ability of a NWP model to simulate its formation is tested by varying model physics, initial conditions and grid spacing. It was hypothesized that latent heating associated with the IP that formed over the Gulf Coast states early on 24 January generated a lower-tropospheric PV maximum that was important to the moisture transport into the Carolinas and Virginia and the track and intensity of the surface cyclone later in the cyclone event. Calculations from a PV budget and piecewise PV inversion found that the IP was associated with the genesis of a lower-tropospheric PV maximum and that the balanced flow associated with the PV maximum contributed significantly to moisture transport into the region of heavy snowfall. Operational NWP models that failed to forecast the IP did not generate the PV maximum or the heavy precipitation over the Carolinas and Virginia. Observational analyses and radar imagery showed that the IP formed in a region of elevated convective symmetric instability (a mixture of gravitational conditional instability and conditional symmetric instability) where forcing for ascent was provided by an approaching upper-level trough/jet streak. Short-range forecasts from NWP models under-forecasted the strength of the forcing and instability, and were unable to generate the IP in the region where it was observed. An 18-member mesoscale model ensemble with 20-km horizontal grid spacing varying initial condition analyses and model physics was unable to generate the IP feature. Variance associated with the cyclone?s sea-level pressure and precipitation distributions due to initial condition variation was larger than that due to variations in model physics, although significant variation was due to poor performance by ensemble members initialized from the Global Data Assimilation System analysis. A high-resolution model simulation with 4-km grid spacing showed that the IP initially formed within a layer of elevated CSI, consistent with analyses. Buckling of absolute geostrophic momentum surfaces indicated adjustment to slantwise convection at later times. Simulations with 12-km and 20-km grid spacing degraded the representation of these features, suggesting that models run with even coarser grid spacing would be unable to capture the initial formation of the IP. Other simulations initialized only three or six hours later showed a marked improvement in the representation of the IP, the cyclone track and intensity, and the final precipitation distribution, confirming the importance of properly representing the IP feature in successful simulations of this event. The current configuration of operational models with CP schemes and grid spacing insufficient to properly resolve the effects of slantwise convection suggests that future cases may occur where NWP models fail to capture the impact of a cold-air precipitation feature (possibly associated with elevated gravitational and slantwise instability), resulting in poor forecasts of downstream moisture transport and cyclone track and intensity. Operational forecasters should be aware of this possibility and be able to anticipate the potential feedbacks from precipitation (in NWP models and in reality) onto atmospheric dynamics. Available observations and high-frequency model analyses can be used to evaluate NWP model forecasts of precipitation and the lower-tropospheric PV distribution, allowing forecasters to recognize instances when model guidance can be adjusted to improve forecasts of high-impact cyclone events.
The Precipitation Mass Sink in Tropical Cyclones
(2004-07-22) Yablonsky, Richard Michael; Allen J. Riordan, Committee Member; Yuh-Lang Lin, Committee Member; Gary M. Lackmann, Committee Chair
Conservation of atmospheric mass is one of the fundamental concepts used in the study of meteorology. Any time precipitation occurs, however, atmospheric mass is not conserved. As precipitation is removed from the atmosphere, the hydrostatic pressure in the precipitating region is reduced. In a tropical cyclone, the heaviest precipitation occurs in the eyewall, and this localized region of heavy precipitation leads to mass and moisture convergence towards the center of the tropical cyclone. The Coriolis force deflects the converging air, thereby contributing to vorticity generation and increasing the cyclonic wind speed. Moisture convergence can also enhance precipitation. In addition, potential vorticity (PV) increases as a result of the precipitation mass sink. To assess the significance of the precipitation mass sink, several hypothesis tests are performed. The MM5 model is used to create a physically realistic dataset of Hurricane Lili (2002) from which mass and PV budgets can be performed. The mass budget reveals that in a 100-km radius cylinder around the model storm center, the 5-h total and 1-h average pressure-equivalent mass loss due to precipitation during forecast hours 30 to 35 are -7.25 hPa and -1.45 hPa h⁻¹, respectively, while the 5-h total and 1-h average model surface pressure change during that time are -2.29 hPa and -0.46 hPa h⁻¹, respectively. Although the surface pressure change in the cylinder is mostly due to large cancellation between strong low-level convergence and stronger upper-level divergence, the continuous removal of mass via precipitation represents a non-negligible effect in the mass budget. The PV budget reveals that in the same cylinder, the average hourly instantaneous mass sink PV tendency during forecast hours 30 to 35 is 0.42 PVU day^-1, while the average hourly instantaneous diabatic PV tendency is -2.19 PVU day⁻¹. The diabatic PV tendency exhibits large spatial and temporal cancellation, so the small but continuously positive PV contribution from the precipitation mass sink has a non-negligible effect on the PV budget. In addition, according to the PV tendency terms, an air parcel rising through the troposphere in the eyewall should experience nearly continuous PV generation via the precipitation mass sink but both PV generation and destruction via latent heat release, leading to large cancellation from the latter. Therefore, the parcel upon reaching the upper troposphere can likely attribute a non-negligible amount of its PV to the precipitation mass sink, but trajectory computations would be needed to quantify the relative contributions of the PV tendency terms on a given parcel. In addition to the mass and PV budgets, the workstation version of the Eta model is used to perform multiple sensitivity experiments with and without the precipitation mass sink. The most realistic of these sensitivity experiments reveals that the precipitation mass sink generally reduces the central pressure by ~5-7 hPa, increases the wind field by ~5-15 kt, and increases the precipitation rate by ~5-25 mm h⁻¹, but the rainfall rate difference in particular exhibits large spatial and temporal variation. PV and geopotential height cross sections show maximum precipitation mass sink-induced PV increase (>5 PVU) and geopotential height reduction (>4 dam) near the surface and near the melting layer. The results of this study suggest that the precipitation mass sink should not be neglected in tropical cyclones. Further research possibilities include detailed analysis of the impact of the precipitation mass sink on the tropical cyclone track, as well as the importance of the precipitation mass sink in heavily precipitating systems other than tropical cyclones.
The Role of Terrain and Convection on Microfront Formation Leading to Severe Low-Level Turbulence
(2003-09-03) Cetola, Jeffrey David; Yuh-Lang Lin, Committee Co-Chair; Michael L. Kaplan, Committee Co-Chair; Allen J. Riordan, Committee Member; Gerald S. Janowitz, Committee Member
Two low-level convectively-induced turbulence (CIT) events east of the Appalachian Mountains are investigated utilizing observations, satellite, radar, and numerical simulations. Both events had an inordinate amount of low-level turbulence reported, but one event had more than twice as many severe or greater reports. The events were compared—to include the 72 hours leading up to the turbulence reports—and similarities and differences at the various scales from the synoptic to meso-alpha, meso-beta, meso-gamma, and microscale are noted. The case of weaker turbulence featured a meridional wave pattern with ridging over the East Coast and a single upper-level jet closely coupled with the large-scale frontal system. The stronger turbulence case possessed a zonal wave pattern with a vortex over eastern Canada and both a polar jet and subtropical jet. These differences are reflected in the low-level temperature and potential vorticity patterns and affected the hydraulic structures as well—with the stronger turbulence environment more prone to a blocking-type regime. Hydrostatic mountain waves were observed for both events. Stronger cross-mountain flow combined with a strong low-level leeside inversion resulted in a more vigorous mountain wave with a stronger downstream isentropic upfold (mid-level cold pool) in the stronger turbulence event. This mid-level cold pool was deformed by the large-scale jet resulting in a mid-level cold front (downstream from the surface cold front), surface pressure rises to the lee of the Allegheny Mountains, and ultimately a surface cold surge (edgewave) that merges with warm air from the south. The phasing of the mid-level cold pool and the convergence with the northerly cold surge and southerly warm air results in kata-frontogenesis and cellular convection that transits the severe turbulence location in space and time. Convection in the weaker turbulence case was lineal in structure and tied to the large-scale cold anafront. Vorticity, enstrophy, turbulent kinetic energy, and Richardson number analyses indicated maxima were lineal in structure and upstream from the convection in the weaker case and arc-like in appearance and downstream from convection in the stronger case. A turbulence index was formulated based on three-dimensional vorticity (enstrophy), vertical wind shear, and static stability.