Observation-Based Estimates of Present-Day and Future Climate Change Impacts on Heavy Rainfall in Harris County

This report describes the results of an extreme value analysis of precipitation in and around Harris County, Texas, in order to determine whether the newly-promulgated NOAA Atlas 14 rainfall design values are valid in a changing climate. The analysis in this report is based on the original NOAA Atlas 14 data set as well as a set of composite stations for the Gulf and Southeast Coasts. As of this writing, this report and its findings have not yet been peer-reviewed. • The recent upward trend in extreme precipitation in the Houston area has contributed to extreme rainfall design values in the area that far exceed those of comparable locations. This is in part due to some stations not having a sufficiently lengthy set of observations and in part due to southeast Texas receiving more than its fair share of storms. We assess that the design values of 100-year rainfall amounts would be 7% smaller if a longer period of record was available at all observation locations. • Coastal southeast Texas has the largest single-day and multi-day return values anywhere along the Gulf and Atlantic coasts for return periods of 100 years or more. This is in part due to some recent storms that could have occurred anywhere along the Gulf coast concentrating their activity around Houston. There is no known factor that would make storms such as Harvey more likely to happen in Texas than elsewhere along the northern Gulf Coast. We assess that extreme rainfall risk in Southeast Texas should consider storms from a broader portion of the Gulf Coast, decreasing return values by an additional 1%-18%, with the larger values applying to the larger return periods. • A robust upward trend in extreme precipitation is present across the southern and southeastern United States. The trend is larger in southeast Texas, but we have no reason to expect that climate change would cause trend variations on such a small scale. Using averaged trends across areas near the Gulf Coast, we assess the best estimate of the climate-driven trend in southeast Texas to be 11%-15% over the past 60 years, with the remainder of the observed trend caused by regionally unusual storms (like Harvey) that are not likely to recur in the same places. • The three factors listed above effectively cancel each other out for 2-year return values. We assess that the present-day nonstationary return values are approximately equal to the stationary estimates of NOAA Atlas 14 for 2-year return periods. • Because of the three factors listed above, the NOAA Atlas 14 100-year and 500-year return values generally overestimate the present-day and near-term future extreme rainfall risk in and around the Houston area. We assess that for 100-year return periods, current nonstationary values are still about 10-12% below the NOAA Atlas 14 values. • The historic upward trend is very likely to continue with global warming. Because of this, we assess that NOAA Atlas 14 return values underestimate the intensity of all future 2-year rainfall events in the Harris County area. We also assess that, depending on the rate of future warming, the nonstationary 100-year return values will exceed the NOAA Atlas 14 values around the middle of the 21st century.Harris County Flood Control Distric

Potential Vorticity Diagnosis of the Severe Convective Regime. Part IV: Comparison with Modeling Simulations of the Moore Tornado Outbreak

© Copyright 2008 American Meteorological Society (AMS). Permission to use figures, tables, and brief excerpts from this work in scientific and educational works is hereby granted provided that the source is acknowledged. Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act September 2010 Page 2 or that satisfies the conditions specified in Section 108 of the U.S. Copyright Act (17 USC §108, as revised by P.L. 94-553) does not require the AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a web site or in a searchable database, or other uses of this material, except as exempted by the above statement, requires written permission or a license from the AMS. Additional details are provided in the AMS Copyright Policy, available on the AMS Web site located at (https://www.ametsoc.org/) or from the AMS at 617-227-2425 or [email protected] potential vorticity (PV) diagnostic framework is used to explore the sensitivity of the 3 May 1999 Oklahoma City tornado outbreak to the strength of a particular PV anomaly proximate to the geographical region experiencing the tornado outbreak. The results derived from the balanced PV diagnosis agree broadly with those obtained previously in a numerical simulation of the same event, while offering additional insight into the nature of the sensitivity. Similar to the findings of other cases, the balanced diagnosis demonstrates that intensifying (removing) the PV anomaly of interest increases (decreases) the balanced CAPE over the southwestern portion of the outbreak region, reduces (increases) the storm-relative helicity, and increases (reduces) ascent. The latter finding, coupled with the results of the modeling study, demonstrates that intensifying a PV anomaly proximate to an outbreak environment can increase the likelihood that more widespread and possibly less tornadic convection will ensue. The overall results of the balanced diagnosis complement those of other case studies, leading to the formulation of a conceptual model that broadly anticipates how the convective regime will respond to changes in intensity of upper-tropospheric weather features.National Science Foundatio

Initial Modeling of the August 2000 Houston-Galveston Ozone Episode

This report describes MM5 modeling work to date at Texas A&M University, sponsored by the Texas Natural Resource Conservation Commission, with the goal of an accurate, fully validated meteorological simulation delivered by February 28, 2002. Specifically, this report describes the meteorological conditions during the August 25-September 1, 2000, Houston-Galveston ozone episode, describes the modeling philosophy being followed to develop high-quality meteorological fields, describes the MM5 modeling system itself, and describes the results of various experiments conducted over the past few months. Results so far indicate that, with suitable modification to land surface and/or radiative forcing, the MM5 is fully capable of simulating the temperatures and vertical extent of the daytime boundary layer. Low-level nighttime temperatures are too warm, which may lead to overly robust turbulent mixing at night. The present nesting scheme produces very good quality large-scale winds and realistic sea breeze evolution. Future work will focus on determining the accuracy of the model-simulated nighttime wind cycle, and, if necessary, investigating ways of improving the nighttime winds. Differences in behavior of various PBL schemes will be further investigated and evaluated. Wind profiler and Doppler lidar data will be assimilated on a coarse scale, and statistical measures of model accuracy will be used to provide objective measures of model performance. Of all model runs that have been conducted so far, the best performance has been exhibited by the dec6grid4 run, with the MRF planetary boundary layer scheme and an extra sigma layer near the ground.Texas Commission on Environmental Qualit

Meteorological Modeling for the August 2000 Houston-Galveston Ozone Episode: PBL Characteristics, Nudging Procedure, and Performance Evaluation

This report describes evaluations of the performance of various configurations of the MM5 modeling system, as compared to planetary boundary layer (PBL) structure and profiler winds. Soundings from the three sounding sites are grouped by time of day and by regime. Systematic differences between the different model runs are found; the differences between the models and observations vary from site to site. Higher vertical resolution did not produce improved boundary layer structure. The MCNC runs had well-mixed PBL’s, even at night, but were too shallow during the day. Most of the runs with the MRF PBL were similar and performed fairly well. One area of possible concern is the systematic underestimate of the strength of the sea breeze inversion, an error which may lead to too much diffusion of constituents into and out of the advancing marine air. The Gayno-Seaman PBL scheme appeared to be more realistic, but its sea breeze inversion was too strong. Wind errors were computed at a variety of heights, grouped by weather regime, and with 24-hour running means and departures from running means. Most of the model error was associated with the departures from the running means. The MRF PBL schemes tended to perform best overall. All model runs except MCNC developed large biases at heights above 1 km. The MCNC run was worst during Regime 1 but was best during Regime 2 when other model runs produced only a small fraction of the observed low-level jet speeds. Based on these and previously-reported evaluations of various model configurations, a particular configuration was chosen. This configuration uses the MRF PBL with 43 vertical levels and one-way nesting. The soil moisture availability is specified to decrease during the model integration, to simulate evaporation of rain that fell just prior to the ozone episode. A new subroutine was added to the MM5 to permit model restarts with updated soil moisture. The nudging strategy is then outlined. The approach followed here uses a large time window for nudging so as to effectively average out possibly erroneous hour-to-hour variations in low-level winds that were introduced during the quality assurance process. The default value for nudging strength is used. No nudging is performed prior to August 25 because the convection on the previous days are not resolved by the profiler network and any attempt at nudging would cause aliasing in the model fields. The final model run, called the “driver” run, also utilizes lower-tropospheric nudging of water vapor on the 12 km grid. This nudging is designed to suppress a robust outflow boundary which sweeps through Houston on August 31 in the model forecasts but not in the observations. The nudging successfully prevents convection from developing on the 4 km grid and reinforcing the outflow, but a weak wind surge does reach Houston during the evening. The thermodynamic performance of the driver run is very similar to its predecessor runs, since no nudging is applied to the temperature field. The wind field is dramatically improved, at least by comparison to the profiler data. Since this data set was used to nudge the model in the first place, further objective verification of the improvement due to nudging is necessary. The wind and temperature fields during the high ozone days of the episode are examined in detail. On two of the days, August 30 and 31, the model wind and 3 temperature fields have realistic large-scale and small scale features and further improvement is unlikely. Three other days, August 26, August 29, and September 1, are generally accurate but have wind errors which are likely to lead to position errors in the simulation of ozone by a photochemical model. The remaining day, August 25, had erroneous or too-light surface winds. On this day, high values of ozone are likely to be simulated by a photochemical model, but it is possible that the mixture of ozone precursors that leads to the high ozone will be fundamentally different due to the transport errors. In summary, the driver model run produces generally accurate daytime lowertropospheric temperatures and winds. On most days of the episode, the meteorological fields appear to be adequate for driving the particular combination of mixing and chemical processes that lead to high ozone on each of those days.Texas Commission on Environmental Qualit

Evaluation and Comparison of Preliminary Meteorological Modeling for the August 2000 Houston- Galveston Ozone Episode

This report is an evaluation of the quality of MM5 simulations of weather phenomena during the August 2000 Houston-Galveston Ozone episode. The report serves two purposes: first, to help guide final selection of a model configuration, and second, to evaluate the viability of MCNC’s real-time forecasting system as an alternative meteorological model for regulatory work. The TAMU and MCNC modeling efforts are both based on the MM5 model. Primary differences involve the incorporation of analysis information, the size of the domains, the boundary layer parameterizations, and the soil moisture specifications. Data used for the model evaluation include profiler data, surface meteorological data, radiosonde data, radar and satellite imagery, and doppler lidar data. The profiler and doppler lidar data sets are still evolving, and subsequent use of profiler data in this modeling effort will require consideration of both quality-controlled and non-qualitycontrolled data. The analysis of weather phenomena is conducted using the GEMPAK software package and TAMU-written converters and scripts. Overall, the TAMU simulations using the MRF PBL scheme (dec6grid4 and dec30grid4) performed best, with the Gayno-Seaman (dec16grid4) and MCNC simulations deficient in various critical errors. Precipitation was simulated remarkably successfully with the MRF PBL schemes; seven to eight days out of ten had no significant precipitation errors. The Gayno-Seaman run had the proper temporal variation but produced too much precipitation. The MCNC model did not properly simulate the squall line on August 24, produced rain in the wrong place on August 25, and failed to produce any rain at all on succeeding days. None of the model runs produced the observed outflow boundary on the evening of September 1, and all but the MCNC model produced an erroneous outflow boundary on the previous evening. Most clouds during the ozone episode were fair weather cumulus which formed at the top of the boundary layer in the morning and dissipated in late afternoon. None of the model runs can resolve these clouds, and as a result the models produce clouds with too large a horizontal extent. Under such circumstances, the simulation with the fewest clouds is usually the best, and in this case it was the runs with the MRF PBL scheme. None of the models had the proper day-to-day variations in cloudiness. The three TAMU runs had essentially zero maximum temperature bias, while the bias for the MCNC run was on the order of 3 C. This bias likely originates from the use of the default soil moisture. All model runs were able to track day-to-day temperature variations. It is recommended that temporally-varying soil moisture be used in subsequent model runs. Large-scale temperature patterns were generally forecasted well, except for the MCNC model in some cases. The land-sea contrast was well-simulated, except for MCNC, which had too small a contrast. Another major source of variability was the urban heat island, which during this episode was observed to be essentially nil during the day and significantly warmer than its surroundings at night. No model simulation produced a warm nighttime heat island, possibly because no simulation was able to get surrounding areas cool enough. Runs with relatively dry urban soil produce too strong a 3 daytime heat island; runs with relatively moist urban soil produce a late afternoon and evening urban “cool” island. Comparative analysis of soundings confirmed that the models’ nighttime temperature inversions were too weak or nonexistent. The MCNC runs were particularly deficient in that regard. During the day, it was found that the MRF PBL runs produced bias-free mixed-layer depths, while the Gayno-Seaman and MCNC simulations were too shallow by nearly 20% on average. Wind simulations on most days were good; frequently the wind field will evolve into a more accurate configuration as the model atmosphere responds to local forcing. A close look at August 25 and August 30 suggests that one or both might be successfully simulated with a photochemical model using the current, preliminary TAMU grids. The MCNC model had erroneously weak sea breeze winds because land areas were not heating up sufficiently. The vertical structure of the wind was dominated by a diurnally-varying component which seems to have large vertical extent on some days and very shallow extent on others. Further analysis of the diurnal wind cycle and the nocturnal low-level jet will be described in a subsequent report.Texas Commission on Environmental Qualit

The 2011 Texas Drought: A Briefing Packet for the Texas Legislature

The 2011 drought in Texas has been unprecedented in its intensity. The year 2010 had been relatively wet across most of the state, except for extreme eastern Texas. Beginning in October 2010, most of Texas experienced a relatively dry fall and winter, but the record dry March 2011 brought widespread extreme drought conditions to the state. A record dry March through May was followed by a record dry June through August, and the 12-month rainfall total for October 2010 through September 2011 was far below the previous record set in 1956. Average temperatures for June through August were over 2 °F above the previous Texas record and were close to the warmest statewide summer temperatures ever recorded in the United States. As the drought intensified, the previous year’s relatively lush growth dried out, setting the stage for spring wildfires. Conditions were so dry during the spring planting season across much of the state that many crops never emerged from the ground. Continued dry weather through the summer led to increasing hardship for ranchers, who generally saw very little warm-season grass growth while stock tanks dried up. The record warm weather during the summer in Texas was primarily a consequence of the lack of rainfall, but the heat and resulting evaporation further depleted streamflow and reservoir levels. By early fall, trees in central and eastern Texas were showing widespread mortality and dry and windy conditions allowed forest fires to burn intensely and spread rapidly in Bastrop and elsewhere. Twelve-month rainfall was driest on record across much of western, central, and southern Texas, and many stations received less than 25% of their normal 12-month precipitation. The area near, north, and east of Dallas was comparatively well off compared to the rest of the state, but still endured serious drought conditions and record heat. This drought has been the most intense one-year drought in Texas since at least 1895 when statewide weather records begin, and though it is difficult to compare droughts of different durations, it probably already ranks among the five worst droughts overall. The statewide drought index value has surpassed all previous values, and it has been at least forty years since anything close to the severity of the present drought has been experienced across Texas. Because of the return of La Niña conditions in the tropical Pacific, a second year of drought in Texas is likely, which will result in continued drawdown of water supplies. Whether the drought will end after two years or last three years or beyond is impossible to predict with any certainty, but what is known is that Texas is in a period of enhanced drought susceptibility due to global ocean temperature patterns and has been since at least the year 2000. The good news is that these global patterns tend to reverse themselves over time, probably leading to an extended period of wetter weather for Texas, though this may not happen for another three to fifteen years. Looking into the distant future, the safest bet is that global temperatures will continue to increase, causing Texas droughts to be warmer and more strongly affected by evaporation

Potential Vorticity Diagnosis of the Severe Convective Regime. Part II: The Impact of Idealized PV Anomalies

© Copyright 2008 American Meteorological Society (AMS). Permission to use figures, tables, and brief excerpts from this work in scientific and educational works is hereby granted provided that the source is acknowledged. Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act September 2010 Page 2 or that satisfies the conditions specified in Section 108 of the U.S. Copyright Act (17 USC §108, as revised by P.L. 94-553) does not require the AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a web site or in a searchable database, or other uses of this material, except as exempted by the above statement, requires written permission or a license from the AMS. Additional details are provided in the AMS Copyright Policy, available on the AMS Web site located at (https://www.ametsoc.org/) or from the AMS at 617-227-2425 or [email protected] numerical experiments are conducted to understand the effect of upper-tropospheric potential vorticity (PV) anomalies on an environment conducive to severe weather. Anomalies are specified as a single isolated vortex, a string of vortices analogous to a negatively tilted trough, and a pair of string vortices analogous to a position error in a negatively tilted trough. The anomalies are placed adjacent to the tropopause along a strong upper-level jet at a time just prior to a major tornado outbreak and inverted using the nonlinear balance equations. In addition to the expected destabilization beneath and adjacent to a cyclonic PV anomaly, the spatial pattern of the inverted balanced streamfunction and height fields is distorted by the presence of the horizontal PV gradient along the upper-tropospheric jet stream. Streamfunction anomalies are elongated in the cross-jet direction, while height and temperature anomalies are elongated in the along-jet direction. The amplitude of the inverted fields, as well as the changes in CAPE associated with the inverted temperature perturbations, are linearly proportional to the amplitudes of the PV anomalies themselves, and the responses to complex PV perturbation structures are approximated by the sum of the responses to individual simple PV anomalies. This is true for the range of PV amplitudes tested, which was designed to mimic typical 6-h forecast or analysis errors and produced changes in CAPE beneath the trough of well over 100 J kg−1. Impacts on inverted fields are largest when the PV anomaly is on the anticyclonic shear side of the jet, where background PV is small, compared with the cyclonic shear side of the jet, where background PV is large.National Science Foundatio

Historic and Future Droughts in the Big Bend Region of the Chihuahuan Desert

The future of ecosystems in the Big Bend region of the Chihuahuan Desert largely depends on the response to future droughts. Most of the precipitation in the region falls in the 6-month period from May to October due in large part to the Southwestern United States monsoon. There is considerably less interannual variability in precipitation during these 6 months than in the rest of the year. The interannual variations tend to occur simultaneously throughout the region, indicating that drought is a regional-scale rather than a local-scale phenomenon despite the hit-or-miss nature of summertime convection. There is uncertainty in the future of precipitation in the region, but the interannual variability of precipitation is expected to far outweigh any change in the long-term trend. However, future temperatures are expected to significantly increase, so past precipitation and temperature records were examined to determine periods that would serve as analogs for drought in a warmer overall environment. The identification of analogs relies in part on a detailed assessment of station metadata to assess the quality of their precipitation and temperature records. There is a greater availability of quality precipitation records in the region than temperature records, which are more influenced by changes in the land surrounding an observation site. Several future drought analogs were identified for the region. Most of the analogs occurred in the late 1990s or early 2000s. It is likely that some future droughts will occur with temperatures well beyond what has previously been observed in the area.World Wildlife Fun

Potential Vorticity Diagnosis of the Severe Convective Regime. Part III: The Hesston Tornado Outbreak

© Copyright 2008 American Meteorological Society (AMS). Permission to use figures, tables, and brief excerpts from this work in scientific and educational works is hereby granted provided that the source is acknowledged. Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act September 2010 Page 2 or that satisfies the conditions specified in Section 108 of the U.S. Copyright Act (17 USC §108, as revised by P.L. 94-553) does not require the AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a web site or in a searchable database, or other uses of this material, except as exempted by the above statement, requires written permission or a license from the AMS. Additional details are provided in the AMS Copyright Policy, available on the AMS Web site located at (https://www.ametsoc.org/) or from the AMS at 617-227-2425 or [email protected] balance potential vorticity (PV) inversion is used to diagnose the sensitivity of the severe convective parameter space to the amplitude of a subsynoptic-scale PV anomaly on 13 March 1990, a day on which a significant tornado outbreak impacted the Great Plains. PV surgery is used to both amplify and remove the PV anomaly, and the contemporaneous impact on various convective parameters is subsequently quantified by using piecewise PV inversion to compute the changes in those parameters attributable to each PV alteration. It is found that amplifying the anomaly increases the CAPE by amounts typically ranging from 20% to 30% within the atmospheric columns experiencing the maximum PV increase. Ascent is increased slightly downshear of the PV anomaly, consistent with extant conceptual models governing synoptic-scale forcing for vertical motion. Amplifying the PV anomaly increases deep-layer shear over the southern half of the outbreak region and reduces storm-relative helicity over the northern half, primarily through changes in the estimated storm motion vector. Removing the anomaly produces complementary changes of the opposite sign. Thresholds of several commonly used convective parameters are chosen on the basis of prior empirical studies, and the horizontal displacement of these threshold contours produced by the PV alterations reveals that relatively modest subsynoptic-scale PV changes would not likely change the predominant convective mode during the Hesston outbreak.National Science Foundatio

A New Homogenized Climate Division Precipitation Dataset for Analysis of Climate Variability and Climate Change

© Copyright 2011 American Meteorological Society (AMS). Permission to use figures, tables, and brief excerpts from this work in scientific and educational works is hereby granted provided that the source is acknowledged. Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act September 2010 Page 2 or that satisfies the conditions specified in Section 108 of the U.S. Copyright Act (17 USC §108, as revised by P.L. 94-553) does not require the AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a web site or in a searchable database, or other uses of this material, except as exempted by the above statement, requires written permission or a license from the AMS. Additional details are provided in the AMS Copyright Policy, available on the AMS Web site located at (https://www.ametsoc.org/) or from the AMS at 617-227-2425 or [email protected] new homogeneous climate division monthly precipitation dataset [based on full network estimated precipitation (FNEP)] was created as an alternative to the National Climatic Data Center (NCDC) climate division dataset. These alternative climate division monthly precipitation values were estimated using an equal-weighted average of Cooperative Observer Program stations that contained serially complete time series. Missing station observations were estimated by a procedure that was optimized through testing on U.S. Historical Climate Network stations. Inhomogeneities in the NCDC dataset arise from two principal causes. The pre-1931 estimation of NCDC climate division monthly precipitation from statewide averages led to a significant time series discontinuity in several climate divisions. From 1931 to the present, NCDC climate division averages have been calculated from a subset of available station data within each climate division, and temporal changes in the location of available stations have caused artificial changes in the time series. The FNEP climate division dataset is recommended over the NCDC dataset for studies involving climate trends or long-term climate variability. According to the FNEP data, the 1895–2009 linear precipitation trend is positive across most of the United States, and trends exceed 10% per century across the southern plains and the Corn Belt. Remaining inhomogeneities from changes in gauge technology and station location may be responsible for an artificial trend of 1%–3% per century