Abstract

It has been proposed that evaporation of precipitation failing from widespread thick masses of nimbostratus derived from cumulonimbus can account for the mesoscale unsaturated downdrafts inferred to occur within certain tropical mesosystems.

This paper discusses experiments made with a numerical model suitable for testing this idea. Dynamics of the explicitly described (large-scale) flow are governed by the hydrostatic unfiltered equations specialized to two dimensions. The value of f is for 10°N. Cumulus convection is parameterized through a one-dimensional plume model which allows for vertical transport of water substance in vapor and liquid form. The water budget for the large scale includes vapor and both cloud and precipitation and allows for transformations between these categories.

Computations are sensitive to the assigned value of β, the ratio of mass flux upward through the bases of convective clouds to the large-scale upward mass flux through 900 mb. For β=1.0, the initial wave disturbance (wavelength 10³ km) weakens. Rapid deepening of the initial disturbance in runs with β=1.35 and 1.50 is a result of low-level warming by “compensating subsidence” between clouds.

That evap6rative cooling can induce a mesoscale downdraft of 10 cm s⁻¹ is demonstrated by a pair of model computations, one including evaporation and the other not. In the former there develops a mesosystem similar to several recently reported in the literature. Furthermore, evaporation is sufficient to terminate deepening of the initial wave disturbance. Close to half the evaporation is the end result of mesoscale ascent within the large-scale (anvil) cloud. Comparison of computations with observation indicates that evaporation can account for much but possibly not all of the mesoscale subsidence underneath the anvil.

Abstract

A severe thunderstorm which spawned at least four tornadoes, one of them anticyclonic, formed over central Iowa during the afternoon of 13 June 1976. This storm moved toward the east-northeast, approximately parallel to but slower than the mean tropospheric flow. The anticyclonic tornado (F3) and the most intense (F5) of the cyclonic tornadoes coexisted for 23 min and traveled on nearly parallel, cycloidal-like tracks, with the anticyclonic tornado 3–5 km southeast of the cyclonic. The major emphasis of this paper is on this pair of tornadoes and their relationship to the structure and evolution of the parent thunderstorm.

Radar recorded the development of a hook echo just prior to the genesis of the intense cyclonic tornado. A strengthening mesolow was centered somewhere south of this tornado soon after it formed. The mesolow is believed to have initiated a new updraft; the anticyclonic tornado formed in association with this updraft, south of the cyclonic tornado. It is hypothesized that the mesolow was responsible (through alteration of the storm-scale airflow) for the nearly simultaneous sharp right turns made by these tornadoes. Each of these tornadoes was observed to diminish in intensity soon after becoming associated with heavy rain.

It is argued that the parent thunderstom's distinctive airflow and thermodynamic structure at low levels provided a more favorable setting for the amplification of anticyclonic vorticity than is typical of most severe thunderstorms.

In this historical paper, we trace the scientific-and engineering-based steps at the National Severe Storms Laboratory (NSSL) and in the larger weather radar community that led to the development of NSSL's first 10-cm-wavelength pulsed Doppler radar. This radar was the prototype for the current Next Generation Weather Radar (NEXRAD), or Weather Surveillance Radar-1998 Doppler (WSR-88D) network.

We track events, both political and scientific, that led to the establishment of NSSL in 1964. The vision of NSSL's first director, Edwin Kessler, is reconstructed through access to historical documents and oral histories. This vision included the development of Doppler radar, where research was to be meshed with the operational needs of the U.S. Weather Bureau and its successor—the National Weather Service.

Realization of the vision came through steps that were often fitful, where complications arose due to personnel concerns, and where there were always financial concerns. The historical research indicates that 1) the engineering prowess and mentorship of Roger Lhermitte was at the heart of Doppler radar development at NSSL; 2) key decisions by Kessler in the wake of Lhermitte's sudden departure in 1967 proved crucial to the ultimate success of the project; 3) research results indicated that Doppler velocity signatures of mesocyclones are a precursor of damaging thunderstorms and tornadoes; and 4) results from field testing of the Doppler-derived products during the 1977-79 Joint Doppler Operational Project—especially the noticeable increase in the verification of tornado warnings and an associated marked decrease in false alarms—led to the government decision to establish the NEXRAD network.

Abstract

This study compares several formulations parameterizing the surface moisture flux and boundary-layer processes using the θ-σ hybrid-b model of the Mesoscale Analysis and Prediction System (MAPS) within both 1D and 3D frameworks.

A modified formula for computing the surface moisture flux is proposed based on the assumption that the layer below the lowest model computational level can be represented by three “physical” layers, of which the bottom one is the molecular layer. This three-layer aerodynamic (3LAD) scheme is compared with two-layer aerodynamic (2LAD) as well as flux matching and Penman-Monteith potential evapotranspiration (PM) schemes. Both a 10-day forecast period (3D) and case simulations demonstrate that the 3LAD scheme gives the best prediction in latent heat flux from the ground and mixing ratio in the atmosphere. The moisture flux produced by the 2LAD scheme is too large, especially over warm and moist surfaces. The mean 12-h forecast rms errors in relative humidity at the surface (10 m AGL) are 15.6%, 21.5%, and 26.0%, respectively, for the 3LAD, PM, and 2LAD schemes in a 10-day parallel test period using MAPS.

For the boundary-layer parameterization, the Mellor-Yamada level 2.0 turbulence scheme (MY) and Blackadar convective scheme are compared. Results show that the MY scheme gives more reasonable boundary-layer structure and smaller rms forecast errors.

Abstract

This study compares three modifications to the one-dimensional planetary boundary layer scheme that is implemented in the σ–θ hybrid-b version of the Mesoscale Analysis and Prediction System (MAPS) and the Rapid Update Cycle (RUC). All three modifications are based on the incorporation of a simple soil model into the basic version to more accurately calculate the moisture and heat fluxes across the ground surface. The presented schemes are of increasing sophistication: the first model combines the soil model with heat and moisture budget equations for the ground surface and uses an explicit numerical scheme to compute the surface fluxes; the second model uses a more energy-conservative implicit solution for the latent and sensible surface fluxes and heat and moisture soil fluxes; the third model further incorporates a simple parameterization of the evapotranspiration process.

The comparison includes the effect of different schemes on diurnal changes of surface temperature and soil heat flux. The schemes are tested for two case studies; a dry case from the O’Neill, Nebraska, Great Plains Turbulence Field Program and a moist case from the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment. Tests are performed to evaluate sensitivity to soil parameters related to thermal diffusivity and to vertical resolution of the soil scheme. Overall, the comparison supports the idea that implementation of a multilevel soil model is competitive with and can even improve the ground surface temperature forecast over that produced by the present MAPS implementation of the force restore method. The case study demonstrates that incorporation of a primitive evapotranspiration model can give positive results.

Abstract

The hypothesis that inertial instability plays a role in the upscale development of mesoscale convective systems (MCSs) is explored by sampling environments that supported the growth of MCSs in the Preliminary Regional Experiment for STORM (Stormscale Operational and Research Meteorology) (PRE-STORM) network with high quality special soundings. Secondary circulations that occurred in the presence of inertial instabilities were analyzed and documented using rawinsonde data with high spatial and temporal resolution from the PRE-STORM field program. Additional examples of MCS environments were examined using data from the Mesoscale Analysis and Prediction System. Results show strong divergence and cross-stream accelerations occurred at upper-tropospheric levels where inertial instabilities were present. These accelerations were not uniform over the domain but were focused in the regions of instability. Also, the analyses of these data showed that regions of inertial instability may be more commonplace than is typically assumed.

The Regional Atmospheric Modeling System was used to increase the understanding of the basic processes and secondary circulations that enhance MCS growth in inertially unstable environments. Model results indicate that the strength of the divergent outflow was strongly linked to the degree of inertial stability in the local environment. The results also showed a strong dependence on the magnitude of the Coriolis parameter. Finally, experiments using varying degrees of vertical stability indicated that there was also significant sensitivity to this parameter.

Abstract

It can be shown, theoretically, that the polarization properties of laser light scattered by a volume of air containing aerosols include considerable information as to the size distribution of the aerosols. A theoretical inversion model, utilizing the above information, is developed, which uses the Stokes parameters of the angularly scattered laser light as input data. These input data are generated theoretically from assumed size distribution functions of the aerosols. Both “perfect” measurements and measurements into which random errors are introduced are employed. These data are then used in the inversion model to generate predicted size distribution functions. Numerical experiments are performed with 0, 1 and 2% random error in the observations, in order to determine what accuracy is required in the lidar measurements. Comparisons between the actual and predicted functions are then made in order to assess the accuracy of the model.

Abstract

Observed regional rainfall characteristics can be analyzed by examining both the frequency and intensity of different categories of rainfall. A complementary approach is to consider rainfall characteristics associated with regional synoptic regimes. These two approaches are combined here to examine daily rainfall characteristics over the Australian region, providing a target for model simulations. Using gridded daily rainfall data for the period 1997–2007, rainfall at each grid point and averaged over several sites is decomposed into the frequency of rainfall events and the intensity of rainfall associated with each event. Daily sea level pressure is classified using a self-organizing map, and rainfall on corresponding days is assigned to the resulting synoptic regimes. This technique is then used to evaluate rainfall in the new Australian Community Climate and Earth-System Simulator (ACCESS) global climate model and separate the influence of large-scale circulation errors and errors due to the representation of subgrid-scale physical processes. The model exhibits similar biases to many other global climate models, simulating too frequent light rainfall and heavy rainfall of insufficient intensity. These errors are associated with particular synoptic regimes over different sectors of the Australian continent and surrounding oceans. The model simulates only weak convective rainfall over land during the summer monsoon, and heavy rainfall associated with frontal systems over southern Australia is also not simulated. As the model captures the structure and frequency of synoptic patterns, but not the associated rainfall intensity or frequency, it is likely that the source of the rainfall errors lies in model physical parameterizations rather than large-scale dynamics.

Abstract

Case studies of heavy snowstorms at Denver and Colorado Springs, Colorado, indicate that they occur under different meteorological conditions. The authors examine the hypothesis that there are in fact fundamental differences between the synoptic evolution of events in these two storm types by compositing a total of 28 cases, 17 (11) of which are defined as heavy snowstorms (at least 20 cm of snowfall) at Denver (Colorado Springs). These composited fields were constructed using data at three times in the history of each case. Results show distinct differences in the composited synoptic evolution of the two groups. At low levels the Denver composite shows low static stabilities, warm advection, and high values of potential temperature in the lee of the Rockies. The Colorado Springs composite, on the other hand, shows cold, stable air and cold advection in the lee. At upper levels an eastward-progressing short-wave trough is found at different longitudes in the two composites.

The implied interaction between lower and upper levels of the two composites is also very different. For the Denver composite, the trajectory of the upper-level trough brings it close to the area of low static stability and high surface potential temperature at low levels. This implies strong interaction between the upper-level system and the warm unstable air at low levels and dramatic cyclogenesis east of the Rocky Mountains, typically in southeast Colorado. In contrast, the upper short-wave trough in the Colorado Springs composite is farther north, and a layer of cool stable air is found on the High Plains of Colorado. Not surprisingly, surface cyclogenesis is notably weaker in this composite. These conclusions, substantiated by inspection of the individual cases, have obvious implications for predicting the location of heavy snow along the Front Range of Colorado.

Abstract

The Rapid Refresh (RAP) and High-Resolution Rapid Refresh (HRRR), both operational at NOAA’s National Centers for Environmental Prediction (NCEP) use the Thompson et al. mixed-phase bulk cloud microphysics scheme. This scheme permits predicted surface precipitation to simultaneously consist of rain, snow, and graupel at the same location under certain conditions. Here, the explicit precipitation-type diagnostic method is described as used in conjunction with the Thompson et al. scheme in the RAP and HRRR models. The postprocessing logic combines the explicitly predicted multispecies hydrometeor data and other information from the model forecasts to produce fields of surface precipitation type that distinguish between rain and freezing rain, and to also portray areas of mixed precipitation. This explicit precipitation-type diagnostic method is used with the NOAA operational RAP and HRRR models. Verification from two winter seasons from 2013 to 2015 is provided against METAR surface observations. An example of this product from a January 2015 south-central United States winter storm is also shown.