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W.-K. Tao, T. Iguchi, and S. Lang

Abstract

The Goddard convective–stratiform heating (CSH) algorithm has been used to retrieve latent heating (LH) associated with clouds and cloud systems in support of the Tropical Rainfall Measuring Mission and Global Precipitation Measurement (GPM) mission. The CSH algorithm requires the use of a cloud-resolving model to simulate LH profiles to build lookup tables (LUTs). However, the current LUTs in the CSH algorithm are not suitable for retrieving LH profiles at high latitudes or winter conditions that are needed for GPM. The NASA Unified-Weather Research and Forecasting (NU-WRF) Model is used to simulate three eastern continental U.S. (CONUS) synoptic winter and three western coastal/offshore events. The relationship between LH structures (or profiles) and other precipitation properties (radar reflectivity, freezing-level height, echo-top height, maximum dBZ height, vertical dBZ gradient, and surface precipitation rate) is examined, and a new classification system is adopted with varying ranges for each of these precipitation properties to create LUTs representing high latitude/winter conditions. The performance of the new LUTs is examined using a self-consistency check for one CONUS and one West Coast offshore event by comparing LH profiles retrieved from the LUTs using model-simulated precipitation properties with those originally simulated by the model. The results of the self-consistency check validate the new classification and LUTs. The new LUTs provide the foundation for high-latitude retrievals that can then be merged with those from the tropical CSH algorithm to retrieve LH profiles over the entire GPM domain using precipitation properties retrieved from the GPM combined algorithm.

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Mark S. Kulie and Yuh-Lang Lin

Abstract

The structure and evolution of a high-precipitation (HP) supercell thunderstorm is investigated using a three-dimensional, nonhydrostatic, cloud-scale numerical model (TASS). The model is initialized with a sounding taken from a mesoscale modeling study of the environment that produced the 28 November 1988 Raleigh tornadic thunderstorm. TASS produces a long-lived convective system that compares favorably with the observed Raleigh tornadic thunderstorm. The simulated storm evolves from a multicell-type storm to a multiple-updraft supercell storm. The storm complex resembles a hybrid multicell-supercell thunderstorm and is consistent with the conceptual model of cool season strong dynamic HP supercells that are characterized by shallow mesocyclones. The origin of rotation in this type of storm is often in the lowest levels.

Interactions between various cells in the simulated convective system are responsible for the transition to a supercellular structure. An intense low-level updraft core forms on the southwest flank of the simulated storm and moves over a region that is rich in vertical vorticity. The stretching of this preexisting vertical vorticity in the storm’s lowest levels is the most important vertical vorticity production mechanism during the initial stages of the main updraft’s development. Interactions with an extensive cold pool created by the storm complex are also important in producing vertical vorticity as the main updraft grows. Overall, the development of vorticity associated with the main updraft appears similar to nonsupercellular tornadic storms. However, classic supercell signatures are seen early in the simulation associated with other updrafts (e.g., formation of vortex couplet due to tilting of ambient horizontal vorticity, storm splitting, etc.) and are deemed important.

In the storm’s supercell stage, rotation is sustained in the lowest levels of the storm despite large amounts of precipitation located near and within the main mesocyclone. Pulsating downdrafts periodically invigorate the storm and the gust front never occludes, thus allowing the main updraft to persist for a prolonged period of time. The storm’s intensity is also maintained by frequent updraft mergers.

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S. Lang, W-K. Tao, J. Simpson, and B. Ferrier

Abstract

Six different convective–stratiform separation techniques are compared and evaluated using 2D numerical simulations of a tropical and a midlatitude continental squall line. The techniques used include a texture algorithm applied to surface rainfall, a similar algorithm but with additional criteria applied to vertical velocity and cloud, a texture algorithm applied to vertical velocities below the melting layer, a simple approach that assumes a constant characteristic width for the convective region, a more sophisticated texture algorithm applied to radar reflectivities below the melting layer, and a new technique based on the premise that the fall speed of precipitation particles is large relative to air velocity in regions of stratiform precipitation. Comparisons are made in terms of rainfall, mass fluxes, apparent heating and moistening, hydrometeor contents, reflectivity and vertical-velocity contoured-frequency-with-altitude diagrams (CFAD), microphysics, and latent heating retrieval. Overall, it was found that the different separation techniques produced results that qualitatively agreed. However, the quantitative differences were significant. The texture algorithm applied to surface rain consistently produced the most stratiform rain while the texture algorithm applied to radar reflectivities below the melting layer and the new method comparing air velocities to terminal velocities consistently produced the most convective rain. The simple constant-area method performed comparably to the others in this squall line setting. Observational comparisons within the context of the model were unable to identify a superior technique. However, all of the methods were able to generate CFADs that were consistent with observations. Latent heating retrieval was shown to be sensitive to the use of separation technique mainly as a result of differences in the stratiform region. Methods that found very little stratiform rain resulted in exaggerated rain-normalized stratiform heating profiles.

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S. Lang, W-K. Tao, J. Simpson, and B. Ferrier
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Yuh-Lang Lin, Roy L. Deal, and Mark S. Kulie

Abstract

In this study, mechanisms of cell regeneration, development, and propagation within a two-dimensional multicell storm are investigated using a numerical cloud model. The cell regeneration is explained by the advection mechanism. The following processes occur periodically during cell regeneration: (i) Near the edge of the gust front, the gust front updraft is formed by low-level convergence ahead of the gust front near the surface. (ii) The upper portion of the gust front updraft grows by midlevel inflow since the gust front propagates faster than the basic wind. (iii) The growing cell tends to produce and is flanked by strong compensating downdrafts. The upstream downdraft tends to cut off the growing cell from the gust front updraft. It is found that the period of cell regeneration is inversely proportional to the midlevel, strong relative wind speed. This advection mechanism is different from that proposed by Yang and Houze, which views the rearward propagating cell as gravity waves generated by the quasi-steady updraft moving through the ambient flow.

Cell development and propagation within a two-dimensional multicell storm may be described in terms of two distinctive modes: (i) a growing mode and (ii) a propagating mode. When a growing cell reaches its maximum intensity, it splits and then propagates downstream without amplification. The dynamics of cell development and propagation is explained here by critical level argument. For the growing mode there is growth because of a conditionally unstable environment leading to steering level propagation, while for the propagating mode there is no growth because of a more stable environment leading to propagation relative to the flow (i.e., absence of critical level). It is found that the phase relationship between w′ and θ′ (w′ and u′) in the growing mode is different from that in the propagating mode and can be explained by the dominance of latent heating in the thermodynamic equation. The propagating mode is dominated by horizontal advection. The propagating mode exhibits gravity wave properties and propagates faster than the growing mode.

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S. Lang, W-K. Tao, J. Simpson, R. Cifelli, S. Rutledge, W. Olson, and J. Halverson

Abstract

The 3D Goddard Cumulus Ensemble model is used to simulate two convective events observed during the Tropical Rainfall Measuring Mission Large-Scale Biosphere–Atmosphere (TRMM LBA) experiment in Brazil. These two events epitomized the type of convective systems that formed in two distinctly different environments observed during TRMM LBA. The 26 January 1999 squall line formed within a sheared low-level easterly wind flow. On 23 February 1999, convection developed in weak low-level westerly flow, resulting in weakly organized, less intense convection. Initial simulations captured the basic organization and intensity of each event. However, improvements to the model resolution and microphysics produced better simulations as compared to observations. More realistic diurnal convective growth was achieved by lowering the horizontal grid spacing from 1000 to 250 m. This produced a gradual transition from shallow to deep convection that occurred over a span of hours as opposed to an abrupt appearance of deep convection. Eliminating the dry growth of graupel in the bulk microphysics scheme effectively removed the unrealistic presence of high-density ice in the simulated anvil. However, comparisons with radar reflectivity data using contoured-frequency-with-altitude diagrams (CFADs) revealed that the resulting snow contents were too large. The excessive snow was reduced primarily by lowering the collection efficiency of cloud water by snow and resulted in further agreement with the radar observations. The transfer of cloud-sized particles to precipitation-sized ice appears to be too efficient in the original scheme. Overall, these changes to the microphysics lead to more realistic precipitation ice contents in the model. However, artifacts due to the inability of the one-moment scheme to allow for size sorting, such as excessive low-level rain evaporation, were also found but could not be resolved without moving to a two-moment or bin scheme. As a result, model rainfall histograms underestimated the occurrence of high rain rates compared to radar-based histograms. Nevertheless, the improved precipitation-sized ice signature in the model simulations should lead to better latent heating retrievals as a result of both better convective–stratiform separation within the model as well as more physically realistic hydrometeor structures for radiance calculations.

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W-K. Tao, S. Lang, W. S. Olson, R. Meneghini, S. Yang, J. Simpson, C. Kummerow, E. Smith, and J. Halverson

Abstract

This paper represents the first attempt to use Tropical Rainfall Measuring Mission (TRMM) rainfall information to estimate the four-dimensional latent heating structure over the global Tropics for one month (February 1998). The mean latent heating profiles over six oceanic regions [Tropical Ocean and Global Atmosphere (TOGA) Coupled Ocean–Atmosphere Response Experiment (COARE) Intensive Flux Array (IFA), central Pacific, South Pacific Convergence Zone (SPCZ), east Pacific, Indian Ocean, and Atlantic Ocean] and three continental regions (South America, central Africa, and Australia) are estimated and studied. The heating profiles obtained from the results of diagnostic budget studies over a broad range of geographic locations are used to provide comparisons and indirect validation for the heating algorithm–estimated heating profiles. Three different latent heating algorithms, the Goddard Space Flight Center convective–stratiform heating (CSH), the Goddard profiling (GPROF) heating, and the hydrometeor heating (HH) algorithms are used and their results are intercompared. The horizontal distribution or patterns of latent heat release from the three different heating retrieval methods are very similar. They all can identify the areas of major convective activity [i.e., a well-defined Intertropical Convergence Zone (ITCZ) in the Pacific, a distinct SPCZ] in the global Tropics. The magnitudes of their estimated latent heating release are also in good agreement with each other and with those determined from diagnostic budget studies. However, the major difference among these three heating retrieval algorithms is the altitude of the maximum heating level. The CSH algorithm–estimated heating profiles only show one maximum heating level, and the level varies among convective activity from various geographic locations. These features are in good agreement with diagnostic budget studies. A broader maximum of heating, often with two embedded peaks, is generally derived from applications of the GPROF heating and HH algorithms, and the response of the heating profiles to convective activity is less pronounced. Also, GPROF and HH generally yield heating profiles with a maximum at somewhat lower altitudes than CSH. The impact of different TRMM Microwave Imager (TMI) and precipitation radar (PR) rainfall information on latent heating structures was also examined. The rainfall estimated from the PR is smaller than that estimated from the TMI in the Pacific (TOGA COARE IFA, central Pacific, SPCZ, and east Pacific) and Indian Oceans, causing weaker latent heat release in the CSH algorithm–estimated heating. In addition, the larger stratiform amounts derived from the PR over South America and Australia consequently lead to higher maximum heating levels. Sensitivity tests addressing the appropriate selection of latent heating profiles from the CSH lookup table were performed.

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Steven A. Amburn, Andrew S. I. D. Lang, and Michael A. Buonaiuto

Abstract

An elegant and easy to implement probabilistic quantitative precipitation forecasting model that can be used to estimate the probability of exceedance (POE) is presented. The model was built using precipitation data collected across eastern Oklahoma and northwestern Arkansas from late 2005 through early 2013. The dataset includes precipitation analyses at 4578 contiguous, 4 km × 4 km grid cells for 1800 precipitation events of 12 h. The dataset is unique in that the meteorological conditions for each 12-h event were relatively homogeneous when contrasted with single-point data obtained over months or years where the meteorological conditions for each rain event could have varied widely. Grid cells were counted and stratified by precipitation amount in increments of 0.05 in. (1.27 mm) up to 10 in. (254 mm), yielding histograms for each event. POEs were computed from the observed precipitation distributions and compared to POEs computed from two gamma probability density functions ( and ). The errors between the observed POEs and gamma-computed POEs ranged between 2% and 10%, depending on the threshold POE selected for the comparison. This accuracy suggests the gamma models could be used to make reasonably accurate estimates of POE, given the percent areal coverage and the mean precipitation over the area. Finally, it is suggested that the areal distribution for each event is representative of the distribution at any point in the area over a large number of similar events. It then follows that the gamma models can be used to make forecasts for the probability of exceedance at a point, given the probability of rain and the expected mean rainfall at that same point.

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Jordan G. Powers, Kelly K. Werner, David O. Gill, Yuh-Lang Lin, and Russ S. Schumacher

Abstract

The Weather Research and Forecasting (WRF) Model is a numerical weather prediction model supported by the National Center for Atmospheric Research (NCAR) to a worldwide community of users. In recognition of the growing use of cloud computing, NCAR is now supporting the model in cloud environments. Specifically, NCAR has established WRF setups with select cloud service providers and produced documentation and tutorials on running WRF in the cloud. Described here are considerations in WRF cloud use and the supported resources, which include cloud setups for the WRF system and a cloud-based tool for model code testing.

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W-K. Tao, S. Lang, J. Simpson, C-H. Sui, B. Ferrier, and M-D. Chou

Abstract

Radiative forcing and latent heat associated with precipitation are the two most important diabatic processes that drive the circulation of the atmosphere. Clouds can affect radiation and vice versa. It is known that longwave radiative processes can enhance precipitation in cloud systems. This paper concentrates on determining the relative importance of three specific longwave radiative mechanisms by comparing cloud-resolving models with and without one or more of these processes. Three of the ways that longwave radiation is thought to interact with clouds are as follows: 1) cloud-top cooling and cloud-base warming may alter the thermal stratification of cloud layers, 2) differential cooling between clear and cloudy regions might enhance convergence into the cloud system, and 3) large-scale cooling could change the environment. A two-dimensional version of the Goddard Cumulus Ensemble model has been used to perform a series of sensitivity tests to identify which is the dominant cloud-radiative forcing mechanism with respect to the organization, structure, and precipitation processes for both a tropical (EMEX) and a midlatitude (PRE-STORM) mesoscale convective system.

The model results indicate that the dominant process for enhancing the surface precipitation in both the PRE-STORM and EMEX squall cases is the large-scale radiative cooling. However, the overall effect is really to increase the relative humidity and not tie convective available potential energy (CAPE). Because of the high moisture in the Tropics, the increase in relative humidity by radiative cooling can have more of an impact on precipitation in the tropical case than in the midlatitude case. The large-scale cooling led to a 36% increase in rainfall for the tropical cast. The midlatitude model squall with a higher CAPE and lower humidity environment was only slightly affected (8%) by any of the longwave mechanisms. Our results also indicated that the squall systems' overall (convective and stratiform) precipitation is increased by turning off the cloud-top cooling and cloud-base warming. Therefore, the cloud-top cooling-cloud-base warming mechanism was not the responsible cloud-radiative mechanism for enhancing the surface precipitation. However, the circulation as well as the microphysical processes were indeed (slightly) enhanced in the stratiform region by the cloud-top cooling and cloud-base warming mechanism for the midlatitude squall case.

For both cases, the model results show that the mechanism associated with differential cooling between the clear and cloudy regions may or may not enhance precipitation processes. However, this mechanism is definitely less important than the large-scale longwave radiative cooling. Solar heating was run from 0900 to 1300 LST in both environments and was found to decrease the precipitation by 7% in each case compared to the runs with longwave radiation only. This result suggests that solar heating may play a significant role in the daytime minimum/nighttime maximum precipitation cycle found over most oceans.

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