Numerous modifications to the Kain‐Fritsch convective parameterization have been implemented over the last decade. These modifications are described, and the motivating factors for the changes are discussed. Most changes were inspired by feedback from users of the scheme (primarily numerical modelers) and interpreters of the model output (mainly operational forecasters). The specific formulation of the modifications evolved from an effort to produce desired effects in numerical weather prediction while also rendering the scheme more faithful to observations and cloud-resolving modeling studies.

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The Kain–Fritsch Convective Parameterization: An Update

From a societal, weather, and climate perspective, precipitation intensity, duration, frequency, and phase are as much of concern as total amounts, as these factors determine the disposition of precipitation once it hits the ground and how much runs off. At the extremes of precipitation incidence are the events that give rise to floods and droughts, whose changes in occurrence and severity have an enormous impact on the environment and society. Hence, advancing understanding and the ability to model and predict the character of precipitation is vital but requires new approaches to examining data and models. Various mechanisms, storms and so forth, exist to bring about precipitation. Because the rate of precipitation, conditional on when it falls, greatly exceeds the rate of replenishment of moisture by surface evaporation, most precipitation comes from moisture already in the atmosphere at the time the storm begins, and transport of moisture by the storm-scale circulation into the storm is vital....

/pdf/the-changing-character-of-precipitation-5gto4ulnge.pdf

The Changing Character of Precipitation

The conservation and distribution of water substance in atmospheric circulations is considered within a frame of continuity principles, model air flows, and models of microphysical processes. The simplest considerations of precipitation involve its vertical distribution in an updraft column, where condensate appears immediately as precipitation with uniform terminal fallspeed. The study also treats steady two-dimensional air circulations in which time-dependent distributions of water vapor, cloud and precipitation respond to model microphysical processes.

On the distribution and continuity of water substance in atmospheric circulations

An approach to forecasting the potential for flash flood-producing storms is developed, using the notion of basic ingredients. Heavy precipitation is the result of sustained high rainfall rates. In turn, high rainfall rates involve the rapid ascent of air containing substantial water vapor and also depend on the precipitation efficiency. The duration of an event is associated with its speed of movement and the size of the system causing the event along the direction of system movement. This leads naturally to a consideration of the meteorological processes by which these basic ingredients are brought together. A description of those processes and of the types of heavy precipitation-producing storms suggests some of the variety of ways in which heavy precipitation occurs. Since the right mixture of these ingredients can be found in a wide variety of synoptic and mesoscale situations, it is necessary to know which of the ingredients is critical in any given case. By knowing which of the ingredients...

/pdf/flash-flood-forecasting-an-ingredients-based-methodology-1l6x7t3b0b.pdf

Flash Flood Forecasting: An Ingredients-Based Methodology

A physically based conceptual framework is put forward that explains why an increase in heavy precipitation events should be a primary manifestation of the climate change that accompanies increases in greenhouse gases in the atmosphere. Increased concentrations of greenhouse gases in the atmosphere increase downwelling infrared radiation, and this global heating at the surface not only acts to increase temperatures but also increases evaporation which enhances the atmospheric moisture content. Consequently all weather systems, ranging from individual clouds and thunderstorms to extratropical cyclones, which feed on the available moisture through storm-scale moisture convergence, are likely to produce correspondingly enhanced precipitation rates. Increases in heavy rainfall at the expense of more moderate rainfall are the conseqUence along with increased runoff and risk of flooding. However, because of constraints in the surface energy budget, there are also implications for the frequency and/or efficiency of precipitation. It follows that increased attention should be given to trends in atmospheric moisture content, and datasets on hourly precipitation rates and frequency need to be developed and analyzed as well as total accumulation.

Conceptual Framework for Changes of Extremes of the Hydrological Cycle with Climate Change

The initiation of thunderstorms is examined through a combined observational and modeling case study. The study is based on Doppler radar, aircraft, mesonet, balloon sounding, and profiler and photographic data from the Convection Initiation and Downburst Experiment (CINDE) conducted near Denver, Colorado. The study examines the initiation of a line of thunderstorms that developed along a preexisting, quasi-stationary boundary-layer convergence line on 17 July 1987. The storms were triggered at the intersection of the convergence line with horizontal rolls where enhanced updrafts were present. The primary effect of the convergence line was to deepen the moist layer locally and provide a region potentially favorable to deep convection. The critical factor governing the time of storm development was apparently related to the attainment of a balance between horizontal vorticity in the opposing flows on either side of the convergence line. The effect was to cause the updrafts in the convergence line ...

https://opensky.ucar.edu:/islandora/object/articles%3A3268/datastream/PDF/view

The Role of Boundary-Layer Convergence Zones and Horizontal Rolls in the Initiation of Thunderstorms: A Case Study

The precipitation efficiency (the ratio of surface rainfall to water vapor inflow) of a small sample of thunderstorms observed in the Cooperative Convective Precipitation Experiment is calculated using surface and cloud-base airflow and moisture measurements and subcloud rainout based on radar reflectivity factor. Highly-resolved vertical flux measurements from aircraft indicate that a significant amount of water vapor inflow may have been overlooked in past work of this kind, resulting in overestimates in precipitation efficiency. Trends in the mass of water vapor influx resolved at intervals of 10 to 30 min are corroborated by the evolution of water vapor flux convergence computed at 5-min intervals from objectively analyzed surface mesonetwork observations. Fluxes of water vapor inflow and precipitated rainwater are integrated over periods exceeding an hour to obtain precipitation efficiencies applicable to the mature storm phase. Precipitated rainwater estimates from radar reflectivity-rainra...

Estimates of Thunderstorm Precipitation Efficiency from Field Measurements in CCOPE

A case study is presented of a persistent thunderstorm of moderate intensity which occurred in northeast Colorado, and which produced a light hailfall at the ground. The storm was intensively monitored by aircraft, radar, dropsondes, and surface and upper air networks involved in the National Hail Research Experiment. The present study emphasizes the measurements obtained by instrumented aircraft in examining the properties of the subcloud airflow. Large-scale analysis shows that the storm formed on a surface confluence line and subsequently moved toward the region of surface moisture. A brief radar analysis of the storm during the mature stage of its ∼5-hr lifetime is presented, and identifies the general features as belonging to the category termed “supercell” by previous workers. A precipitation analysis is carried out, and rain and hailfall are correlated with the track of the storm. Liberal use of a time-space conversion technique results in detailed mesoscale pressure, temperature, moisture...

Airflow and Moisture Budget Beneath a Northeast Colorado Hailstorm

In typical squall-line situations wherein the wind veers strongly with height, individual convective storms move as much as 60 deg right or 30 deg left of the direction of the mean wind in the cloud layer. It is shown that, on the average, the radar echoes having largest diameters move farthest to right of the wind. This behavior is consistent with physical considerations and with supply-and-demand requirements of the storm water budget. For a given rainfall intensity, the amount of water precipitated by a storm is proportional to its area or to the diameter squared. The amount of water vapor intercepted is proportional directly to the diameter, and to the velocity of the storm relative to the winds of the lower-tropospheric moist layer. A large storm must intercept more vapor in proportion to its diameter than a small one, requiring a larger migration velocity relative to the moist layer. This requirement is satisfied if (wind veering with height) large storms move toward the right of the mean w...

On the Movements of Convective Storms, with Emphasis on Size Discrimination in Relation to Water-Budget Requirements

Aircraft measurements at low- and mid-cloud levels near two isolated and persistent Great Plains thunderstorms concurrently scanned by radar are analyzed to determine the kinematic properties of the near-cloud air flow, the moisture budget, and the dynamical interactions between the cloud and its surroundings. Time variation in velocity divergence, relative vorticity, and moisture flux convergence in the subcloud layer relate well to changes in storm development and translation. Aircraft winds and radar chaff trajectories substantiate the premise in some models that mature thunderstorms, moving more slowly than ambient winds, divert and distort mid-tropospheric air motion in a manner similar to solid obstacles in relative streaming flow. Observed ingestion of mid-tropospheric air motion tracers demonstrates that, at the same time, internal circulations aloft are not entirely insulated from the environment. A graphical synthesis of three-dimensional air flow within and around a typical Great Plain...

James C. Fankhauser

Papers

The Role of Boundary-Layer Convergence Zones and Horizontal Rolls in the Initiation of Thunderstorms: A Case Study

Estimates of Thunderstorm Precipitation Efficiency from Field Measurements in CCOPE

Airflow and Moisture Budget Beneath a Northeast Colorado Hailstorm

On the Movements of Convective Storms, with Emphasis on Size Discrimination in Relation to Water-Budget Requirements

Thunderstorm-environment interactions determined from aircraft and radar observations