Atmospheric Convection: Research and Operational Forecasting Aspects

The nature of atmospheric convection is briefly reviewed. We can assess the stability of a single air parcel with respect to vertical displacements by comparing the lapse rate of the parcel’s environment to the rate of temperature change within the displaced parcel owing to adiabatic expansion or compression and latent heating or chilling. We also can examine the tendency for convective overturning in a global sense, when buoyancy sources are distributed over a large area and the entire fluid is engaged in convective overturning. In this case, the onset of global dry convection due to thermal instability is determined by the Rayleigh number. Within the atmospheric boundary layer on a sunny day, the Rayleigh number is several orders of magnitude larger than the critical Rayleigh number; thus, convective overturning is a ubiquitous characteristic of the atmospheric boundary layer in sunny conditions. The structure of dry atmospheric convection depends to a large degree on the vertical wind shear within the atmospheric boundary layer, and quite possibly also is sensitive to surface characteristics and mean vertical motions.

The origins of the buoyancy force and the vertical perturbation pressure gradient force are reviewed. A discussion of parcel theory and its limitations follows. The quantitative determination of updraft velocity from environmental soundings is complicated by factors such as environmental heterogeneity, hydrometeor loading, freezing, entrainment, and vertical perturbation pressure gradients.

The momentum equations can be written in a form whereby pressure is replaced by a deviation of pressure from a hydrostatic, horizontally homogeneous base state. This pressure perturbation has both hydrostatic and nonhydrostatic parts owing to density deviations from the horizontally homogeneous base state and three-dimensional wind velocity gradients, respectively. The pressure perturbation also can be decomposed into what are referred to as a dynamic pressure perturbation and a buoyancy pressure perturbation. It is shown that dynamic pressure perturbations arise when deformation or vorticity are present in the velocity field. Buoyancy pressure perturbations arise when vertical buoyancy gradients exist.

The conditions necessary for the development of deep moist convection are discussed. This material is followed by an overview of the ways in which convective storms may be organized, with emphasis on the environmental characteristics favoring the various organizational modes.

The environments and characteristics of supercell thunderstorms are reviewed, followed by a theoretical discussion of the dynamical origins of supercell rotation and propagation. Supercell thunderstorm environments are characterized by large vertical wind shear. The large wind shear promotes longevity, organization, and severity for several reasons: (1) large storm-relative winds associated with the wind shear minimize the degree to which precipitation interferes with the updraft; (2) large horizontal vorticity associated with the wind shear is tilted into the vertical to produce a midlevel mesocyclone; (3) the rotation within the updraft and the interaction of the updraft with the large environmental wind shear give rise to strong dynamic pressure gradients which can feed back to the intensity and organization of the supercell.

Tornado structure and dynamics are reviewed. The conditions leading to tornadogenesis also are discussed, as are some of the latest forecasting techniques used to discriminate between tornadic and nontornadic supercells.

In this document the dynamical aspects of topography is being discussed and the consequences for forcing or damping convection. Besides general flow modification due to topography foehn and Bora are discussed, followed by a chapter on fronts and orography.

In this document the thermodynamic aspects of topography is being discussed and the consequences for forcing or damping convection. After the discussion of the surface energy budget in dependence of elevation, the generation of thermally driven slope wind circulations is treated. Finally the differential heating of air in valleys due to the volume reduction is derived and its consequences for valley wind systems. Concluding some concepts of convection over complex terrain are discussed.

In this document the basic characteristics of topography on our planet and its impact on the atmosphere is discussed. Fundamental thermodynamic as well as dynamic consequences of mountains with regard to the atmospheric processes are being treated. Relevant scales of phenomena are investigated and their implication on damping or forcing of convection.

An important tool in understanding the relationship between environments and observed severe thunderstorm events are vertical profiles of environmental conditions collected in the vicinity of the storms. These relationships can help in the future forecasting of weather. In this paper, the use and cautions associated with these so-called proximity soundings are discussed.

Estimates of the occurrence (“climatologies”) of convective phenomena in time, space, and intensity can be useful in a variety of contexts. They provide background for forecasters, and the risk management and meteorological research communities. In part, because of the different needs of those user groups, caution must be applied when developing and using climatologies, especially if the intended application is outside of the original intent of the developers.

Forecasting the weather can be thought of as a problem in extracting a small signal from a noisy background field. Much information is available to the forecaster, but, frequently, only a small amount of that information is of importance for solving the forecast problem(s) of the day. As a result, an approach to forecasting must maximize the efficiency of the process. An effective way, particularly for hazardous weather, is to identify the ingredients required to produce a particular weather event and then to focus on the processes that can affect the presence of those ingredients. This allows the forecaster to narrow the range of aspects of the observations and model guidance that are considered during the forecast shift and, it is hoped, identify crucial developments as they occur.

The first stage of forecasting convective weather involves forecasting the evolution of conditions that are favorable for the development of storms and their probable initiation. The scale of the forecasts are typically on the order of 100 km or larger and the lead time between the forecast and storms is 1-48 hours. In the United States, procedures have evolved so that the Storm Prediction Center of the National Weather Service has the responsibility for issuing these forecasts for the contiguous 48 states (the part of the US excluding Alaska and Hawaii.)

In order to protect life and property, forecasts of severe convection are critical on short time and space scales (on the order of 1 hour or less and a few 10s of km or less). Accurate assessment of the environment and monitoring of high-resolution observational data, frequently focusing on radar-observed evolution, are essential in this process. In the United States, these short-term time and space scale forecasts are referred to as warnings and are prepared by local forecast offices of the National Weather Service, who have responsibility for forecasters on the order of 100,000 km².

In this lecture we will deal with the general aspects of an operational forecast of convective severe weather in the medium and short range, that is from 72^h to 24^h ahead the occurrence of the severe weather event. The attention will be focused on the information available to the forecaster, their reliability and their use. The role of the numerical model outputs generally available in the daily operational forecast activity are described and their limits are stressed. It is shown how the subjective contribution of the forecaster integrates the model outputs information. The main elements that characterize a severe weather occurrence are schematically described and their identification is explored by means of the useful information available at the medium and short range.

In this lecture we deal with the general aspects of the convective severe weather forecast in the short range, that is from 24^h ahead the occurrence of the event down to the threatening phenomena onset. When the forecast is very close to the event it relates to, lets say a few hours before, or it aims to track the event evolution in real time, the word nowcasting is used instead of forecast. The main elements relevant to issue a local severe weather forecast in the short range are here described, furthermore the information available for that purpose is analyzed with special attention to the limits and the constrains imposed by the operational forecasting activity. Some examples of environments prone to the severe weather onset and evolution are presented and discussed.

These notes are voted to stress the importance as well as the intrinsic difficulty of the weather forecasts verification, giving some hints to solve specific problems and some tools to face various situations. In general weather forecasts cannot be fully wrong but they cannot be neither fully right, this because they are trying to represent a future state of an extremely complex system, which is defined by too many aspects to be fully well described. There is a quite general confidence on the fact that it is at least possible try to quantify the amount of good and bad information that forecasters are trying to give on that future state. Nevertheless it is not possible to define in a unique way this quantification process, then different verifications procedures might give different results even if correctly realized. The standardization of definitions and of procedures is generally still poor and sometimes contraddictory. This fact makes, if possible, even more difficult to deal with the weather forecast verification. Facing the verification of rare weather events, as can be the case for the phenomena related to deep moist convection, extra difficulty arise by the fact that the powerful tool represented by statistics becomes less effective and the intepretation of results becomes in those cases even more tricky. In any case the verification of weather forecasts is an extremely important and structural aspect of the forcasting activity, that cannot be considered complete without it. Moreover the verification of weather forecasts can be an important opportunity to have a different look to the atmospheric aspect toward which we are pointing our attention and, for whom it might interest, to have a different look at the forecasters mind.