Extratropical cyclones produce extensive regions of cloud and precipitation on scales ranging from intense convective storms to synoptic-scale cloud bands. Cloud and precipitation structures within extratropical cyclones are organized by synoptic and mesoscale structures such as warm and cold conveyor belts, dry intrusions and tropopause anomalies (see Browning, 1986, 2003, for a summary). Exchanges of heat due to diabatic processes in clouds are in turn likely to modify these flow structures, but the precise manner by which this takes place is not well understood. In addition, the parametrization of these processes in numerical weather prediction (NWP) models is uncertain and may be a major source of model error in forecasts (e.g. Ehrendorfer, 1997).
Potential vorticity (PV) provides a compact means to describe the dynamics of atmospheric flow on large scales. PV is also a convenient diagnostic for analysing the relationship between cloud-scale processes and the larger-scale flow. If the PV anomalies associated with non-conservative processes can be identified and located, then their impact on the structure and evolution of an extratropical weather system may be inferred (e.g. Stoelinga, 1996). Brennan et al. (2008) suggested that by examining non-conservative sources of PV due to latent heating in output from NWP models, forecasters could identify features that are associated with increased uncertainty. Piecewise PV inversion (PPVI) is a method that is often used to assess the influence of particular PV anomaly (e.g. Davis and Emanuel 1991; Pomroy and Thorpe, 2000). In PPVI, the total PV at a given time is divided spatially according to user-defined criteria. These criteria may, for example, isolate the PV generated at a given elevation. Inversion techniques may then be applied to the cut-away PV anomaly in order to establish which flow anomalies can be associated with it. PPVI does not directly indicate the origin of a given anomaly, nor does it unambiguously distinguish diabatically generated PV from the conservative background PV. A method for analysing the origin of diabatic PV using a set of PV tracers is presented in this paper. The technique is similar to that employed by Davis et al. (1993) and Stoelinga (1996) in which diabatic tendencies of PV are accumulated and advected. The approach based on diabatic PV clarifies the relationship between cloud-scale physical processes and larger-scale dynamics.
The tropopause is typically defined as a PV surface (except in the Tropics). There are two possible ways by which a diabatic process could alter the structure of the tropopause: (i) directly by heating or cooling in the vicinity of the tropopause generating PV at the tropopause level (i.e. mass converges across isentropic surfaces and dilutes PV substance at the tropopause); or (ii) indirectly by diabatically generated PV near the tropopause (e.g. in the upper troposphere within the downstream ridge) altering the winds and the gradient of PV, which in turn affects the development of waves on the jet stream. If these diabatic processes are not properly represented and the simulated PV anomaly is of insufficient magnitude or is in the wrong place, then the result is an error in the trough structure and potentially in the downstream forecast.
A particular bias that has been identified in global NWP systems concerns the structure of mid-latitude upper-level troughs on their downstream side. Didone (2006) demonstrated that simulated troughs have insufficient meridional slope and too weak a gradient of PV on their downstream side when compared to analysed troughs. The downstream side of a trough is typically located in the vicinity of the warm conveyor belt (WCB). The cloud and precipitation within the WCB generate significant heating in the mid troposphere. Heating applied against an environment having non-zero absolute vorticity provides a dipole in diabatic PV tendency (e.g. positive beneath the heating and negative above). Air mass trajectories flowing through WCBs experience an increase in PV below the maximum of latent heat release and decrease above, resulting in negative diabatic PV near the tropopause in the ridge downstream of the WCB (Pomroy and Thorpe, 2000; Grams et al., 2011).
The purpose of this paper is to isolate the influence of various diabatic processes on PV structure within a simulated extratropical cyclone using a PV tracer technique. The cyclone chosen was typical for the east Atlantic in October–intense but not exceptionally so. Simulations are performed in both a coarse-resolution global model and in a higher-resolution limited-area model. Each PV tracer is a three-dimensional prognostic field that is integrated online within the modelling system. At each time step, the tracers are advected, but they also accumulate tendencies attributable to various processes represented in the model. Each PV tracer therefore represents a history of the total PV accumulated along air mass trajectories contributed by a specific modelled process. The procedure not only allows one to identify precisely which PV structures are diabatically generated, but it also identifies the specific modelled processes that are responsible. In this paper, PV tracers are employed to answer the following questions:
- (i)What is the structure of the PV that arises due to diabatic processes in an extratropical cyclone?
- (ii)How does diabatic PV modify the structure of the tropopause?
- (iii)Which specific modelled processes contribute to this PV anomaly?
The structure of the paper is as follows. Section 2 describes the methods of analysis, including the PV tracer method. Section 3 provides an overview of the case. Section 4 examines the diabatic PV in the simulated cyclone and presents a partitioning of PV into contributions from various physical processes. Section 5 provides a summary of results and discussion of their implications.