Frequency Of Circulation Types Research Articles

Increasing frequency and changing nature of Saharan dust storm events in the Carpathian Basin (2019–2023) – the new normal?

The number and intensity of Saharan dust storm events identified in Europe has been increasing over the last decade. This can be explained by the role of ongoing climate change. An extension of previous studies covering a 40-year period is presented in this paper, with new data on the frequency, synoptic meteorological background, source areas, grain size, grain shape and general mineralogy of deposited dust for the period 2019–2023 in the Carpathian Basin. A total of 55 dust storm episodes have been identified in the region over the five-year period, which is significantly higher than the long-term average. The classification based on synoptic meteorological background clearly showed that the frequency of circulation types with a more pronounced meridional component increased and dust material reached further north more frequently than before. In several cases, large amounts of dust were deposited, from which samples were collected and subjected to detailed granulometric analysis. The varied grain size data showed that coarse silt (20–62.5 μm) and sand (62.5 < μm) fractions were also present in large quantities in the transported dust material.

Heat Waves Amplify the Urban Canopy Heat Island in Brno, Czechia

This study used homogenised mean, maximum, and minimum daily temperatures from 12 stations located in Brno, Czechia, during the 2011–2020 period to analyse heat waves (HW) and their impact on the canopy urban heat island (UHI). HWs were recognized as at least three consecutive days with Tx ≥ 30 °C and urban–rural and intra-urban differences in their measures were analysed. To express the HWs contribution to UHI, we calculated the UHI intensities (UHII) separately during and outside of HWs to determine the heat magnitude (HM). Our results show that all HW measures are significantly higher in urban areas. UHII is mostly positive, on average 0.65 °C; however, day-time UHII is clearly greater (1.93 °C). Furthermore, day-time UHII is amplified during HWs, since HM is on average almost 0.5 °C and in LCZ 2 it is even 0.9 °C. Land use parameters correlate well with UHII and HM at night, but not during the day, indicating that other factors can affect the air temperature extremity. Considering a long-term context, the air temperature extremity has been significantly increasing recently in the region, together with a higher frequency of circulation types that favour the occurrence of HWs, and the last decade mainly contributed to this increase.

The Impact of Atmospheric Circulation on Air Temperature Rise in Estonia

In comparison to the global average, the climate in the Baltic Sea region, including in Estonia, has warmed up particularly fast. From synoptic climatology's point of view, a question can be posed: is this warming caused by changes in the frequency of particular circulation types or warmer weather started accompanying these types? The main aim of present study is to analyze the relationship between increase of air temperature and changes in atmospheric circulation during the period of 1966-2015. Changes in the frequency of circulation types belonging to 12 classifications from COST733 data set and changes in accompanying air temperature were analyzed. The circulation types were divided into 'warm' and 'cold' for a given season according to daily temperature anomalies in three Estonian meteorological stations. On the basis of the similar air flow direction circulation types from different classifications were selected for intercomparison. Linear trend analysis showed that there were only a few statistically significant (p<0.05) changes in the frequency of circulation types. The major changes occurred in spring – the frequency of eastern and northeastern flow types has decreased, and the frequency of types related to northwestern flow has increased. However, the positive temperature anomalies have increased for practically all circulation types. A particularly great warming has taken place in winter 'cold' types. In conclusion, the increase of Estonian air temperature during the analyzed period is more likely associated with the changes of temperature within circulation types than of their frequency.

Classifications of winter atmospheric circulation patterns: validation of CMIP5 GCMs over Europe and the North Atlantic

Winter atmospheric circulation over the Euro-Atlantic domain and three subdomains (British Isles, Central Europe, and Eastern Mediterranean) is validated in outputs of historical runs of 32 global climate models (GCMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Eight automated classifications of daily SLP patterns from five reanalysis datasets are produced for each domain in order to analyse the effect of the choices of methods and reference data on results. The results show that the ranking of GCMs fundamentally depends on which classification is used; therefore, only parallel usage of multiple classifications can provide robust rankings of models. Considering all eight classifications, three models (HadGEM2-CC, MIROC4h, and CNRM-CM5) are among the best in simulating the frequency of circulation types (CTs) over all four domains. Regardless the domain, the bias in CT frequency of the worst GCMs is larger than 50% of the frequency in the reference reanalysis dataset. Conversely, the best GCM for each domain differs from the reference reanalysis by about 10–20%, which is nearly the same result as found for the NOAA-CIRES Twentieth Century Reanalysis (version 2). The persistence of circulation is simulated better than the frequency with errors rarely exceeding 15%. The GCMs overestimate the frequency of westerly circulation over all domains (by about 7% over the British Isles, 21% over Central Europe, and almost 70% over the Eastern Mediterranean) and also cyclonic CTs, while easterly and anticyclonic CTs are typically underestimated by 30–40%.

A new pattern of the moisture transport for precipitation related to the drastic decline in Arctic sea ice extent

Abstract. In this study we use the term moisture transport for precipitation for a target region as the moisture coming to this region from its major moisture sources resulting in precipitation over the target region (MTP). We have identified changes in the pattern of moisture transport for precipitation over the Arctic region, the Arctic Ocean, and its 13 main subdomains concurrent with the major sea ice decline that occurred in 2003. The pattern consists of a general decrease in moisture transport in summer and enhanced moisture transport in autumn and early winter, with different contributions depending on the moisture source and ocean subregion. The pattern is statistically significant and consistent with changes in the vertically integrated moisture fluxes and frequency of circulation types. The results of this paper also reveal that the assumed and partially documented enhanced poleward moisture transport from lower latitudes as a consequence of increased moisture from climate change seems to be less simple and constant than typically recognised in relation to enhanced Arctic precipitation throughout the year in the present climate.

Rola typów cyrkulacji atmosferycznej w kształtowaniu stężeń pyłu zawieszonego PM10 w Poznaniu = The role of the atmospheric circulation types on PM10 concentrations in Poznań

Emissions from various sources and favorable meteorological conditions associated with the advection of air masses, promote the occurrence of high concentration of particulate matter. Using data of mean daily values of PM10 and calendar of circulation types for Western Poland drafted by T. Niedźwiedź (2003), made it possible to show the most and least favorable situations occurrence of high particulate matter concentration. The most preferred situation for pollution accumulation is the advection from the southern sector. The lowest values of PM10 appeared in the case of northern types of circulation. It was also shown that exceeding the permissible daily values of 50 μg m−3 (according to Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe) occur much more frequently in the case of anticyclonic types of circulation. Mean values of PM10 in different seasons, confirmed the presence of the highest concentrations in winter, what is associated with an increase of emissions from municipal sector, as well as increase of frequency of circulation types from the South at this time of year.

Projected evolution of circulation types and their temperatures over Central Europe in climate models

The study deals with changes in large-scale atmospheric circulation (represented by circulation types) and associated surface air temperatures as projected in an ensemble of regional climate models (RCMs) from the ENSEMBLES project. We examine changes of circulation type frequencies and means of daily maximum and minimum temperatures within circulation types in individual seasons for two time slices of transient runs under the SRES A1B scenario (2021–2050 and 2071–2100) with respect to the control period (1961–1990). To study the influence of driving data, simulations of the driving general circulation models (GCMs) also are evaluated. We find that all models project changes of atmospheric circulation that are statistically significant for both future time slices. The models tend to project strengthening of the westerly circulation in winter and its weakening in summer. We show that increases of daily maximum and minimum temperatures in all seasons differ for individual circulation types. There are, however, only few features of the projected changes in the future circulation–temperature links that are common among the models, in particular relatively smaller warming for westerly types. Only in winter, projected changes in circulation types tend to contribute to the projected overall warming. This effect is negligible and mostly opposite in the other seasons. We also detect a strong influence of driving data on RCMs’ simulation of atmospheric circulation and temperature changes.

Spring and summer extreme temperatures in Iberia during last century in relation to circulation types

In the Iberian Peninsula the raise of temperatures has been notable from mid-1970s to mid-2000s, especially in spring and summer. This study analyses spatial and temporal relationships between extreme temperatures and atmospheric circulation types (CTs) defined over the Iberian Peninsula (IP) in these seasons. Station series (29) of maximum and minimum temperature are considered, starting from 1905 until 2006. The CTs (9 for spring and 8 for summer) are derived by a cluster method applied to daily mean SLP grids covering the period 1850–2003. Changes in the seasonal frequency of extreme temperatures and of CTs are analysed. Subsequently, the CTs are examined for their effectiveness in leading to moderately extreme temperatures (at each location) using an index that measures the contribution to extreme days with respect to the contribution to non-extreme days. Correlation between regional extreme series and CTs frequency is also tested.In spring, the decrease in cold nights, which is notable in the 1970s onwards, can be partially attributable to a downtrend in the frequency of Northerly flow. High frequency of Anticyclone in North Iberia in the 1980s and 1990s has contributed to an increase in warm days in West and North stations. To the SE quadrant of the IP, a great part of warm days is related to south-westerly flow, (both) presenting a higher frequency in the 1950s and the 1960s.In summer warm nights increased remarkably to the SE and SW, and may be in part related to uptrends in Iberian thermal low pattern (1950–2003) and North Atlantic Anticyclone (1850–2003) respectively. Warm days have increased remarkably to the NE especially in the 1990s and 2000s, but this is not found to be related to changes in CTs' frequency.Furthermore, the existence of within type changes (variations in Tmax, Tmin and extreme indices within the CTs) points to the identification of other physical factors operating on inter-annual to multidecadal time scales. Thus, the consideration of Sea Surface Temperature (SST) of the East Atlantic Ocean and Iberian soil moisture conditions (by means of a drought index) helps to explain the evolution of extreme temperatures.

Are there weekly cycles in occurrence frequencies of large‐scale circulation types?

AbstractThe existence of weekly cycles in occurrence frequency distributions of circulation types is examined on the basis of a comprehensive data base of automated circulation‐type classifications for the greater North Atlantic European region and sub‐domains during the period 1957–2002. The main result is that the percentage of circulation types for which a statistical significant weekly cycle (95% level of confidence) could be detected by a chi‐square goodness‐of‐fit test does not exceed the number that would be expected by chance. Thus it is concluded that no consistent weekly cycles in occurrence frequencies of circulation types exist. Copyright © 2012 Royal Meteorological Society

Atmospheric Circulation Influence on the Winter Thermal Conditions in Poland in 2021-2050 Based on the RACMO2 Model

Abstract Thermal conditions are largely determined by atmospheric circulation. Therefore, projection of future temperature changes should be considered in relation to changes in circulation patterns. This paper assess to what extent changes in circulation correspond to spatial variability of the winter temperature increase in Poland in 2021-2050 period based on the RACMO2 model. The daily data of the mean temperature and sea level pressure (SLP) from selected regional climate model and observations were used. SLP data were used to determine the advection types and circulation character. Firstly, changes in frequency of circulation types between 2021-2050 and 1971-2000 periods were examined. Then changes in air temperature for specific circulation type in relation to reference period were studied. Finally, the influence of atmospheric circulation on spatial temperature variation was discussed. Considerably high increase in cyclonic situation of more than 18%, especially from the west and south-west direction, and decrease in anticyclonic situation mainly from the west and northwest in winter was noticed. Changes in frequency of circulation types result in temperature growth. For some types it is predicted that warming can reach even 3-4°C. The cyclonic (Ec, SEc, Sc) and anticylonic (SEa, Sa, Ea) types are likely to foster the highest warming in the scenario period.

Changes in Present and Future Circulation Types Frequency in Northwest Iberian Peninsula

The aim of the work described herein was to study projection scenarios in order to find changes in the synoptic variability of the northwest Iberian Peninsula in the 21st century. To this end, we investigated the changes in the frequency of the different circulation types computed for the study area using three different models used in the IPCC 4th assessment report. The circulation types were computed using the procedure known as Lamb circulation types. The control simulation for the late 20th century was evaluated objectively from the results obtained using data from the NCEP/NCAR reanalysis, as to evaluate the ability of the model to reproduce the present climate. We have compared not only seasonal mean sea level pressure fields but also the mean seasonal frequency of circulation types. The results for the end of the 21st century show a decrease in the frequency of cyclonic, W, and SW circulation types in the spring and summer months. This trend also appears in the autumn, with a concomitant increase in the anticyclonic types.

Circulation vs. climatic changes over the Czech Republic: A comprehensive study based on the COST733 database of atmospheric circulation classifications

We have analyzed the effect of changes in the frequency of circulation types on significant seasonal trends of eight climatic variables (maximum, minimum, and average daily temperature, precipitation amount and occurrence, relative humidity, cloudiness, and sunshine duration) at 21 Czech stations in the period 1961–1998 using 24 objective and two subjective atmospheric circulation classifications collected and developed within the COST733 Action. Using two independent methods, we show that recent climatic trends can be only partly explained by changes in the frequency of circulation types. In winter, such changes in the objective catalogues explain around 30% of long-term linear temperature trends and around 50% of temperature changes that took place between the first and the second half of the study period. Changing internal properties of individual circulation types (i.e. within-type climatic changes) are responsible for a major part of the observed climatic trends in spring, summer, and autumn; but also in winter for variables other than temperature. The subjective catalogues are usually more tightly connected with the observed climatic changes than the objective ones. These results are consistent with the fact that except for winter, very few circulation types in the objective catalogues yield significant long-term trends in their seasonal occurrence. On the contrary, both used versions of the subjective Hess–Brezowsky catalogue bear a substantial proportion of significant trends in the occurrence of circulation types in all seasons. Very high variability in the results, caused both by the variability within stations and within the circulation classifications used, suggests that such a comparative approach is highly desirable in synoptic-climatological studies.

Influence of circulation types on temperature extremes in Europe

The aim of this study is to determine the influence of atmospheric circulation on the recently observed changes in the number of warm days and cold days in Europe. The temperature series for stations in the European Climate Assessment and Data set project and the Grosswetterlagen (GWL) were used here. The temperature series were first adjusted for global warming before determining the indices for cold and warm extremes. The 29 GWLs were grouped in ten circulation types. Then, the number of days a certain circulation type occurred was determined for each winter (December, January and February) and summer (June, July and August). The relation between the circulation type frequencies and the temperature indices was modelled with a multi-regression fit over the period 1947–1974 and tested for the period 1974–2000. The difference between the observed indices and the calculated indices in the second period (using the fit coefficients for the first period) shows a warming effect for both winter and summer and for at least the warm day index, which is unaccounted for by the global warming trend. A simple snow model shows that variations in the European snow cover extent are likely influencing the cold and warm day indices in winter: there is a correlation between the decreasing trend of the snow cover extent in Europe and the increasing (decreasing) trend of the number of warm (cold) days for stations throughout Europe.

Classification of circulation types: a new flexible automated approach applicable to NCEP and GCM datasets

This study is motivated by an interest in obtaining a new automated classification scheme of daily circulation types suitable for use throughout Europe. The classification scheme is performed on 500 hPa geopotential height anomalies (NCEP Reanalysis data, 2.5°×2.5°). Nine grid points represent the study area. Five anticyclonic types (Anw, Ane, A, Asw and Ase) and seven cyclonic types (C, Cnnw, Cwnw, Cwsw, Cssw, Cse, Cne) are defined. Each of the circulation types has a distinct underlying synoptic pattern that produces the expected type and direction of flow over the study area. The classification scheme is applied to three different case studies in the Mediterranean Basin: Greece, Cyprus and central Italy. The precipitation percentage of the cyclonic type and the mean seasonal correlation coefficients for all circulation types are the two criteria used to evaluate the performance of the classification scheme. The ability of the HadAM3P general circulation model to reproduce the mean pattern and frequency of circulation types at the 500 hPa level in comparison to the NCEP dataset for the period 1960–1990 is also evaluated. The percentage of rainfall that corresponds to the cyclonic circulation types is greater than 85% for the three study regions. Furthermore, the correlation coefficients for the three classifications are very encouraging, for nearly all days of the study period. Compared to observations, the GCM is able to capture the mean patterns but not able to replicate exactly the observed variability of the circulation types over the three study regions.

Performance of the general circulation HadAM3P model in simulating circulation types over the Mediterranean region

AbstractThe capability of the general circulation model(GCM) Hadley Center atmospheric general circulation model 3P (HadAM3P) to reproduce the mean pattern and the frequency of circulation types concerning the 500 hPa geopotential height fields and 1000–500 hPa thickness fields over Europe and the Mediterranean region are evaluated against the NCEP/NCAR dataset for the period 1960–1990. Daily catalogues of the circulation types describing the regional circulation over Greece at the 500 hPa and 1000–500 hPa thickness fields are constructed based on the two datasets, and the corresponding seasonal frequencies are also estimated. The evaluation is performed for the mean seasonal fields of the two parameters, as well as for those derived for each circulation type separately. It is demonstrated that the HadAM3P model generally succeeds in reproducing the two fields over the examined area, while their seasonal variability is underestimated. The mean patterns of the circulation types are well represented by the model but the seasonal simulated frequencies of the circulation types are not in good agreement with the national centers for environmental predictions (NCEP) ones. The systematic errors of the frequency estimation and the variability in some anticyclonic or cyclonic types should be considered seriously, because it can introduce some degree of uncertainty in the results of a downscaling method based on a circulation type approach. Nevertheless, the links between the precipitation, temperature and circulation types frequency seem to hold in the GCMs. It is therefore safe to use the frequencies of circulation types as predictor in a downscaling technique in order to obtain future precipitation and temperature. Copyright © 2007 Royal Meteorological Society

Circulation types and extreme temperature changes in Greece

ABSTRACT: We examined changes in extreme temperature over the Greek area in relation to thegeopotential thickness field (1000 to 500 hPa). Daily maximum and minimum temperature series from20 Greek stations evenly distributed over Greece were used for the period 1958–2000. The atmos-pheric circulation over the Greek area was classified with the aid of a calendar of daily circulationtypes based on the geopotential thickness field of 1000 to 500 hPa for the same period using dailyNCEP and HadAM3P datasets. There was a general increase in frequency of anticyclonic circulationtypes and a decrease in frequency of cyclonic types. In addition, there was an overall interannualincrease (decrease) in maximum (minimum) extreme temperatures over the Greek area, with spatialand seasonal variations. The investigation of the temperature anomalies corresponding to each circu-lation type, along with the trends of the former, revealed that the increase in frequency of the anticy-clonic types, which form positive anomalies, and their positive trends in the majority of the stations,can explain the upward trend of maximum temperature in summer. In contrast, the trend of decreas-ing minimum temperatures in the central and SW Greek area cannot only be interpreted by changesin frequency of circulation types. A statistical downscaling model for simulating temperature changesover Greece (both for NCEP and HadAM3P datasets) was developed following a circulation-typeapproach. This demonstrated high correspondence with the observed extreme temperature values inall seasons, with only slightly lower year-to-year variability.KEY WORDS: Greece · Trend analysis · Extreme temperature · Circulation types · Multipleregression · Downscaling · GCM

On the relationships between circulation types and changes in rainfall variability in Greece

AbstractAn attempt is made to examine rainfall variability over the Greek area in relation to 500 hPa atmospheric circulation. Daily precipitation series from 22 evenly distributed Greek stations have been used for the period 1958–2000, along with the classification scheme of daily circulation types at 500 hPa for the same period. The seasonal frequency and the trends of circulation types have been calculated. It was found that there is a general positive trend of anticyclonic circulation types and a negative one for cyclonic types. The seasonal trends of rainy days and the precipitation totals have also been calculated and analysed. A general decreasing tendency of winter rainfall is observed; the decreasing trend during autumn and spring is less significant. Concerning the frequency and intensity of rainfall per circulation type, a decreasing tendency becomes evident for the majority of the stations during winter, whereas there is an increasing tendency during autumn. A multiple regression–cross‐validation model was developed using the seasonal frequency of circulation types as predictors and the seasonal rainfall totals as predictants. Only the winter modelled precipitation shows a good agreement with the observed precipitation, whereas for the other seasons the agreement is relatively poor. This could be caused by the influence of different factors that are not captured by the classification scheme used. The proposed model could serve as a circulation‐based downscaling method that could be further applied to geopotential data available from general circulation models in order to study regional climatological consequences of future climate scenarios. Copyright © 2004 Royal Meteorological Society

Disaggregating climatic trends by classification of circulation patterns

AbstractThe trends in occurrence frequencies of circulation types over Europe and in nine climate variables in the Czech Republic conditioned by the types are examined for period 1949–1980. The circulation types are determined by an objective procedure from daily 500 hPa heights. Both in summer and winter, anticyclonic types have become more frequent at the expense of cyclonic types. The circulation changes are shown to be unrelated to the trends in surface climate elements in summer, whereas in winter, trends in circulation explain a part of the observed warming and strengthening of southerly winds.The trends in climate elements are not uniform among circulation types. In summer, the trend pattern consisting of decreasing maximum and daily mean temperatures, daily temperature range (DTR) and sunshine duration, and increasing cloudiness and relative humidity is observed under the cyclonic types and the types with a well‐pronounced jet, but is missing under types with a blocking anticyclone over Europe. Two possible mechanisms causing this trend pattern are proposed: increasing cloudiness, and a process responsible for the reduction of sunshine without a concurrent increase of cloudiness. The latter mechanism can possibly be identified with increasing aerosol concentrations. In winter, the degree of warming is governed by changes in zonal wind. The mechanism of change in DTR seems to vary with elevation: at the lowland station (Prague‐Klementinum), the increase in DTR is related to the warming trend, and consequently with zonal wind changes, while at the mountain station (Milešovka), the increase in DTR reflects the increase in precipitating clouds.The changes in DTR are related much more to mid‐tropospheric circulation than to cloud cover in summer, whereas in winter, cloud cover plays a more important role in affecting DTR trends. Copyright © 2001 Royal Meteorological Society

A monthly circulation climatology for Sweden and its application to a winter temperature case study

Atmospheric circulation is important in determining the surface climate and environment. To quantify its effect, circulation indices or classifications of circulation type are often used. In this study, the classification system developed by Lamb (1950. Quarterly Journal of the Royal Meteorological Society76: 393–438) is applied to obtain circulation information for Sweden on a monthly basis. For that purpose, monthly mean sea-level pressure (MSLP) data from 1873 to 1995 is used to derive six circulation indices and to provide a circulation catalogue with 27 circulation types. The frequency of circulation types over different periods is computed and described. Four major types (cyclonic, C; west, W; southwest, SW; anticyclonic, A) have been identified. The catalogue and the associated indices provide a tool for interpreting the regional climate and for developing statistical downscaling models to derive regional climate change scenarios for Sweden. An example is given to illustrate the application and usefulness of this climatology. It deals with the linkage between the mean regional temperature in southwestern Sweden and the large-scale circulation. It is shown that directional flows with a westerly component clearly favour a positive temperature anomaly, while the anticyclonic condition often induces a negative anomaly. Moreover, it is the westerly or southwesterly flows that accompany extreme positive temperature anomalies, whereas anticyclonic conditions play an important role in creating extreme negative temperature anomalies. Further, the derived indices have a relationship with temperature anomalies, established via a statistical model. Stepwise multiple regression was used, leading to a successful model with only three indices. Using the model, 70% of the total variance in the temperature anomalies between 1887 and 1994 has been reconstructed from the pressure, which leads to the conclusion that the circulation is a critical determinant of the January temperature and that the derived climatology is useful in explaining the temperature anomalies. Copyright © 2000 Royal Meteorological Society

Modification of a correlation-based circulation pattern classification to reduce within-type variability of temperature and precipitation

When a correlation-based circulation pattern classification was applied to daily 700 hPa heights for 25 summer seasons, and the circulation types were related to surface temperature and precipitation for the Lake Superior basin, within-type variability was found to be high. Regarding temperature, the difference between warm and cold days of a type is a positive height anomaly centred to the east of the lake, which is very similar to the height anomaly for winter temperatures identified in an earlier study. The difference between wet and dry days of a type is a pattern quite unlike that for temperature. It is a dipole pattern consisting of a negative anomaly to the northwest of the lake and a positive anomaly to the east and southeast. These small-scale circulation components were missed by the correlation-based classification because they are much smaller than the differences in height between circulation types. Regression models, relating monthly mean temperature anomalies to circulation type frequencies, produced results comparable to those obtained for winter temperature. For precipitation, the variance explained by the regression models is only half that obtained for temperature. Two approaches were used to reduce within-type variability and to improve the regression results: the division of the first few circulation types into warm/cold and wet/dry subtypes on the basis of differences in 700 hPa vorticity (an approach introduced in the earlier paper), and a modified correlation-based circulation pattern classification. The modification consisted of raising the correlation threshold for a few selected grid-points to capture the small-scale circulation component and also, of lowering the threshold for the comparison between maps and map sectors in order to minimize the number of unclassified days. Of the two approaches used to reduce within-type variability, the modified classification scheme produced superior results. In particular, the variance in precipitation explained by the regression models is about 30% higher than that achieved using vorticity to create subtypes. Copyright © 2000 Royal Meteorological Society