Monthly Weather Review

Gelaro, Ronald; Langland, Rolf H.; Pellerin, Simon; Todling, Ricardo

doi: 10.1175/2010MWR3393.1pmid: N/A

An experiment is being conducted to directly compare the impact of all assimilated observations on short-range forecast errors in different forecast systems using an adjoint-based technique. The technique allows detailed comparison of observation impacts in terms of data type, location, satellite sounding channel, or other relevant attributes. This paper describes results for a ““baseline”” set of observations assimilated by three forecast systems for the month of January 2007. Despite differences in the assimilation algorithms and forecast models, the impacts of the major observation types are similar in each forecast system in a global sense. However, regional details and other aspects of the results can differ substantially. Large forecast error reductions are provided by satellite radiances, geostationary satellite winds, radiosondes, and commercial aircraft. Other observation types provide smaller impacts individually, but their combined impact is significant. Only a small majority of the total number of observations assimilated actually improves the forecast, and most of the improvement comes from a large number of observations that have relatively small individual impacts. Accounting for this behavior may be especially important when considering strategies for deploying adaptive (or ““targeted””) components of the observing system.

Straus, David M.

doi: 10.1175/2010MWR3333.1pmid: N/A

A method to incorporate synoptic eddies into the diagnosis of circulation regimes using cluster analysis is illustrated using boreal winter reanalyses of the National Centers of Environmental Prediction (hereafter observations) over the Pacific–North American region. The motivation is to include the configuration of the high-frequency (periods less than 10 days) transients as well as the low-frequency (periods greater than 10 days) flow explicitly into the definition of the regimes. Principle component analysis is applied to the low-frequency 200-hPa height field, and also to the low-frequency “envelope” modulations of the rms of high-frequency meridional velocity at 200 hPa. A maximum covariance analysis of the height and envelope fields, carried out using the appropriate principal components, defines three modes as explaining most of the covariance. This defines the minimum dimensionality of the space in which to apply k -means cluster analysis to the covariance coefficients. Clusters found using this method agree with results of the previous work. Significance is assessed by comparing cluster analyses with results from synthetic datasets that have the same spectral amplitudes (but random phases) of seasonal means and, separately, intraseasonal fluctuations as do the original observed time series. This procedure ensures that the synthetic series have similar autocovariance structures to the observations. Building on earlier work, the clusters obtained are newly tested to be highly significant without the need for quasi-stationary prefiltering.

Jeglum, Matthew E.; Steenburgh, W. James; Lee, Tiros P.; Bosart, Lance F.

doi: 10.1175/2010MWR3432.1pmid: N/A

The topography in and around the Intermountain West strongly affects the genesis, migration, and lysis of extratropical cyclones. Here intermountain (i.e., Nevada or Great Basin) cyclone (IC) activity and evolution are examined using the ECMWF Re-Analysis Interim (ERA-Interim) the North American Regional Reanalysis (NARR), and the NCEP–NCAR reanalysis from 1989 to 2008, the period during which all three are available. The ICs are defined and tracked objectively as 850-hPa geopotential height depressions of ≥40 m that persist for ≥12 h. The monthly distribution of IC center and genesis frequency in all three reanalyses is bimodal with spring (absolute) and fall (secondary) maxima. Although the results are sensitive to differences in resolution, topographic representation, and reanalysis methodology, both the ERA-Interim and NARR produce frequent IC centers and genesis in the Great Basin cyclone region, which extends from the southern “high” Sierra to northwest Utah, and the Canyonlands cyclone region, which lies over the upper Colorado River basin of southeast Utah. The NCEP–NCAR reanalysis fails to resolve these two distinct cyclone regions and produces less frequent IC centers and genesis than the ERA-Interim and NARR. An ERA-Interim-based composite of strong ICs generated in cross-Sierra (210°–300°) 500-hPa flow shows that cyclogenesis is preceded by the development of the Great Basin confluence zone (GBCZ), a regional airstream boundary that extends downstream from the Sierra Nevada across the Intermountain West. Cyclogenesis occurs along the GBCZ as large-scale ascent develops over the Intermountain West in advance of an approaching upper-level trough. Flow splitting around the high Sierra and the presence of low-level baroclinicity along the GBCZ suggest that IC evolution may be better conceptualized from a potential vorticity perspective than from traditional quasigeostrophic models of lee cyclogenesis. Although these results provide new insights into IC activity and evolution, analysis uncertainty and the cyclone identification criteria are important sources of ambiguity that cannot be fully eliminated.

Bryan, George H.; Parker, Matthew D.

doi: 10.1175/2010MWR3359.1pmid: N/A

Rawinsonde data were collected before and during passage of a squall line in Oklahoma on 15 May 2009 during the Second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2). Nine soundings were released within 3 h, allowing for unprecedented analysis of the squall line’s internal structure and nearby environment. Four soundings were released in the prestorm environment and they document the following features: low-level cooling associated with the reduction of solar isolation by a cirrus anvil; abrupt warming (1.5 K in 30 min) above the boundary layer, which is probably attributable to a gravity wave; increases in both low-level and deep-layer vertical wind shear within 100 km of the squall line; and evidence of ascent extending at least 75 km ahead of the squall line. The next sounding was released ∼5 km ahead of the squall line’s gust front; it documented a moist absolutely unstable layer within a 2-km-deep layer of ascent, with vertical air velocity of approximately 6 m s −1 . Another sounding was released after the gust front passed but before precipitation began; this sounding showed the cold pool to be ∼4 km deep, with a cold pool intensity C ≈ 35 m s −1 , even though this sounding was located only 8 km behind the surface gust front. The final three soundings were released in the trailing stratiform region of the squall line, and they showed typical features such as: “onion”-shaped soundings, nearly uniform equivalent potential temperature over a deep layer, and an elevated rear inflow jet. The cold pool was 4.7 km deep in the trailing stratiform region, and extended ∼1 km above the melting level, suggesting that sublimation was a contributor to cold pool development. A mesoscale analysis of the sounding data shows an upshear tilt to the squall line, which is consistent with the cold pool intensity C being much larger than a measure of environmental vertical wind shear Δ U . This dataset should be useful for evaluating cloud-scale numerical model simulations and analytic theory, but the authors argue that additional observations of this type should be collected in future field projects.

Xu, Jing; Wang, Yuqing

doi: 10.1175/2010MWR3335.1pmid: N/A

The multiply nested, fully compressible, nonhydrostatic tropical cyclone model version 4 (TCM4) is used to examine and understand the sensitivity of the simulated tropical cyclone (TC) inner-core size to its initial vortex size. The results show that although the simulated TC intensity at the mature stage is weakly dependent on the initial vortex size for the general settings, the simulated TC inner-core size is largely determined by the initial vortex size. The initial vortex size is critical to both the energy input from the ocean and the effectiveness of the inward angular momentum transport by the transverse circulation driven by eyewall convection and diabatic heating in spiral rainbands. Strong outer winds in a storm with a large initial size lead to large entropy fluxes to a large radial extent outside the eyewall, favoring the development of active spiral rainbands. Latent heat released in spiral rainbands plays a key role in increasing the low-level radial inflow and accelerating tangential winds outside the eyewall, leading to outward expansion of tangential wind fields and thus increasing the inner-core size of the simulated storm. On the contrary, a storm with a small initial size has weaker outer winds and smaller surface entropy fluxes outside the eyewall and is accompanied by less active spiral rainbands and thus a much slower increase in the inner-core size. The effectiveness of the inward transport of absolute angular momentum to increase the tangential winds outside the eyewall is largely determined by the radial extent of the vertical absolute vorticity, which is shown to be higher in a large size vortex. The relative importance of the initial vortex size and the environmental relative humidity (RH) to the TC inner-core size is also evaluated. It is found that the inner-core size of the simulated storm at the mature stage depends more heavily on the initial vortex size than on the initial RH of the environment.

Yasunaga, Kazuaki; Yoneyama, Kunio; Moteki, Qoosaku; Fujita, Mikiko; Takayabu, Yukari N.; Suzuki, Junko; Ushiyama, Tomoki; Mapes, Brian

doi: 10.1175/2010MWR3469.1pmid: N/A

A field observational campaign i.e., the Mirai Indian Ocean cruise for the Study of the MJO-convection Onset (MISMO) was conducted over the central equatorial Indian Ocean in October–December 2006. During MISMO, large-scale organized convection associated with a weak Madden–Julian oscillation (MJO) broke out, and some other notable variations were observed. Water vapor and precipitation data show a prominent 3–4-day-period cycle associated with meridional wind υ variations. Filtered υ anomalies at midlevels in reanalysis data i.e., the Japan Meteorological Agency (JMA) Climate Data Assimilation System (JCDAS) show westward phase velocities, and the structure is consistent with mixed Rossby–gravity waves. Estimated equivalent depths are a few tens of meters, typical of convectively coupled waves. In the more rainy part of MISMO (16–26 November), the 3–4-day waves were coherent through the lower and midtroposphere, while in the less active early November period midlevel υ fluctuations appear less connected to those at the surface. SST diurnal variations were enhanced in light-wind and clear conditions. These coincided with westerly anomalies in prominent 6–8-day zonal wind variations with a deep nearly barotropic structure through the troposphere. Westward propagation and structure of time-filtered winds suggest n = 1 equatorial Rossby waves, but with estimated equivalent depth greater than is common for convectively coupled waves, although sheared background flow complicates the estimation somewhat. An ensemble reanalysis i.e., the AGCM for the Earth Simulator (AFES) Local Ensemble Transform Kalman Filter (LETKF) Experimental Reanalysis (ALERA) shows enhanced spread among the ensemble members in the zonal confluence phase of these deep Rossby waves, suggesting that assimilating them excites rapidly growing differences among ensemble members.

van der Velde, I. R.; Steeneveld, G. J.; Wichers Schreur, B. G. J.; Holtslag, A. A. M.

doi: 10.1175/2010MWR3427.1pmid: N/A

A case of a severe radiation fog during frost conditions is analyzed as a benchmark for the development of a very high-resolution NWP model. Results by the Weather Research and Forecasting model (WRF) and the High-Resolution Limited-Area Model (HIRLAM) are evaluated against detailed observations to determine the state-of-the-art in fog forecasting and to derive requirements for further research and development. For this particular difficult case, WRF is unable to correctly simulate the fog for any of the parameterizations and model configurations utilized. Contrary, HIRLAM does model the onset of fog, but is unable to represent it beyond the lowest model layer, which leads to an early dispersal of fog in the morning transition. The sensitivity of fog forecasts to model formulation is further analyzed with a high-resolution single-column version of HIRLAM, and with the Duynkerke single-column model as a reference. The single-column results are found to be sensitive to the proper specification of the external forcings. It is reconfirmed that high vertical resolution is essential for modeling the fog formation, the growth of the fog layer, and when the fog lifts for the maintenance of a stratus deck. The properly configured column models are able to accurately model the onset of fog and its maturation, but fail in the simulation of fog persistence and subsequent dispersal. Details of the turbulence parameterization appear to be important in this process. It is concluded that, despite all of the advances in numerical weather prediction, fog forecasting is still a major challenge.

Molthan, Andrew L.; Petersen, Walter A.; Nesbitt, Stephen W.; Hudak, David

doi: 10.1175/2010MWR3485.1pmid: N/A

The Canadian CloudSat /Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) Validation Project (C3VP) was a field campaign designed to obtain aircraft, surface, and radar observations of clouds and precipitation in support of improving the simulation of snowfall and cold season precipitation, their microphysical processes represented within forecast models, and radiative properties relevant to remotely sensed retrievals. During the campaign, a midlatitude cyclone tracked along the U.S.–Canadian border on 22 January 2007, producing an extensive area of snowfall. Observations of ice crystals from this event are used to evaluate the assumptions and physical relationships for the snow category within the Goddard six-class, single-moment microphysics scheme, as implemented within the Weather Research and Forecasting (WRF) model. The WRF model forecast generally reproduced the precipitation and cloud structures sampled by radars and aircraft, permitting a comparison between C3VP observations and model snowfall characteristics. Key snowfall assumptions in the Goddard scheme are an exponential size distribution with fixed intercept and effective bulk density, and the relationship between crystal diameter and terminal velocity. Fixed values for the size distribution intercept and density did not represent the vertical variability of naturally occurring populations of aggregates, and the current diameter and fall speed relationship underestimated terminal velocities for all sizes of crystals.

Candille, Guillem; Beauregard, Stéphane; Gagnon, Normand

doi: 10.1175/2010MWR3349.1pmid: N/A

Previous studies have shown that the raw combination (i.e., the combination of the direct output model without any postprocessing procedure) of the National Centers for Environmental Prediction (NCEP) and Meteorological Service of Canada (MSC) ensemble prediction systems (EPS) improves the probabilistic forecast both in terms of reliability and resolution. This combination palliates the lack of reliability of the NCEP EPS because of the too small dispersion of the predicted ensemble and the lack of probabilistic resolution of the MSC EPS. Such a multiensemble, called the North American Ensemble Forecast System (NAEFS), especially shows bias reductions and dispersion improvements that could only come from the combination of different forecast errors. It is then legitimate to wonder whether these improvements in terms of biases and dispersions, and by extension the skill improvements, are only due to the balancing between opposite model errors. In the NAEFS framework, bias corrections “on the fly,” where the bias is updated over time, are applied to the operational EPSs. Each model of the EPS components (NCEP/MSC) is individually bias corrected against its own analysis with the same process. The bias correction improves the reliability of each EPS component. It also slightly improves the accuracy of the predicted ensembles and thus the probabilistic resolution of the forecasts. Once the EPSs are combined, the improvements due to the bias correction are not so obvious, tending to show that the success of the multiensemble method does not only come from the cancellation of different biases. This study also shows that the combination of the raw EPS components (NAEFS) is generally better than either the bias corrected NCEP or MSC ensembles.