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T. N. Krishnamurti
,
Wei Han
,
Bhaskar Jha
, and
H. S. Bedi

Abstract

The main theme of this paper is on the intensity forecast of a hurricane (Opal) and interpretation of factors contributing toward it. The paper illustrates the results of assimilation and prediction for Hurricane Opal of 1995 from a very high-resolution global model. The assimilation makes use of a detailed physical initialization that vastly improves the nowcasting skill of rainfall and the model-based outgoing longwave radiation. Some of the interesting aspects of Hurricane Opal’s history occurred between 1200 UTC 1 October 1995 and 1200 UTC 5 October 1995. During this period the storm made landfall over the Florida panhandle. The storm reached maximum wind speed of over 130 kt on 4 October 1995. The intensity issue of Opal has drawn much attention. Issues such as the potential vorticity impact from a middle-latitude trough, the angular momentum of the lower-tropospheric inflow layer, the warm ocean temperature anomalies of the northern Gulf of Mexico, and the possible role of mesoconvective concentric eyewall are discussed in this paper.

The main finding of this study is that a reduction of the gradient of angular momentum occurs above the regions of maximum convective heating. This contributes toward stronger cyclonic spinup of parcels that enter the storm environment from the middle latitudes. Another major contributor is the import of angular momentum along the lower-tropospheric inflow channels of the storm. These channels were found to be open, that is, uncontaminated with a plethora of deep convection and heavy rain. This permitted the high angular momentum to advance toward the storm’s interior thus contributing to its intensification.

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T. N. Krishnamurti
,
S. Pattnaik
, and
D. V. Bhaskar Rao

Abstract

This paper addresses physical initialization of precipitation rates for a mesoscale numerical weather prediction model. This entails a slight modification of the vertical profile of the humidity variable that provides a close match between the satellite and model-based rain rates. This is based on the premise that the rain rate from a cumulus parameterization scheme such as the Arakawa–Schubert scheme is most sensitive to the vertical profiles of moist static stability. It is possible to adjust the vertical profile of moisture by a small linear perturbation by making it wetter (or drier) in the lower levels and the opposite at levels immediately above. This can provide a change in the moist static stability in order to achieve the desired rain rate. The procedure is invoked in a preforecast period between hours −24 and 0 following Krishnamurti et al. The present study is the authors’ first attempt to bring in this feature in a mesoscale model. They first noted that the procedure does indeed provide a much closer match between the satellite estimate of initial rain and that from the physical initialization for a mesoscale model. They have examined the impacts of this procedure for the initialization and short-range forecasts of a monsoon rainfall event and a hurricane. In both of these examples it became possible to improve the forecasts of rains compared with those from control runs that did not include the initialization of rains. Among these two examples, the results for the monsoon forecasts that deployed a uniform resolution of 25 km and the Grell and Devenyi scheme over the entire domain had the largest positive impact. The hurricane forecasts example also show improvement over the control run but with less impact, which may be due to heavy rains from explicit clouds in the nonhydrostatic model. Here the results did convey a strong positive impact from the use of the physical initialization; however, forecasts of very heavy rains carry smaller equitable threat scores. These require development of a more robust precipitation initialization procedure.

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Robert S. Ross
,
T. N. Krishnamurti
, and
S. Pattnaik

Abstract

This paper defines a mechanism for the genesis of tropical cyclones from African easterly waves (AEWs) over the eastern Atlantic, the so-called Cape Verde storms. Convective “superbursts” produce strong diabatic heating, which then strengthens the African easterly jet (AEJ), leading to enhanced barotropic energy conversions, which occur at the critical developmental stages of the system.

Diabatic heating is calculated using the Ertel isentropic potential vorticity (IPV) equation, while energy conversions are determined using energy equations first derived by Lorenz. The genesis mechanism is developed from studying Hurricane Bill (2009), as well as Tropical Storm Debby, Hurricane Helene, and a nondeveloping AEW, all from the 2006 NASA African Monsoon Multidisciplinary Analysis (NAMMA) field experiment, using the NCEP Final (FNL) analyses and the Advanced Research Weather Research and Forecasting model (WRF-ARW) simulations.

A striking and singular maximum in the diabatic heating due to the convective superburst is shown to precede by 24–36 h a pronounced maximum in positive barotropic energy conversion, which is demonstrated to occur simultaneously with the strengthening of the AEJ. The maximum in barotropic energy conversion is documented to occur in the developmental stages of the system, typically in the depression or early storm stages.

A physical mechanism is developed to explain how a mesoscale convective superburst can lead subsequently to an enhanced synoptic-scale AEJ over the eastern Atlantic, an enhanced jet that is critical to the genesis mechanism.

The findings agree with cited idealized studies by other investigators who found that moist AEWs grow 3 times stronger than dry waves as a result of faster AEJ development and larger barotropic energy conversions.

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T. N. Krishnamurti
,
J. Sanjay
,
A. K. Mitra
, and
T. S. V. Vijaya Kumar

Abstract

This paper addresses a procedure to extract error estimates for the physical and dynamical components of a forecast model. This is a two-step process in which contributions to the forecast tendencies from individual terms of the model equations are first determined using an elaborate bookkeeping of the forecast. The second step regresses these estimates of tendencies from individual terms of the model equations against the observed total tendencies. This process is executed separately for the entire horizontal and vertical transform grid points of a global model. The summary of results based on the corrections to the physics and dynamics provided by the regression coefficients highlights the component errors of the model arising from its formulation. This study provides information on geographical and vertical distribution of forecast errors contributed by features such as nonlinear advective dynamics, the rest of the dynamics, deep cumulus convection, large-scale condensation physics, radiative processes, and the rest of physics. Several future possibilities from this work are also discussed in this paper.

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T. S. V. Vijaya Kumar
,
T. N. Krishnamurti
,
Michael Fiorino
, and
Masashi Nagata

Abstract

Using currently available operational forecast datasets on the tracks and intensities of tropical cyclones over the Pacific Ocean for the years 1998, 1999, and 2000 a multimodel superensemble has been constructed following the earlier work of the authors on the Atlantic hurricanes. The models included here include forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF), the National Centers for Environmental Prediction/Environmental Modeling Center [NCEP/EMC, the Aviation (AVN) and Medium-Range Forecast (MRF) Models], the U.S. Navy [Naval Operational Global Atmospheric Prediction System, (NOGAPS)], the U.K. Met Office (UKMO), and the Japan Meteorological Agency (JMA). The superensemble methodology includes a collective bias estimation from a training phase in which a multiple-regression-based least squares minimization principle for the model forecasts with respect to the observed measures is employed. This is quite different from a simple bias correction, whereby a mean value is simply shifted. These bias estimates are described by separate weights at every 12 h during the forecasts for each of the member models. Superensemble forecasts for track and intensity are then constructed up to 144 h into the future using these weights. Some 100 past forecasts of tropical cyclone days are used to define the training phase for each basin. The findings presented herein show a marked improvement for the tracks and intensities of forecasts from the proposed multimodel superensemble as compared to the forecasts from member models and the ensemble mean. This note includes detailed statistics on the Pacific Ocean tropical cyclone forecasts for the years 1998, 1999, and 2000.

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T. N. Krishnamurti
,
C. P. Wagner
,
Tina J. Cartwright
, and
Darlene Oosterhof

Abstract

In this paper the authors illustrate wave trains that are excited during the equatorial passage of the annual cycle of monsoonal convection. Twice a year, during roughly the months of December–January and March–April, the annual cycle of monsoonal convection crosses the equator. A principle axis of annual cycle monsoon precipitation extends from the Java Sea to the eastern Himalayas. Monsoonal convection makes a north–south seesaw roughly along this axis each year. Near-equatorial convection provides a tropospheric heat source somewhat akin to that of El Niño over the equatorial Pacific Ocean. This equatorial passage of the monsoonal heat source excites a wave train, somewhat similar to the familiar Pacific–North American pattern. Monsoonal wave trains were extracted from a 9-yr dataset, and a composite geometry was constructed. This note also illustrates excitation of short-period wet and dry spells associated with excitation of this wave train. This is illustrated for several trough and ridge locations of the wave train by examining rainfall for a sequence of days some 10 days prior to and 10 days subsequent to passage of this wave train. There is a strong suggestion that equational passage of monsoon convection does influence short-term dry and wet spells along the wave train; that is, beneath upper troughs (ridges), wet (dry) weather prevails.

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T. N. Krishnamurti
,
S. K. Roy Bhowmik
,
Darlene Oosterhof
,
Gregg Rohaly
, and
Naomi Surgi

Abstract

This paper presents some recent results on physical initialization from the use of a very high resolution global model. Fundamentally this procedure improves the model-based initial rainfall, surface fluxes, and diagnostic cloud amount. Physical initialization is a useful procedure for the nowcasting of rainfall. Correlation between model-based initialized rain and satellite/rain gauge-based rain over the Tropics (for 6-h averages and averaged over transform grid squares) is of the order 0.85. This compares with a correlation of around 0.3 for models that do not include physical initialization. The day 1 tropical rainfall forecast skill is also relatively high for the physically initialized experiments; the correlation is of the order 0.55. It should be noted that the lifetime of mesoconvective systems is approximately 1 day, whereas more organized tropical disturbances may last substantially longer. A major portion of the tropical rainfall is associated with these short-lived systems, hence the skill beyond 1 day degrades somewhat. However, the model does seem to capture the 1-day passage of mesoconvective systems and their coupling to the large-scale, synoptic environment. The mesoconvective systems illustrated exhibit a robust vertical structure of divergence, heating, and vertical motion, which is absent without physical initialization.

The organization of mesoconvective systems (advected by the large-scale circulations and coalescence of the mesoscale elements) appears to play an important role in the formation of tropical storms. The vorticity associated with these mesoscale elements, however, does not exhibit any interesting organization during the forecast as the storms form. The Florida State University atmospheric global circulation model at the resolution T213 discerns the tight central circulation features and the outer rainbands of Hurricane Andrew (1992), which appear similar to the radar imagery; however, the storm as seen from the model is not on the exact scale as that of the radar that is shown. Further enhancement of resolution is needed to model tropical storms on a more realistic scale, which is well known in the modeling community. Overall this study demonstrates that mesoconvective elements are in fact simulated by very high resolution global models. It appears that very high resolution models with an augmented analysis using satellite data may soon aid in resolving the formation issue associated with tropical cyclones and cyclogenesis.

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T. N. Krishnamurti
,
H. S. Bedi
,
G. D. Rohaly
, and
D. Oosterhof

Abstract

The emphasis of this paper is on residue-free budgets of seasonal climate forecasts. It is possible to ask the following question: given a seasonal mean geopotential height simulation from a climate model, what is a breakdown of that contribution from different areas of the model physics and dynamics? In that context, the authors have examined the maintenance of a monsoonal 500-mb ridge, the eastward shift of the Tibetan anticyclone during an El Nin˜o year, and the Pacific-North American pattern. The salient results of this study include a substantial contribution from the advective nonlinear dynamics toward the maintenance (positive or negative) of the seasonal climate.

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D. W. Blake
,
T. N. Krishnamurti
,
S. V. Low-Nam
, and
J. S. Fein

Abstract

In May of 1979 a unique data set was obtained over the desert area in Saudi Arabia near the surface heat low as part of the GARP Monsoon Experiment (MONEX). Analysis of the data reveals that during the day a well-mixed layer extends up to 650 mb, while a stable layer forms above the desert surface during the night. Calculations of the divergence and kinematic vertical velocity show that the entire troposphere is dynamically active with strong descending motions during the day down to a 1 km deep surface layer which has ascending motion in it. There is daytime surface convergence, divergence around 850 mb, and convergence above 700 mb. During the pre-sunrise hours there is descending motion down to the surface with divergence at nearly all levels from the surface up to 550 mb and convergence above this level.

The roles of vertical and horizontal advective, as well as radiative, processes in the thermal budget have been investigated. During the pre-sunrise hours the horizontal advective and longwave radiative processes contribute a cooling which is nearly balanced by the heating due to vertical advective processes. During the midday hours there is net radiative heating and heating due to vertical advective processes. The horizontal advective processes give a cooling up to 400 mb and a heating above. Approximately half of the sensible beat estimated to leave the earth's surface is transferred to the atmosphere between the surface and 975 mb and half between 975 and 650 mb.

Satellite observations show that desert areas experience a net loss of radiation to outer space while the thermal stratification remains nearly invariant from one day to the next. This study indicates that a crucial element in the maintenance of stratification over the desert region in Saudi Arabia is the importation of heat into the upper troposphere coupled with subsidence above this region. This energy supply most likely comes from planetary-scale divergent circulations.

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T. N. Krishnamurti
,
Y. Ramanathan
,
Hua-Lu Pan
,
Richard J. Pasch
, and
John Molinari

Abstract

Modeling of convective rainfall rates is a central problem in tropical meteorology. Toward numerical weather prediction efforts the semi-prognostic approach (i.e., a one time-step prediction of rainfall rates) provides a relevant test of cumulus parameterization methods. In this paper we compare five currently available cumulus parameterization schemes using the semi-prognostic approach. The calculated rainfall rates are compared with observed estimates provided in the recent publication of Hudlow and Patterson (1979). Among these the scheme proposed by Kuo (1974) provides the least root-mean-square error between the calculated and the observed estimates, slightly better than that of Arakawa and Schubert (1974), which was used by Lord (1978a). The simplicity of the approach holds promise for numerical weather prediction. Unlike some of the other schemes this method is not sensitive to and does not require computation of internal parameters such as profiles of cloud mass flux updrafts and downdrafts, detrainment of cloud matter and entrainment of environmental air. The present paper does not address the prognostic evolution and verification of the vertical distribution of temperature, humidity or momentum. These will be compared for the different methods in more detail separately.

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