This page has only limited features, please log in for full access.

Unclaimed
Jan Westerholm
Faculty of Science and Engineering, Åbo Akademi University, FI-20500 Turku, Finland

Honors and Awards

The user has no records in this section


Career Timeline

The user has no records in this section.


Short Biography

The user biography is not available.
Following
Followers
Co Authors
The list of users this user is following is empty.
Following: 0 users

Feed

Journal article
Published: 27 May 2021 in Remote Sensing
Reads 0
Downloads 0

Seawaters exhibit various types of cyclic and trend-like temporal alterations in their biological, physical, and chemical processes. Surface water dynamics may vary, for instance, when the timings, durations, or amplitudes of seasonal developments of water properties alter between years and locations. We introduce a workflow using remote sensing to identify surface waters undergoing similar dynamics. The method, called ocean surface dynamics partitioning, classifies pixels based on their temporal change patterns instead of their properties at successive time snapshots. We apply an efficient parallel computing method to calculate Dynamic Time Warping (DTW) time series distances of large datasets of Earth Observation MERIS-instrument reflectance data Rrs(510 nm) and Rrs(620 nm), and produce a matrix of time series distances between 12,252 locations/time series in the Baltic Sea, for both wavelengths. We define cluster prototypes by hierarchical clustering of distance matrices and use them as initial prototypes for an iterative process of partitional clustering in order to identify areas that have similar reflectance dynamics. Lastly, we compute distances from the time series of the reflectance data to selected physical factors (wind, precipitation, and changes in sea surface temperature) obtained from Copernicus data archives. The workflow is reproducible and capable of managing large datasets in reasonable computation times and identifying areas of distinctive dynamics. The results show spatially coherent and logical areas without a priori information about the locations of the satellite image time series. The alignments of the reflectance time series vs. the observational time series of the physical environment clarify the causalities behind the cluster formation. We conclude that following the changes in an aquatic realm by biogeochemical observations at certain temporal intervals alone is not sufficient to identify environmental shifts. We foresee that the changes in dynamics are a sensitive measure of environmental threats and therefore they will be important to follow in the future.

ACS Style

Tapio Suominen; Jan Westerholm; Risto Kalliola; Jenni Attila. Partition of Marine Environment Dynamics According to Remote Sensing Reflectance and Relations of Dynamics to Physical Factors. Remote Sensing 2021, 13, 2104 .

AMA Style

Tapio Suominen, Jan Westerholm, Risto Kalliola, Jenni Attila. Partition of Marine Environment Dynamics According to Remote Sensing Reflectance and Relations of Dynamics to Physical Factors. Remote Sensing. 2021; 13 (11):2104.

Chicago/Turabian Style

Tapio Suominen; Jan Westerholm; Risto Kalliola; Jenni Attila. 2021. "Partition of Marine Environment Dynamics According to Remote Sensing Reflectance and Relations of Dynamics to Physical Factors." Remote Sensing 13, no. 11: 2104.

Article
Published: 05 March 2020 in Physical Review Letters
Reads 0
Downloads 0

We discuss quantum annealing of the two-dimensional transverse-field Ising model on a D-Wave device, encoded on L×L lattices with L≤32. Analyzing the residual energy and deviation from maximal magnetization in the final classical state, we find an optimal L dependent annealing rate v for which the two quantities are minimized. The results are well described by a phenomenological model with two powers of v and L-dependent prefactors to describe the competing effects of reduced quantum fluctuations (for which we see evidence of the Kibble-Zurek mechanism) and increasing noise impact when v is lowered. The same scaling form also describes results of numerical solutions of a transverse-field Ising model with the spins coupled to noise sources. We explain why the optimal annealing time is much longer than the coherence time of the individual qubits.

ACS Style

Phillip Weinberg; Marek Tylutki; Jami M. Rönkkö; Jan Westerholm; Jan A. Åström; Pekka Manninen; Päivi Törmä; Anders W. Sandvik. Scaling and Diabatic Effects in Quantum Annealing with a D-Wave Device. Physical Review Letters 2020, 124, 090502 .

AMA Style

Phillip Weinberg, Marek Tylutki, Jami M. Rönkkö, Jan Westerholm, Jan A. Åström, Pekka Manninen, Päivi Törmä, Anders W. Sandvik. Scaling and Diabatic Effects in Quantum Annealing with a D-Wave Device. Physical Review Letters. 2020; 124 (9):090502.

Chicago/Turabian Style

Phillip Weinberg; Marek Tylutki; Jami M. Rönkkö; Jan Westerholm; Jan A. Åström; Pekka Manninen; Päivi Törmä; Anders W. Sandvik. 2020. "Scaling and Diabatic Effects in Quantum Annealing with a D-Wave Device." Physical Review Letters 124, no. 9: 090502.

Journal article
Published: 01 April 2016 in Computers & Geosciences
Reads 0
Downloads 0

Digital elevation models (DEMs) are widely used in the modeling of surface hydrology, which typically includes the determination of flow directions and flow accumulation. The use of high-resolution DEMs increases the accuracy of flow accumulation computation, but as a drawback, the computational time may become excessively long if large areas are analyzed. In this paper we investigate the use of graphical processing units (GPUs) for efficient flow accumulation calculations. We present two new parallel flow accumulation algorithms based on dependency transfer and topological sorting and compare them to previously published flow transfer and indegree-based algorithms. We benchmark the GPU implementations against industry standards, ArcGIS and SAGA. With the flow-transfer D8 flow routing model and binary input data, a speed up of 19 is achieved compared to ArcGIS and 15 compared to SAGA. We show that on GPUs the topological sort-based flow accumulation algorithm leads on average to a speedup by a factor of 7 over the flow-transfer algorithm. Thus a total speed up of the order of 100 is achieved. We test the algorithms by applying them to the Revised Universal Soil Loss Equation (RUSLE) erosion model. For this purpose we present parallel versions of the slope, LS factor and RUSLE algorithms and show that the RUSLE erosion results for an area of 12 km x 24 km containing 72 million cells can be calculated in less than a second. Since flow accumulation is needed in many hydrological models, the developed algorithms may find use in many other applications than RUSLE modeling. The algorithm based on topological sorting is particularly promising for dynamic hydrological models where flow accumulations are repeatedly computed over an unchanged DEM.

ACS Style

Johan Sten; Harri Lilja; Jari Hyväluoma; Jan Westerholm; Mats Aspnäs. Parallel flow accumulation algorithms for graphical processing units with application to RUSLE model. Computers & Geosciences 2016, 89, 88 -95.

AMA Style

Johan Sten, Harri Lilja, Jari Hyväluoma, Jan Westerholm, Mats Aspnäs. Parallel flow accumulation algorithms for graphical processing units with application to RUSLE model. Computers & Geosciences. 2016; 89 ():88-95.

Chicago/Turabian Style

Johan Sten; Harri Lilja; Jari Hyväluoma; Jan Westerholm; Mats Aspnäs. 2016. "Parallel flow accumulation algorithms for graphical processing units with application to RUSLE model." Computers & Geosciences 89, no. : 88-95.

Journal article
Published: 01 October 2015 in Journal of Transport Geography
Reads 0
Downloads 0

Utilization of movement data from mobile sports tracking applications is affected by its inherent biases and sensitivity, which need to be understood when developing value-added services for, e.g., application users and city planners. We have developed a method for generating a privacy-preserving heat map with user diversity (ppDIV), in which the density of trajectories, as well as the diversity of users, is taken into account, thus preventing the bias effects caused by participation inequality. The method is applied to public cycling workouts and compared with privacy-preserving kernel density estimation (ppKDE) focusing only on the density of the recorded trajectories and privacy-preserving user count calculation (ppUCC), which is similar to the quadrat-count of individual application users. An awareness of privacy was introduced to all methods as a data pre-processing step following the principle of k-Anonymity. Calibration results for our heat maps using bicycle counting data gathered by the city of Helsinki are good (R2 > 0.7) and raise high expectations for utilizing heat maps in a city planning context. This is further supported by the diurnal distribution of the workouts indicating that, in addition to sports-oriented cyclists, many utilitarian cyclists are tracking their commutes. However, sports tracking data can only enrich official in-situ counts with its high spatio-temporal resolution and coverage, not replace them.

ACS Style

Juha Oksanen; Cecilia Bergman; Jani Sainio; Jan Westerholm. Methods for deriving and calibrating privacy-preserving heat maps from mobile sports tracking application data. Journal of Transport Geography 2015, 48, 135 -144.

AMA Style

Juha Oksanen, Cecilia Bergman, Jani Sainio, Jan Westerholm. Methods for deriving and calibrating privacy-preserving heat maps from mobile sports tracking application data. Journal of Transport Geography. 2015; 48 ():135-144.

Chicago/Turabian Style

Juha Oksanen; Cecilia Bergman; Jani Sainio; Jan Westerholm. 2015. "Methods for deriving and calibrating privacy-preserving heat maps from mobile sports tracking application data." Journal of Transport Geography 48, no. : 135-144.

Journal article
Published: 24 September 2015 in ISPRS International Journal of Geo-Information
Reads 0
Downloads 0

The breakthrough of GPS-equipped smartphones has enabled the collection of track data from human mobility on massive scales that can be used in route recommendation, urban planning and traffic management. In this work we present a fast map server that can generate and visualize heat maps of popular routes online from massive sports track data based on client preferences, e.g., running routes lasting less than an hour. The heat maps shown respect user privacy by not showing routes with less than a predefined number of different users, for instance five. The results are represented to the client using a dynamic tile layer. The current implementation uses data collected by the Sports Tracker mobile application with over 800,000 different tracks and 2.8 billion GPS data points. Stress tests indicate that the server can handle hundreds of simultaneous client requests in a single server configuration.

ACS Style

Jani Sainio; Jan Westerholm; Juha Oksanen. Generating Heat Maps of Popular Routes Online from Massive Mobile Sports Tracking Application Data in Milliseconds While Respecting Privacy. ISPRS International Journal of Geo-Information 2015, 4, 1813 -1826.

AMA Style

Jani Sainio, Jan Westerholm, Juha Oksanen. Generating Heat Maps of Popular Routes Online from Massive Mobile Sports Tracking Application Data in Milliseconds While Respecting Privacy. ISPRS International Journal of Geo-Information. 2015; 4 (4):1813-1826.

Chicago/Turabian Style

Jani Sainio; Jan Westerholm; Juha Oksanen. 2015. "Generating Heat Maps of Popular Routes Online from Massive Mobile Sports Tracking Application Data in Milliseconds While Respecting Privacy." ISPRS International Journal of Geo-Information 4, no. 4: 1813-1826.

Journal article
Published: 01 June 2010 in Journal of Computational Biology
Reads 0
Downloads 0

A central task in protein sequence characterization is the use of a sequence database homology search tool to find similar protein sequences in other individuals or species. PSI-BLAST is a widely used module of the BLAST package that calculates a position-specific score matrix from the best matching sequences and performs iterated searches using a method to avoid many similar sequences for the score. For some queries and parameter settings, PSI-BLAST may find many similar high-scoring matches, and therefore up to 80% of the total run time may be spent in this procedure. In this article, we present code optimizations that improve the cache utilization and the overall performance of this procedure. Measurements show that, for queries where the number of similar matches is high, the optimized PSI-BLAST program may be as much as 2.9 times faster than the original program.

ACS Style

Mats Aspnäs; Kimmo Mattila; Kristoffer Osowski; Jan Westerholm. Code Optimization of the Subroutine to Remove Near Identical Matches in the Sequence Database Homology Search Tool PSI-BLAST. Journal of Computational Biology 2010, 17, 819 -823.

AMA Style

Mats Aspnäs, Kimmo Mattila, Kristoffer Osowski, Jan Westerholm. Code Optimization of the Subroutine to Remove Near Identical Matches in the Sequence Database Homology Search Tool PSI-BLAST. Journal of Computational Biology. 2010; 17 (6):819-823.

Chicago/Turabian Style

Mats Aspnäs; Kimmo Mattila; Kristoffer Osowski; Jan Westerholm. 2010. "Code Optimization of the Subroutine to Remove Near Identical Matches in the Sequence Database Homology Search Tool PSI-BLAST." Journal of Computational Biology 17, no. 6: 819-823.

Journal article
Published: 01 September 2008 in Computer Physics Communications
Reads 0
Downloads 0

We describe the parallel full-f gyrokinetic particle-in-cell plasma simulation code ELMFIRE and the issue of solving an electrostatic potential from particle data distributed across several MPI (Message Passing Interface) processes. The potential is solved through a linear system with a strongly sparse matrix and ELMFIRE stores data of the estimated non-zero diagonals of the whole matrix in every MPI process. We present and compare several memory efficient structures for gathering the matrix data while keeping only a local part of the matrix in each process. We also demonstrate that these alternative structures improve scalability, thus enabling ELMFIRE to use more MPI processes and a finer time and space scale than before without sacrificing performance.

ACS Style

Artur Signell; Francisco Ogando; Mats Aspnäs; Jan Westerholm. Scalable plasma simulation with ELMFIRE using efficient data structures for process communication. Computer Physics Communications 2008, 179, 330 -338.

AMA Style

Artur Signell, Francisco Ogando, Mats Aspnäs, Jan Westerholm. Scalable plasma simulation with ELMFIRE using efficient data structures for process communication. Computer Physics Communications. 2008; 179 (5):330-338.

Chicago/Turabian Style

Artur Signell; Francisco Ogando; Mats Aspnäs; Jan Westerholm. 2008. "Scalable plasma simulation with ELMFIRE using efficient data structures for process communication." Computer Physics Communications 179, no. 5: 330-338.